CH-20 Crustaceans – Integrated Principles of Zoology : Chapter Notes

This document contains Chapters 20 to 21 CHAPTER 20 CRUSTACEANS CHAPTER OUTLINE 20.1. Overview (Figure 20.1) A. Over 67,000 living species and probably several times that number have not been described. B. Together, insects and crustacea compose over 80% of all named animal species. C. Probably the most abundant animals in the world are members of the copepod genus Calanus. D. Crustacea and Hexapoda share five derived features and are united in clade Pancrustacea E. Crustaceans are divided among three subgroups: Oligostraca, Xenocarida, and Vericrustacea F. Oligostraca contains members of the former phylum Pentastomida. 1. Pentastomids are also called tongue worms. 2. They are parasites of vertebrates, living in lungs or nasal cavities. 3. They are closely related to fish lice. 20.2. Subphylum Crustacea (Figure 20.2) A. General Nature of a Crustacean 1. The main distinguishing characteristic of crustaceans is that they have two pairs of antennae. 2. The head also has a pair of mandibles and two pairs of maxillae. 3. There is one pair of appendages on each additional segment; some segments may lack appendages. 4. All appendages, except perhaps the first antennae, are biramous (they have two main branches). 5. Primitive crustaceans may have up to 60 segments; derived crustaceans have fewer segments. 6. The tagmata are usually head, thorax and abdomen but they are not homologous across taxa. 7. In most Crustacea, one or more thoracic segments are fused with the head as a cephalothorax. 8. The arrangement of tagmata in Malacostraca is the ancestral plan. a. The head has five fused somites, the thorax has eight and the abdomen has six somites. b. The anterior end is a non-segmented rostrum. c. The posterior telson, with the last abdominal somite and its uropods, forms a tail. 8. The dorsal covering is the carapace; it may cover most of the body or just the cephalothorax. B. Form and Function 1. External Features (Figure 20.3) a. The secreted cuticle is made of chitin, protein and calcareous material. b. Heavy plates have more calcareous deposits; the joints are soft and thin, allowing flexibility. c. Dorsal tergum and ventral sternum plates on each somite lacking a carapace. (Figure 19.2) d. The telson is not a somite; it bears the anus and may be homologous to the pygidium. e. In some species, the telson may bear a pair of processes, the caudal furca. f. Gonopores may be at the base of appendages, at the tail, or on somites without legs. 2. Appendages (Figures 20.3 − 20.5; Table 20.1) a. Members of Malacostraca and Remipedia have appendages on each somite. b. Other classes may not bear appendages on the abdominal somites. c. Specialization of appendages is based on the basic biramous plan. d. The maxilliped has a basal protopod, a lateral exopod, and a medial endopod. e. On crustacean limbs, endites are medial processes, exites are lateral processes and the epipod is an exite on the protopod. f. Crayfish appendages represent serial homology; they have evolved a wide variety of walking legs, mouthparts, swimmerets, etc. from modification of the basic biramous appendage. 3. Internal Features a. Muscular and nervous systems and segmentation show metamerism of annelid-like ancestors. b. Hemocoel 1) The major body space in arthropods is not a coelom but a persistent blastocoel that becomes blood-filled hemocoel. 2) In crustaceans, coelomic compartments remain as end sacs of excretory organs and gonads. 3) Thus, arthropods are coelomates only in the technical sense of the term. c. Muscular System (Figure 20.6) 1) Striated muscles make up a major portion of the crustacean body. 2) Most muscles are arranged as antagonistic groups; flexors draw a limb toward the body and extensors straighten a limb out. 3) Abdominal flexors of a crayfish allow it to swim backward. 4) Strong muscles located on each side of the stomach control the mandibles. d. Respiratory System (Figure 20.7) 1) Smaller crustaceans may exchange gases across thinner areas of cuticle on their appendages, etc. 2) Larger crustaceans use featherlike gills for gas exchange. 3) The decapod carapace overlaps the gill cavity, leaving anterior and posterior openings. 4) The “bailer” of the second maxilla draws water over the gill filaments. 5) Gills may project from the pleural wall, the articulation of thoracic legs with body, or thoracic coxae. e. Circulatory System (Figure 20.7) 1) Crustaceans and other arthropods have an “open” circulatory system; there is no system of veins to separate blood from interstitial fluid. 2) Hemolymph leaves the heart by arteries but washes through hemocoel to return to the heart via sinuses. 3) The dorsal heart is the propulsive organ; it is a single-chambered sac of striated muscle. 4) Hemolymph enters the heart from the surrounding pericardial sinus. 5) Valves in the arteries prevent backflow of hemolymph. 6) Small arteries empty into tissue sinuses that discharge into the large sternal sinus. 7) Afferent sinus chambers carry hemolymph to the gills, if present, for gas exchange. 8) Hemolymph may be colorless, reddish, or bluish and contains ameboid cells that may help prevent clotting. 9) Hemocyanin and/or hemoglobin are respiratory pigments. f. Excretory System (Figures 20.6, 20.8) 1) Antennal or maxillary glands open at the base of those structures. 2) Decapods have antennal glands called green glands. 3) The end sac of an antennal gland has a small vesicle and a spongy labyrinth. 4) Labyrinth connects by an excretory tubule to a dorsal bladder that opens to exterior pore. 5) Hydrostatic pressure within hemocoel provides force for filtration of fluid into end sac. 6) Resorption of salts and amino acids occurs as the filtrate passes the excretory tubule and bladder; this mainly regulates the ionic and osmotic composition of body fluids. 7) Nitrogenous wastes, mostly ammonia, are excreted across thin areas of cuticle in the gills. 8) Freshwater crustaceans are constantly threatened with over-dilution with water; gills must actively absorb Na+ and Cl-. 9) Marine crustaceans have urine that is isosmotic with the blood. g. Nervous and Sensory Systems (Figures 20.6, 20.9) 1) Though there is much in common between the nervous systems of annelids and crustaceans, the nervous system of crustaceans has more fusion of ganglia. 2) A pair of supra-esophageal ganglia connects to the eyes and the two pairs of antennae. 3) Neuron connectives join this brain to the subesophageal ganglion, a fusion that supplies nerves to the mouth, appendages, esophagus and antennal glands. 4) The double ventral nerve cord has pairs of ganglia at each somite to control appendages. 5) Eyes and statocysts are the largest sensory organs. 6) Tactile hairs occur on the body, especially on the chelae, mouthparts and telson. 7) Chemical sensing of taste and smell occurs in hairs on antennae, antennules, within mouthparts, and other locations. 8) The statocyst opens at the base of each first antenna in crayfishes, and is lined with sensory hairs that detect changes in the position of grains of sand (that serve as statoliths) to detect changes in body position. 9) Eyes are compound, made of many units called ommatidia. (Figure 20.9) a. A transparent cornea focuses light down each columnar ommatidium.that is surrounded by a sleeve of distal retinal, proximal retinal and reflecting pigment cells. b. In bright light, each ommatidium sees a restricted visual area, resulting in a mosaic. image. c. In dim light, distal and proximal pigments separate to produce a continuous image. 4. Reproduction, Life Cycles, and Endocrine Function a. Diversity of Reproduction 1) Barnacles are monoecious but generally cross-fertilize. 2) In some ostracods, males are scarce and reproduction is by parthenogenesis. 3) Most crustaceans brood eggs in brood chambers, in brood sacs attached to the abdomen, or attached to abdominal appendages. (Figure 20.16) 4) Crayfishes develop directly without a larval form. 5) Most crustaceans have a larva unlike the adult in form, and undergo metamorphosis. 6) The nauplius is a common larval form with uniramous first antennae, and biramous second antennae and mandibles that all aid in swimming. (Figures 20.10, 20.20) 7) Appendages and somites are added in a series of molts. 8) Metamorphosis of a barnacle proceeds from a free-swimming nauplius to a larva with a bivalve carapace and finally to a sessile adult with plates. b. Molting and Ecdysis (Figures 20.11, 20.12) 1) Molting is necessary for a crustacean to increase in size; the exoskeleton does not grow. 2) The physiology of molting affects reproduction, behavior and many metabolic processes. 3) The underlying epidermis secretes the cuticle. 4) An outermost epicuticle is very thin lipid-impregnated protein. 5) Most of the cuticle is several layers of the procuticle. 6) Exocuticle is beneath the epicuticle and contains protein, calcium salts and chitin. 7) Endocuticle has heavily calcified principal layer and an uncalcified membranous layer. 8) Molting animals grow in the intermolt phases, or instars; soft tissue increases in size until there is no space within the cuticle. 9) When the body fills the cuticle, the animal is in the premolt phase. 10) Epidermal cells enlarge before ecdysis. 11) They secrete a new epicuticle and begin secreting a new exocuticle. 12) Enzymes released into the area above the new epicuticle dissolve the old endocuticle. 13) Some calcium salts are stored in gastroliths in the walls of the stomach. 14) When only old exocuticle and epicuticle remain, the animal swallows water to expand and burst the old cuticle. 15) The soft new cuticle stretches and then hardens with the deposition of inorganic salts. 16) Molting occurs often in young animals and may cease in adults. c. Hormonal Control of Ecdysis 1) Temperature, day length or other stimuli trigger central nervous system to begin ecdysis. 2) This induces X-organ within medulla terminalis of brain to release less molt-inhibiting hormone. 3) This promotes release of molting hormone from the Y-organs; leading to ecdysis. d. Other Endocrine Functions 1) Removing eyestalks accelerates molting and prevents color changes to match the background. 2) Hormones from neurosecretory cells in the eyestalk control dispersal of cell pigment. 3) Neurosecretions from the pericardial organs cause an increase in heartbeat. 4) Androgenic glands in male amphipods stimulate expression of male characteristics. 5. Feeding Habits (Figure 20.13) a. The same fundamental mouthparts are adapted to a wide array of feeding habits. b. Suspension feeders generate water currents in order to eat plankton, detritus and bacteria. c. Predators consume larvae, worms, crustaceans, snails and fishes. d. Scavengers eat dead animal and plant matter. e. Crayfishes have a two-part stomach; a gastric mill grinds up food in the first compartment. 20.3. Brief Survey of Crustaceans Crustaceans are an extensive group of over 67,000 species and many subdivisions, however classification within Crustacea is in flux with traditional classes and subclasses no longer supported by molecular phylogenies. A. Oligostraca (Figure 20.14) 1. Ostracoda (Figure 20.14A) are enclosed in a bivalve carapace, resemble tiny clams, 0.25–10 mm long a. There is considerable fusion of trunk somites; thoracic appendages are reduced to two or one. b. Most are benthic or climb onto plants, but some are planktonic, parasitic or burrowing. c. They are widespread in both marine and freshwater habitats. d. There are 6,000 known species. Most are dioecious, but some are parthenogenetic. e. Development is by gradual metamorphosis. 2. Mystacocarida (Figure 20.14B) is a class of tiny crustaceans (less than 0.5 mm long) a. They live in interstitial marine water between sand grains. b. Only 10 species have been described, by they are widely distributed through many parts of the world. 3. Branchiura (Figures 20.15A and 20.15B) a. Branchiurans lack gills; most are ectoparasites of fish. b. Found on both marine and freshwater fish, they are 5–10 mm long. c. They have a broad, shield-like carapace, compound eyes, four biramous thoracic swimming appendages and a short unsegmented abdomen. d. The second maxillae are modified as suction cups to hold on to the host fish. e. Development is direct with no nauplius stage. 4. Pentastomida (Figures 20.15C, 20.15D, and 20.15E) a. Tongue worms consist of about 90 species that parasitize vertebrate respiratory systems. b. Most infect reptile lungs; a few infect air sacs of birds or mammals. c. Are more common in tropical regions extending into North America, Europe and Australia. d. They range from one to 13 cm in length; rings provide a segmented appearance. e. A chitinous cuticle that is regularly molted covers them. f. Five protuberances on the anterior end provide the phylum name. g. Four of the protuberances bear claws; the fifth has the mouth and two pairs of hooks for attachment. h. The simple straight digestive system is adapted for sucking. i. The nervous system has paired ganglia along the ventral nerve cord. j. They lack any circulatory, excretory or respiratory organs. k. Sexes are separate and females are larger than males. l. Female emits millions of eggs that enter host’s trachea, are swallowed and leave with feces. m. Larvae hatch out as oval, tailed creatures with four stumpy legs. n. Most life cycles require an intermediate vertebrate host, usually a fish or reptile. o. After eaten by an intermediate host, larvae penetrate intestine and migrate until they change into nymphs; nymph eventually becomes encapsulated and remains dormant until eaten. p. When eaten, the juvenile finds its way to the lung, feeds on blood and tissue, and matures. B. Xenocarida (Figure 20.16) 1. Remipedia (Figure 20.16A) a. This class is small, with only 10 described species; all are found in caves connected to the sea. b. Primitive features include 25–38 segments with similar, paired, biramous, swimming appendages. c. Antennules are also biramous. d. Maxillae and maxillipeds are prehensile and specialized for feeding. e. Swimming legs are directed laterally rather than ventrally as is found in copepods and cephalocarids. 2. Cephalocarida (Figure 20.16B) a. This is another small class, with only nine species described. b. They live in coastal bottom sediments from intertidal zones to 300 meters depth. c. Thoracic limbs and the second maxillae are very similar. d. Cephalocarids lack eyes, a carapace or abdominal appendages. e. They are true hermaphrodites and unique in discharging eggs and sperm through the same duct. C. Vericrustacea (Figure 20.17-20.27) 1. Branchiopoda (Figure 20.15) a. There are over 10,000 species of Branchiopoda. b. There are four orders in this class. 1) Order Anostraca includes fairy shrimp and brine shrimp that lack a carapace. 2) Order Notostraca includes tadpole shrimp; the carapace forms a large dorsal shield. 3) Order Diplostraca includes water fleas (cladocerans) with a carapace that encloses the body but not the head and clam shrimp (conchostracans) which are enclosed by a bivalved carapace. b. All have flattened and leaf-like legs that are the chief respiratory organs. c. Legs assist in suspension feeding and, except for cladocerans, function in locomotion. d. Most are freshwater organisms; cladocerans are an important part of the freshwater zooplankton. e. Similar to rotifers, many use parthenogenesis to rapidly boost summer populations and then use sexual reproduction with the onset of unfavorable conditions. f. Fertilized eggs are highly resistant to cold and are critical for winter survival of the population. g. Cladocerans have mostly direct development; other branchiopods have gradual metamorphosis. 2. Copepoda (Figure 20.18A) a. This group is third in numbers of species. b. They lack a carapace and retain the simple, median, nauplius eye in the adult. c. They have a single pair of uniramous maxillipeds and four pairs of flattened, biramous, thoracic swimming appendages. d. A major joint separates posterior from anterior, appendage-bearing portion of the body. e. Antennules are often longer than other appendages. f. Parasitic forms are highly modified and reduced, often unrecognizable as arthropods. g. Free-living copepods may be the dominant consumer. h. The marine copepod Calanus is the most abundant organism in the zooplankton by biomass. i. Cyclops and Diaptomus form important elements of freshwater plankton. j. Some free-living copepods are intermediate hosts of human parasitic tapeworms and nematodes. k. Development is indirect and some highly modified parasites have unusual metamorphoses. 3. Tantulocarida (Figure 20.18B) a. This group has only recently been described and it includes only about 12 species. b. They are tiny copepod-like ectoparasites of deep-sea benthic crustaceans. c. There are no head appendages beyond the one pair of antennae in sexual females. d. They likely alternate between a parthenogenetic cycle and a bisexual cycle with fertilization. e. Tantalus larvae penetrate the cuticle of the host by a mouth tube. f. The abdomen and all thoracic limbs are lost during metamorphosis to an adult. 4. Thecostraca (Cirripedia) (Figures 20.19 and 20.20) a. Thecostraca includes barnacles (order Thoracica) and three orders of burrowing or parasitic forms. b. Barnacle adults are sessile and attach directly (acorn barnacles) or by a stalk (goose barnacles). c. The carapace surrounds the body and secretes a set of calcareous plates. d. The head is reduced, the abdomen is absent and the thoracic legs are long with hairlike setae. e. Many-jointed cirri that bear setae are extended from the plates to feed on small particles. f. Intertidal barnacles may be exposed to drying; the plates close to protect them. g. Most non-parasitic barnacles are hermaphroditic and undergo metamorphosis during development. h. Most hatch as nauplii and become cyprid larvae with a bivalve carapace and compound eyes. i. They attach to the substrate by their first antennae and adhesive glands. j. They secrete calcareous plates, lose eyes and change swimming appendages to filtering cirri. k. Parasitic forms may have a kentrogon stage that injects cells into the hemocoel of the host. 5. Malacostraca a. This is largest class of Crustacea with over 20,000 species, and is the most diverse. b. This the class contains three subclasses, 14 orders, and many suborders. c. Order Isopoda (Figures 20.21A, 20.21B, 20.22) 1) These are the only truly terrestrial crustaceans; they also have marine and freshwater forms. 2) They are dorsoventrally flattened, lack a carapace and have sessile compound eyes. 3) The first pair of thoracic limbs are maxillipeds; the remaining thoracic limbs lack exopods. 4) Abdominal appendages bear gills, except for the uropods. 5) Common land forms include the sow bugs and pill bugs. 6) The cuticle lacks the protection of insect cuticle and they must live in moist conditions. 7) Some isopods are highly modified as parasites of fishes or crustaceans (Figure 20.22). 8) Development is typically direct but may be metamorphic in parasitic forms. d. Amphipoda (Figures 20.23 and 20.24) 1) Amphipods resemble isopods; they lack a carapace, have sessile compound eyes, and one pair of maxillipeds. 2) However, they are compressed laterally, and gills are in the thoracic position. 3) Abdominal and thoracic limbs are grouped for jumping and swimming. 4) Many are marine; others are beach-dwelling, freshwater, or parasitic. 5) Development is direct. e. Euphausiacea (Figure 20.25) 1) This order only has about 90 species but includes the important ocean plankton called “krill.” 2) They have a carapace that does not completely enclose the gills. 3) They lack maxillipeds and have all limbs equipped with exopods. 4) Most are bioluminescent with a light-producing organ called a photophore. 5) They form a major component of the diet of baleen whales and of many fishes. 6) Eggs hatch as nauplii and development is direct. f. Decapoda (Figures 20.26, 20.27) 1) Decapods have five pairs of walking legs and three pairs of maxillipeds. In crabs the first pair of walking legs forms pincers. 2) Range from a few millimeters to the largest arthropod, a Japanese crab with 4 meter leg-span. 3) About 18,000 species known; including crayfishes, lobsters, crabs and true shrimp. 4) Crabs have a broader cephalothorax and reduced abdomen, compared to crayfish or lobsters. 20.4. Phylogeny and Adaptive Diversification A. Phylogeny (Figure 20.1) 1. Using molecular characters over morphological characters changes the crustacean phylogenies. 2. A new arrangement of crustacean taxa places pentastomids, branchiurans, mystacocarids, and ostracods as a clade that diverged from other crustaceans at the base of the tree. Morphologists had assumed that members of Remipedia were the most ancient group. 3. Fossils of an arthropod in the Mississippian period are likely the sister group to remipedians. 4. One theory is that each modern somite represents two ancestral somites that fused together, forming the biramous appendage. 5. It is now known that modulation in expression of the Distal-less (Dll) gene is involved. This suggests that uniramous limbs are not necessarily ancestral to biramous limbs and the uniramous condition in Remipedia is a derived state. 6. The pentastomids were placed in Ecdysozoa because of the similarities in larvae, molting of the cuticle, and sperm morphology. 7. Phylogenies based on ribosomal RNA genes and affirmed in sequences of mitochondrial DNA confirm that pentastomids are crustaceans. 8. Molecular phylogenies sometimes place insects (Hexapoda) within Crustacea, but there is no general agreement as to where they belong. B. Adaptive Diversification 1. Crustaceans are unquestionably the dominant arthropod in marine environments. 2. They also share dominance in freshwater environments with the insects. 3. The class Malacostraca is most diverse and members of Copepoda are most abundant. 4. Copepods are particularly successful as parasites of both vertebrates and invertebrates; some are herbivorous and are critical to food webs within marine ecosystems. Lecture Enrichment 1. The anatomy of living Daphnia and Cyclops can be viewed using 2x2 slide projection cells. 2. The many anatomical terms will require either labwork or substantial visuals for student understanding. 3. It is possible to point out how it is easier to establish the evolutionary lineage of a narrowly adapted and monotonous group such as Pycnogonida, while the evolution of an ancient and highly diverse group such as the Crustacea is far more difficult to decipher. 4. Crustaceans are often labeled the “insects of the sea,” although some have made inroads to freshwater and a few terrestrial environments. Meanwhile, the insects appear to be fairly well restricted from marine environments. The reason for this partitioning is not well understood and is controversial, but students at this level of understanding can begin to knowledgeably speculate. Commentary/Lesson Plan Background: Some students will have “experienced” and “internalized” the internal muscular anatomy of lobsters, crabs, shrimp, prawns, and even crayfish when they have eaten seafood, although many do not intellectually focus on these structures at the time. Some students have raised “sea monkeys” which are brine shrimp. Others may be familiar with smaller shrimp as fish food if they have maintained a home aquarium. Misconceptions: The belief that evolution leads to ever more segments, etc. can be countered with the examples given here; the most primitive classes such as Remipedia have the most segments (although they are mostly uniform) and the derived groups have fused body segments and have generally lost appendages during specialization. The parasitic forms are most extreme in this reduction–and most recent! Schedule: HOUR 1 20.1. Overview 20.2. Subphylum Crustacea A. General Nature of a Crustacean B. Form and Function 20.3. Brief Survey of Crustaceans A. Ostracoda B. Mystacocarida C. Branchiura D. Pentastomida HOUR 2 E. Remipedia F. Cephalocarida G. Branchiopoda H. Copepoda I. Tantulocarida J. Thecostraca K. Malacostraca 20.4. Phylogeny and Adaptive Diversification A. Phylogeny B. Adaptive Diversification ADVANCED CLASS QUESTIONS: 1. Consider a lab experiment where a crayfish was placed in a clean aquarium free of sand but provided with a bottom coating of small iron filings. If enough time were allowed for the crayfish to shed its exoskeleton, an iron filing would have to be incorporated into the statocyst as a statolith. What would happen if you then held a magnet above the crayfish? Answer: If a crayfish were placed in an aquarium with a bottom coating of small iron filings, and it shed its exoskeleton, incorporating an iron filing into the statocyst as a statolith, holding a magnet above the crayfish could potentially affect its behavior and orientation. The statocyst in crustaceans like crayfish is a sensory organ responsible for detecting gravity and acceleration, helping the crayfish maintain balance and orientation in the water. The statolith, typically composed of calcium carbonate or other dense materials, rests on sensory hairs within the statocyst, providing information about the crayfish's orientation relative to gravity. When a magnet is held above the crayfish, the magnetic field would exert a force on the iron filing incorporated into the statocyst, potentially causing the filing to move or alter the sensory input received by the crayfish. Depending on the strength and orientation of the magnetic field, this could disrupt the normal functioning of the statocyst, leading to aberrant sensory feedback and affecting the crayfish's ability to maintain balance and orientation. In essence, holding a magnet above the crayfish could interfere with its ability to sense gravity and orient itself properly in the water, potentially causing disorientation or changes in behavior. However, the extent of the effect would depend on factors such as the strength and proximity of the magnet, as well as the size and position of the iron filing within the statocyst. 2. A “softcraw” is a crayfish that has just shed its exoskeleton and not yet hardened its new cuticle. What behaviors would you expect this softcraw to exhibit in the wild? Answer:A "softcraw," or a crayfish that has just shed its exoskeleton and has not yet hardened its new cuticle, is in a vulnerable state. In the wild, softcraws exhibit specific behaviors to protect themselves and facilitate the hardening process of their new exoskeleton. Here are some behaviors you might expect to observe in a softcraw: 1. Seeking Shelter: Softcraws are more susceptible to predation and environmental stressors due to their soft and fragile exoskeleton. Therefore, they tend to seek shelter in concealed areas such as under rocks, logs, or vegetation to minimize exposure to potential predators and abrasive surfaces. 2. Reduced Activity: Softcraws typically exhibit reduced activity levels compared to mature crayfish. They may move cautiously and remain relatively immobile to conserve energy and avoid unnecessary risks that could damage their soft exoskeleton. 3. Feeding Avoidance: Softcraws may temporarily reduce or avoid feeding until their new exoskeleton hardens sufficiently to provide adequate protection. This minimizes the risk of injury or predation during the vulnerable post-molt period. 4. Social Avoidance: Softcraws may exhibit avoidance behaviors towards conspecifics (other crayfish) and potential predators to reduce the likelihood of encounters that could result in injury or predation. They may actively avoid areas with high crayfish densities or predators until their exoskeleton hardens. 5. Thigmotaxis: Softcraws may exhibit thigmotactic behavior, seeking close physical contact with solid surfaces such as the substrate or sheltering structures. This behavior provides additional support and stability, helping them navigate and protect themselves during the vulnerable post-molt period. 6. Increased Vulnerability: Despite their avoidance behaviors, softcraws remain more vulnerable to predation and environmental hazards until their new exoskeleton fully hardens. Predators such as fish, birds, and larger crayfish may actively target softcraws during this period, contributing to high mortality rates. Overall, softcraws exhibit a suite of behaviors aimed at minimizing exposure to predation, reducing energy expenditure, and facilitating the hardening process of their new exoskeleton. These behaviors help ensure their survival during the vulnerable post-molt period until they regain full functionality and protection from their hardened exoskeleton. 3. Over a century ago, it was noted that whenever the eyestalks of crustaceans were removed, they were unable to adjust their body coloration to remain camouflaged. This was interpreted as proof that they detected their visual environment and mentally controlled the color pattern. Why do we now know that this complex reasoning is not involved? Answer:We now know that the complex reasoning involving mental control of coloration in crustaceans, as inferred from the effects of removing eyestalks, is not accurate due to several reasons: 1. Neurophysiological Understanding: Advances in neurophysiology have revealed that the primary function of crustacean eyestalks is sensory perception rather than cognitive processing. Eyestalks contain photoreceptors that detect light and transmit visual information to the brain, where it is processed. Removing the eyestalks disrupts this sensory input, affecting the crustacean's ability to respond to changes in light and adjust its coloration accordingly. 2. Simple Reflexive Responses: Crustaceans, like many other animals, exhibit reflexive responses to external stimuli without requiring complex cognitive processing. The ability to change coloration in response to environmental cues, known as physiological color change, is often mediated by hormonal and nervous systems that regulate pigment distribution in the skin. These responses can occur without conscious awareness or cognitive reasoning. 3. Experimental Evidence: Experimental studies have demonstrated that crustaceans are capable of physiological color change even in the absence of intact eyestalks or visual input. For example, researchers have observed color changes in crustaceans kept in constant darkness or with blinded eyestalks, indicating that visual perception is not necessary for color adjustment. Instead, hormonal and nervous mechanisms control color change in response to other environmental cues, such as changes in temperature, substrate, or social interactions. 4. Evolutionary Perspective: The ability to change coloration for camouflage or communication purposes is a widespread trait among animals, including crustaceans. This ability has evolved through natural selection to enhance survival and reproductive success in diverse habitats. While visual input may play a role in triggering color changes in some species, the underlying mechanisms are primarily physiological rather than cognitive. Overall, while the effects of removing eyestalks on coloration in crustaceans were once interpreted as evidence of complex mental control, our understanding of crustacean biology has since advanced, revealing the neurophysiological basis and evolutionary origins of color change behaviors. This research underscores the importance of considering multiple factors, including sensory perception, physiological mechanisms, and evolutionary history, in interpreting animal behaviors and adaptations. 4. Throughout the animal kingdom, we tend to find the female body plan is “basic.” An animal will develop into a female unless something alters this developmental pathway. How does the functioning of the androgenic glands support or refute this rule? Answer:The observation that the female body plan is often considered "basic" across the animal kingdom, with individuals typically developing into females unless influenced by specific factors, reflects a widespread phenomenon known as female default development. In this developmental pathway, the absence or minimal presence of certain factors leads to the development of a female phenotype. However, the functioning of androgenic glands can both support and refute this rule, depending on the context and species involved. 1. Supporting Female Default Development: - In many species, including mammals and some reptiles, the presence of androgenic glands (such as testes in males) is necessary to induce the development of male characteristics. Without the influence of androgens, individuals default to the female developmental pathway. - For example, in mammals, the absence of the SRY gene on the Y chromosome results in the development of ovaries and a female phenotype by default. In the absence of androgenic stimulation, the reproductive system develops along the female pathway. 2. Refuting Female Default Development: - In some species, such as birds and certain fish, the default developmental pathway may differ from mammals. In these cases, factors other than androgens may play a primary role in determining sex differentiation. - Additionally, in species where androgenic glands are present in both sexes but exert different effects, the concept of female default development may not apply universally. For example, in some reptiles, both male and female individuals possess gonads with the potential to differentiate into testes or ovaries, but the presence or absence of environmental cues may determine the ultimate sex phenotype. In summary, while the functioning of androgenic glands can support the concept of female default development in certain species, it may not universally apply across the animal kingdom. The interplay of genetic, hormonal, and environmental factors can result in diverse mechanisms of sex determination and differentiation, leading to variations in the expression of male and female traits. Therefore, while the default female developmental pathway is a common phenomenon, exceptions exist, and the role of androgenic glands in sex determination can vary depending on the species and context. 5. The textbook provides the scenario where a rhizocephalan larva exploits the host female crab’s care of its egg mass for the parasite’s survival. It is easy to imagine the variations in the parasite’s life cycle that allowed selection for this life style. What possible sequence of selective steps or evolutionary scenario would lead to the parasite converting the male host to a behavioral female? Answer:The scenario you're describing, where a rhizocephalan parasite converts its male host into a behavioral female, is indeed fascinating and involves several evolutionary steps. Here's a possible sequence of selective steps or evolutionary scenario that could lead to this transformation: 1. Initial Parasitic Strategy: The rhizocephalan parasite starts as a larva that infects a male host crab. Initially, the parasite's primary goal is likely to ensure its own survival and reproduction within the host environment. This may involve hijacking the host's resources for its own growth and development without significantly altering the host's behavior or reproductive physiology. 2. Manipulation of Host Physiology: Over time, through natural selection, the parasite evolves mechanisms to manipulate the host's physiology and behavior to its advantage. This may involve the secretion of chemical cues or hormones that influence the host's endocrine system and reproductive pathways. 3. Proliferation and Transmission: As the parasite matures within the host, it may produce large numbers of offspring, which need to be dispersed to infect new hosts. The parasite may evolve strategies to increase its transmission success, such as modifying the host's behavior to enhance mate attraction or reproductive output. 4. Exploitation of Host Reproductive Behavior: The parasite exploits the host's natural reproductive behaviors to ensure the survival of its offspring. For example, if the male host is responsible for guarding and caring for the eggs, the parasite may evolve mechanisms to induce the host to exhibit female-like behaviors associated with egg care and protection. 5. Gradual Conversion to Behavioral Female: Through successive generations of selection, the parasite refines its ability to manipulate the host's reproductive physiology and behavior. This may involve the gradual conversion of the male host into a behavioral female, where it assumes female-like roles in reproduction, such as egg care, while still maintaining some male physiological traits. 6. Enhanced Fitness for Parasite: The parasite benefits from converting the male host into a behavioral female by increasing the likelihood of successful egg fertilization, protection, and dispersal. This enhances the parasite's fitness by ensuring the survival and transmission of its offspring to new hosts. 7. Co-evolutionary Dynamics: The co-evolutionary arms race between the parasite and its host continues, with hosts evolving defenses against parasitic manipulation and parasites evolving counter-strategies to overcome host defenses. This ongoing dynamic process drives further adaptations and refinements in the parasite's life cycle. Overall, the evolution of a rhizocephalan parasite converting its male host into a behavioral female likely involves a series of incremental steps driven by natural selection, ultimately leading to the exploitation of host reproductive behaviors for the parasite's benefit. 6. Examine the different adaptations of the mollusc body plan, and compare the body parts that differ within the groups. Describe how these body parts differ from those of an annelid or an arthropod. If it is a problem in your region, spend some time to illustrate the case of the introduced (exotic) zebra mussel. Answer: Mollusks exhibit a diverse array of adaptations in their body plans, with distinct variations among different groups such as gastropods (snails and slugs), bivalves (clams, mussels, oysters), and cephalopods (squid, octopus, nautilus). Here's a comparison of the body parts that differ within these groups and how they contrast with those of annelids and arthropods: 1. Gastropods: - Gastropods typically have a single, coiled shell or no shell at all. In shelled gastropods like snails, the shell is often asymmetrical and spiraled. - Their muscular foot is well-developed and adapted for crawling or swimming. - Radula: A rasping tongue-like structure used for feeding, composed of chitinous teeth arranged in rows. - Differences from annelids: Unlike annelids, gastropods typically lack segmented bodies and appendages. They also possess a radula, which is absent in annelids. - Differences from arthropods: Gastropods lack jointed appendages and an exoskeleton, which are characteristic features of arthropods. 2. Bivalves: - Bivalves have two hinged shells that enclose the soft body, providing protection and support. - They lack a distinct head region and typically have reduced sensory structures. - Bivalves use a muscular foot for burrowing, anchoring, or locomotion. - Differences from annelids: Bivalves lack segmentation and parapodia, which are present in annelids. They also have a distinct shell, absent in annelids. - Differences from arthropods: Bivalves lack jointed appendages and a well-defined head region, which are characteristic features of arthropods. 3. Cephalopods: - Cephalopods have a distinct head region with large, complex eyes and well-developed sensory organs. - They lack an external shell or have a reduced internal shell (except for nautiluses). - Cephalopods possess a highly developed nervous system and complex behaviors, including learning and problem-solving abilities. - Differences from annelids: Cephalopods have a well-defined head region with eyes, a feature absent in most annelids. They lack segmentation and parapodia. - Differences from arthropods: Cephalopods lack jointed appendages and an exoskeleton, which are characteristic features of arthropods. Now, regarding the case of the introduced zebra mussel (Dreissena polymorpha), this invasive species has caused significant environmental and economic problems in regions where it has been introduced. Originally native to the Caspian and Black Seas, zebra mussels were inadvertently introduced to North America and Europe through ballast water discharged by ships. Key points about the zebra mussel invasion include: 1. Ecological Impact: Zebra mussels are highly invasive and can reproduce rapidly, forming dense colonies that outcompete native species for food and habitat. They filter large volumes of water, leading to increased water clarity but also disrupting aquatic ecosystems by altering nutrient cycles and reducing plankton populations. 2. Economic Costs: Zebra mussels attach themselves to hard surfaces such as pipes, boat hulls, and water intake structures, causing significant damage to infrastructure and increasing maintenance costs for industries such as power plants and water treatment facilities. 3. Management Efforts: Management strategies for controlling zebra mussel populations include chemical treatments, physical removal, and the introduction of natural predators or competitors. However, these efforts are often challenging and costly, and eradication is rarely feasible once established. The case of the zebra mussel illustrates the significant ecological and economic impacts that can result from the introduction of invasive species. It underscores the importance of effective management strategies and prevention measures to mitigate the spread of invasive species and protect native ecosystems. CHAPTER 21 HEXAPODS CHAPTER OUTLINE 21.1. Diversity and Characteristics A. Phylum Arthropoda: Subphylum Hexapoda (Figure 21.1) 1. Members of Hexapoda are named for the presence of six legs; all legs are uniramous. 2. Hexapods have three tagmata (head, thorax, and abdomen). 3. Appendages are attached to head and thorax. 4. There are two classes within Hexapoda: Entognatha and Insecta. B. Characteristics of Entognatha 1. Entognatha is a small group in which bases of mouthparts are enclosed within the head capsule. 2. There are three orders of entognathans: Protura, Diplura, and Collembola. a. Members of Protura and Diplura are tiny, eyeless, and inhabit soils or dark, damp places. b. Members of Collembola are commonly called springtails because of their ability to leap. 1) A springtail 4 mm long may leap 20 times its body length. 2) Live in soil, decaying plant matter, on freshwater pond surfaces, and along the seashore. 3) Can be very abundant, reaching millions per hectare. C. Characteristics of Insecta 1. Insecta is an enormous class whose members have ectognathous mouthparts, however, the bases of mouthparts lie outside the head capsule. 2. Winged insects are called pterogotes and wingless insects are called apterogotes. 21.2. Class Insecta A. Diversity 1. Insecta are the most diverse and abundant of all arthropods. 2. Estimated to be 1.1 million species worldwide, but may be as many as 30 million. 3. Fossil record indicates insects are an evolutionarily stable group; with continued evolution among modern insects. 4. Insects not only ecologically important, but also are economically important. B. Characteristics 1. Have ectognathous mouthparts and often two pair of wings on the thoracic tagma. 2. Vary in size from < 1 mm to 20 cm long; the larger insects are found in tropical environments. C. Distribution 1. Insects are found in nearly all habitats except the sea. 2. Insects are common in freshwater, brackish water and salt marshes. 3. Insects are abundant in soils, forest canopies, and can be found in deserts and wastelands. 4. Many insects are either ecto- or endoparasites of many animals and plants. 5. Adaptive Traits a. Flight and small size facilitate dispersal; consequently, many are widely distributed . b. Well-protected eggs withstand rigorous conditions and are readily dispersed. c. Many structural and behavioral adaptations facilitate access to every possible niche. D. Adaptability 1. Most structural modifications are in wings, legs, antennae, mouthparts and alimentary canal. 2. Specialization for eating only one part of a plant allows many insect species to coexist on a plant. 3. Survival in deserts is possible due to a hard, protective exoskeleton that retains water as well as metabolic adaptations that conserve water. E. External Form and Function (Figures 21.2–21.8) 1. The exoskeleton is made of complex plates, or sclerites, connected by hinge joints. a. Muscles attaching to sclerites allow precise movement. b. The rigidity is due to scleroproteins and not mineral matter; this lightness allows flight. 2. Tagmatization in insects is homogenous in contrast to the variability found among crustaceans. 3. A somite’s cuticle is composed of a dorsal notum, a ventral sternum and a pair of lateral pleura. 4. Head a. Usually there is a pair of large compound eyes. b. One pair of antennae; they varies greatly in form across taxa; they can feel, taste and hear. c. Mouthparts consist of a labrum, a pair of mandibles and maxillae, a labium and a hypopharynx. 5. Thorax a. The thorax consists of the prothorax, mesothorax and metathorax; each has a pair of legs. b. Wings 1) If two pairs of wings are present, they are on the mesothorax and metathorax. 2) Wings are cuticular, double-membraned extensions formed by the epidermis.. 3) Veins serve to strengthen the wing; the vein pattern is used to identify insect taxa. c. Legs 1) Walking legs end in terminal pads and claws. 2) Hindlegs of grasshoppers and crickets are enlarged for jumping. 3) Mole crickets have front legs adapted for burrowing in the ground. 4) Forelegs of the praying mantis allow it to grasp prey. 5) Honeybees have leg adaptations for collecting pollen. 6. Abdomen a. The insect abdomen has from nine to 11 segments; the last is reduced to a pair of cerci. b. Larval and nymphal forms may have abdominal appendages lacking in adults. c. The external genitalia are usually at the end of the abdomen. 7. Variations in Body Form a. Land beetles are thick and shielded. b. Aquatic beetles are streamlined. c. Cockroaches are flat and live in crevices. d. Antennae vary widely from long to short, plumed to knobbed. 8. Locomotion: Walking (Figure 21.9) a. Insects walk using the first and last leg on one side and the middle leg on the opposite side in alteration with the reverse; this provides stability. b. A water strider has non-wetting footpads that do not break the surface water tension. 9. Power of Flight (Figures 21.10 − 21.11) a. Insect wings are not homologous with bird and flying mammal wings. b. Insect wings are outgrowths of cuticle from the mesothoracic and metathoracic segments. c. Recent fossil evidence suggests that the insects may have evolved fully functional wings over 400 million years ago. d. Most flying insects have two pairs of wings; the Diptera (true flies) have one pair. e. Halteres are reduced wings that provide the fly with balance during flight. f. Non-reproductive ants and termites are wingless; lice and fleas have also lost their wings. g. Modifications of Wings 1) Wings for flight are thin and membranous. 2) The thick and horny front wings of beetles are protective. 3) Butterflies have wings covered with scales; caddisflies have wings covered with hairs. h. Flight Muscles of Insects 1) Direct flight muscles attach to a wing directly. 2) Indirect flight muscles alter the shape of the thorax to cause wing movement. 3) The wing is hinged on a pleural process that forms a fulcrum; all insects cause the upstroke with indirect muscles that pull the tergum downward. 4) Dragonflies and cockroaches contract direct muscles to pull the wing downward. 5) Bees, wasps and flies arch the tergum to cause the downstroke indirectly. 6) Beetles and grasshoppers use combination of direct and indirect muscles to move wings. i. Flight Muscle Contraction 1) Synchronous muscle control uses a single volley of nerve impulses to stimulate wing stroke. 2) Asynchronous muscles stretch the antagonistic muscle and cause it to contract in response. 3) Asynchronous muscles only need occasional nervous stimulation. 4) Potential energy can be stored in resilient tissues. 5) Wing beats may vary from a slow 4/second in butterflies to over 1000/second in midges. j. Wing Thrust 1) Direct flight muscles also alter the angle of wings to twist the leading edge to provide thrust. 2) This figure-8 movement moves the insect forward. 3) Fast flight requires long, narrow wings and a strong tilt, as in dragonflies and horse flies. F. Internal Form and Function (Figures 21.12–21.18) 1. Nutrition a. Digestive System 1) The foregut consists of the mouth with salivary glands, esophagus, crop and gizzard. 2) Some digestion, but no absorption, occurs in the crop as salivary enzymes mix with food. 3) The gizzard grinds food before it enters the midgut, the main site of digestion and absorption. 4) The ceca may increase the digestive and absorptive area. 5) The hindgut is primarily a site for water absorption. b. Most insects feed on plant tissues or juices and are herbivorous or phytophagous. c. Many caterpillars are specialized to eat only certain species of plants. d. Certain ants and termites cultivate fungus gardens for food. e. Many beetles and other insect larvae eat dead animals and are saprophagous. f. Some insects are predaceous on other insects or other animals. g. Many species are parasitic as adults and/or larvae. h. Many parasitic insects, in turn, have parasites, which is a condition called hyperparasitism. i. Parasitoids live inside a host until they eventually kill it; they are important in pest control. j. Mouthparts 1) Sucking mouthparts form a tube to pierce tissues of animals or plants. 2) Houseflies and blowflies have sponging mouthparts; the soft lobes at the tip absorb food. 3) Biting mouthparts can seize and crush food. 2. Circulation (Figure 21.12) a. A tubular heart in the pericardial cavity moves hemolymph forward through the dorsal aorta. b. The heartbeat is a peristaltic wave. c. Accessory pulsatile organs help move the hemolymph into wings and legs. d. Hemolymph has plasma and amebocytes but does not function in oxygen transport in most insects. e. In some insects, particularly aquatic immature in low oxygen environments, hemoglobin is present in the hemolymph and functions in oxygen transport. 3. Gas Exchange (Figure 21.18) a. Terrestrial animals are faced with the dilemma of exchanging gases but preventing water loss. b. The tracheal system is a network of thin-walled tubes that branch throughout the insect body. c. The tracheal system of insects evolved independently of that of other arthropod groups. d. Spiracles open to the tracheal trunks; there are two on the thorax and 7–8 on the abdomen. e. Spiracle valve often reduces water loss; the spiracle may also serve as a dust filter. f. Tracheae are composed of a single layer of cells lined with cuticle that is shed at each molt. g. Spiral thickenings of cuticle, called taenidia, prevent the tracheae from collapsing. h. Tracheae branch out into fluid-filled tubules called tracheoles that reach individual body cells. i. This system provides gas transport without use of oxygen-carrying pigments. j. Diving beetles use abdominal hairs as “artificial gill” to maintain a bubble under their wings. k. Mosquito larvae use short breathing tubes to snorkel surface air. l. Air sacs in insects are dilated tracheae without taenidia. m. Contraction of muscles in the jaw or limbs causes increased pressure inside the exoskeleton; this elevated pressure causes contraction of tracheae for exhalation. n. Muscular movements may assist in moving air in and out of air sacs. o. Very small insects transport all gases by simple diffusion. p. Aquatic insect nymphs may use tracheal gills or rectal gills. 4. Excretion and Water Balance (Figures 21.12, 21.19, 21.33A) a. Both insects and spiders utilize Malpighian tubules in conjunction with rectal glands. b. Malpighian tubules vary in number but join between the midgut and hindgut. c. The blind ends of the tubules float freely in the hemocoel bathed in hemolymph. d. Potassium is actively secreted into the tubules; other solutes follow the gradient. e. The main waste product is uric acid; it flows across at the upper end that is mildly alkaline. f. In the lower end of the tubule, potassium combines with carbon dioxide and is reabsorbed. g. Rectal glands then reabsorb chloride, sodium and water; the wastes pass on out. 5. Nervous System (Figure 21.12) a. Insect nervous systems resemble that of larger crustaceans, with fusion of ganglia. b. Some have a giant fiber system. c. A stomadeal system corresponds to the autonomic system of vertebrates. d. Neurosecretory cells in the brain function to control molting and metamorphosis. 6. Sense Organs a. Many insects have keen sensory perception. b. Most sense organs are microscopic and located in the body wall. c. Different organs respond to mechanical, auditory, chemical, visual and other stimuli. d. Mechanoreception 1) Touch, pressure, vibration, etc. are detected by sensilla. 2) A sensillum may be a single hair-like seta or a complex organ. 3) They are distributed widely over the antennae, legs and body. e. Auditory Reception (Figure 21.2) 1) Sensitive setae (hair sensilla) or tympanal organs may detect airborne sounds. 2) Tympanal organs occur in Orthoptera, Hemiptera and Lepidoptera. 3) Organs in the legs can detect vibrations of the substrate. f. Chemoreception 1) These usually are bundles of sensory cell processes located in sensory pits. 2) They may occur on mouthparts, antennae and legs. 3) Some insects can detect odors for several kilometers. 4) Feeding, mating, habitat selection and host-parasite relationships are mediated through chemical senses. g. Visual Reception (Figure 21.20) 1) Insects have two types of eyes: simple and compound. 2) Honeybee studies indicate that the ocelli monitor light intensity but do not form images. 3) Compound eyes may contain thousands of ommatidia. 4) Ommatidia structure is similar to that of crustaceans. 5) Insects can see simultaneously in almost all directions; the image is myopic and fuzzy. 6) Flying insects have higher flicker-fusion rate; can distinguish 200–300 flashes/second. 7) A bee can distinguish ultraviolet light we cannot see, but cannot detect shades of red. h. Other Senses 1) Insects are very sensitive to temperature, especially by cells in antennae and legs. 2) Insects also detect humidity, proprioception, gravity and other physical properties. 7. Neuromuscular Coordination a. Active insects require excellent neuromuscular coordination. b. Arthropod muscles are cross-striated. c. Strength of muscle is related to its cross-sectional area. d. A flea can jump 100 times its length by storing energy in an elastic resilin protein. 8. Reproduction (Figures 21.21–21.23) a. Parthenogenesis occurs predominantly in some Hemiptera and Hymenoptera. b. Sexual reproduction is the norm and sexes are separate. c. Sexual Attraction 1) Female moths secrete a powerful pheromone to attract males from a great distance. 2) Fireflies use flashes of light to detect mates. 3) Some insects use sounds, color signals and other courtship behaviors. d. Fertilization is usually internal. e. Sperm may be released directly or packaged into spermatophores; spermatophores are the result of an evolutionary transition from marine to terrestrial existence. f. Spermatophores may be transferred both without copulation and during copulation. g. The female may only mate once and store the sperm to fertilize eggs throughout her life. h. Females may lay a few eggs and provide care of young, or lay huge numbers. i. Butterflies and moths must lay eggs on the host plant if the caterpillars are to survive. j. Wasps may have to locate a specific species that is the only host to their young. G. Metamorphosis and Growth (Figures 21.22) 1. Various forms of metamorphosis produce degrees of change among different insect groups. a. Most insects change form after hatching from an egg. b. Each stage between molts is called an instar. c. Insects develop wings during the last stage where they are useful in reproduction. 2. Ametabolous (Direct) Development a. Silverfish and springtails have young similar to adults except in size and sexual maturation. b. Stages are egg-juveniles-adult. c. These are primitively wingless insects. 3. Hemimetabolous Metamorphosis (Figures 21.24 − 21.26) a. Some insects undergo a gradual metamorphosis. b. Occurs in insects such as grasshoppers, cicadas, mantids, true bugs, mayflies and dragonflies. c. Young are called nymphs; stages are egg-nymph-adult. d. Bud-like growths in early instars show where the adult wings will eventually develop. 4. Holometabolous Metamorphosis (Figure 21.22) a. About 88% of insects undergo this complete metamorphosis. b. This separates the physiology of larval growth, pupal differentiation and adult reproduction. c. Larvae and adults often live in completely different environments and therefore do not compete. d. After several larval instars, moth or butterfly becomes a pupa inside a cocoon or chrysalis. e. Pupae often overwinter in this stage; the final molt occurs and the adult emerges in spring. f. Stages are egg-larva-pupa-adult. 5. Physiology of Metamorphosis a. Hormones regulate insect metamorphosis. b. The brain and nerve cord ganglia produce brain hormone or ecdysiotropin. c. Neurosecretory cells send axons to the corpora cardiaca that stores and ultimately releases these hormones. d. Brain hormone circulates in the hemolymph to the prothoracic gland in the head or prothorax. e. The prothoracic gland produces molting hormone or ecdysone in response. f. Ecdysone starts the molting process. g. The corpora allata produces juvenile hormone. h. Molting continues as long as juvenile hormone (neotenine) is sufficiently present. i. In later instars, the corpora allata release less and less juvenile hormone. j. When juvenile hormone reaches a low level, the larva molts to become a pupa. k. Cessation of juvenile hormone production in the pupa leads to an adult at the last molt. l. In hemimetabolous insects, cessation of juvenile hormone occurs in the last nymphal instar. m. In adults, the corpora allata becomes active again in normal egg production. n. The prothoracic glands degenerate in adult insects and adults do not molt. H. Diapause 1. Diapause is a period of dormancy in the annual life cycle that is independent of conditions. 2. Winter dormancy is called hibernation; summer dormancy is called estivation. 3. Any stage (eggs, larvae, pupae or adults) may remain dormant to survive adverse conditions. 4. This allows them to synchronize with the environment. 5. Diapause is internally controlled but it may be triggered by environmental cues such as day length. 6. Diapause always occurs after an active growth stage; the insect is then ready for another molt. 7. Therefore, many larvae do not develop beyond this point until spring in spite of mild temperatures. I. Defense (Figures 21.27, 21.28) 1. Protective coloration, warning coloration and mimicry are protective adaptations. 2. Stink bugs and others have repulsive odors and tastes. 3. Some insects are aggressive (e.g., bees and ants). 4. The monarch caterpillar incorporates a poisonous substance from its food plant, milkweed. 5. The bombardier beetle can spray an attacking enemy with irritating chemicals. J. Behavior and Communication (Figures 21.29–21.30) 1. Due to very sensitive perception, many insects respond to many environmental stimuli. 2. Responses are governed by both the physiological state of the animal and its nerve pathways. 3. Many insect behaviors are complex sequences of responses. 4. Most insect behavior is innate but some involve simple learning. 5. Pheromones a. These chemicals are secreted by one individual to affect the behavior of another individual. b. Pheromones attract the opposite sex, trigger aggregation, fend off aggression and mark trails. c. Bees, wasps and ants can recognize nestmates and signal an alarm if strangers enter the nest. d. Pheromones can be used to trap insects to monitor populations. 6. Sound Production and Reception a. Sounds are used as warning devices, advertisement of territory, and courtship songs. b. Crickets chirp for courtship and aggression. c. The male cicada vibrates paired membranes on its abdomen to attract females. 7. Tactile Communication a. Tactile communication involves tapping, stroking, grasping and antennae touching. b. Some beetles, flies and springtails use bioluminescence. c. Some female fireflies mimic another species’ flash pattern and attract males and then eat them. 8. Social Behavior (Figures 21.31-21.33) a. Some social communities are temporary and uncoordinated. b. Other social groups are highly organized and depend on chemical and tactile communication. c. Caste differentiation is common in the most organized social groups. d. Honeybees 1) Honeybees have a few male drones, a fertile female queen and many sterile female workers. 2) Males come from unfertilized eggs; fertilized eggs produce males in this haplodiploid system. 3) The development of a fertile queen occurs because she alone is fed royal jelly. 4) A queen secretes “queen substance” to prevent workers from maturing or feeding larvae any royal jelly. 5) A honeybee hive of 60,000–70,000 individuals continues indefinitely. 6) Honeybee scouts can inform workers on the location of food. e. Termites 1) A fertile king and queen fly away to start a new colony; they mate and lose their wings. 2) Immature members are wingless and become workers and soldiers. 3) Soldiers have large heads and defend the colony. 4) Reproductive individuals secrete inhibiting pheromones that produce sterile workers. 5) Nymphs feed from each other in trophallaxis, thus spreading the pheromone about. 6) Worker castes also produce worker and soldier substances; drops in these pheromone levels result in more of the needed caste developing in the next generation. f. Ants 1) Ants differ from termites; ants are darker, hard-bodied, and have a thread-like waist. 2) In ant colonies, the male ant dies after mating. 3) Ants have wingless soldiers and workers, and often have variations of these castes. 4) Ants have also evolved slavery, fungus farming, sewing nests together, tool use, and herding. 21.3. Insects and Human Welfare (Figures 21.34–21.36) A. Beneficial Insects 1. Insects produce honey, beeswax, silk and shellac. 2. Of more economic importance, bees pollinate $10 billion worth of food crops in the U.S. annually. 3. Pollinating insects and flowering plants are tightly co-evolved. 4. Predaceous and parasitoid insects are vital in controlling many pest insect populations. 5. Dead animals are rapidly consumed by fly maggots. 6. Insects are critical components of most food chains and a central food for many fish and birds. 7. In a science called forensic entomology, the succession of insects living within a corpse can be used to estimate time of death. B. Harmful Insects 1. Harmful insects eat and destroy our food, clothing and property. 2. Nearly every cultivated crop has several insect pests,and insect control is expensive. 3. Bark beetles, spruce budworms, the gypsy moth and others are serious forest pests. 4. Medically important insects include vectors for disease agents. a. Ten percent of all arthropod species are parasites or “micropredators”. b. Warble and bot flies attack humans and domestic livestock. c. Malaria is carried by Anopheles mosquitos and is the most common major world disease. d. Mosquitoes also carry yellow fever, lymphatic filariasis and West Nile virus. e. Fleas carry plague, a disease that changed human history in the Middle Ages. f. Lice carry typhus fever. g. Tsetse flies carry African sleeping sickness. C. Control of Insects 1. Broad-spectrum insecticides damage beneficial insect populations along with the targeted pest. 2. Some chemical pesticides persist and accumulate as they move up the food chain. 3. Some strains of insects have evolved a resistance to common insecticides. 4. Biological control is the use of natural agents, including diseases, to suppress an insect population. 5. Bacillus thuringiensis is a bacterium that controls lepidopteran pests; the gene coding for the “B.t.” toxin has been introduced to other bacteria and transferred to crop plants themselves. 6. Some viruses and fungi may be economical pesticides. 7. Some natural predators or parasites of insect pests can be raised and released to control the pest. 8. Release of sterile males can eradicate the few insect species that only mate once. 9. Pheromones can monitor pests and hormones may have a role in disrupting their life cycle. 10. Integrated pest management is the combined use of all possible, practical techniques listed above, to reduce the reliance on chemical insecticides. D. Classification of Class Entognatha and Class Insecta (Figures 21.37-21.41)
Class Entognatha Order Protura Order Diplura Order Collembola Class Insecta Subclass Apterygota Order Thysanura Subclass Pterygota Infraclass Paleoptera Order Ephemeroptera Order Odonata Intraclass Neoptera Order Orthoptera Order Blattodea Order Phasmatodea Order Mantodea Order Dermaptera Order Plecoptera Order Isoptera Order Emiidina Order Psocoptera Order Zoraptera Order Phthiraptera Order Thysanoptera Order Hemiptera Order Neuroptera Order Coleoptera Order Strepsiptera Order Mecoptera Order Lepidoptera Order Diptera Order Trichoptera Order Siphonaptera Order Hymenoptera
21.4. Phylogeny and Adaptive Diversification (Figure 21.42-21.43) A. Phylogeny 1. Understanding of the relationships among arthropods has changed over the past decade. 2. Using molecular data, members of former subphylum Uniramia are now divided between subphylum Myriapoda and Hexapoda. 3. The nature of the relationship, however, between hexapods and crustaceans is not well understood. 4. Some phylogenies support a sister-taxon relationship between them, but others indicate that hexapods arose within Crustacea. 5. Future studies may support that subphylum Crustacea is paraphyletic. 6. Phylogenies that support hexapods arising from within Crustacea, find that hexapods are most similar to brachiopod, cephaplocarid, and remipedian crustaceans. 7. Within Hexapoda, Entognatha is the sister taxon to class Insecta. 8. However, some research indicates that entognathous mouthparts may have evolved several times and that some entognathans are closer to insects than to other entognathans. B. Adaptive Diversification 1. The first terrestrial arthropods were scorpions and millipedes that appeared in the Silurian period 2. The ancestral insect had a head and trunk of similar somites period. 3. Insects have specialized the first three post-cephalic somites as thorax and lost the remaining appendages. 4. Some modern apterygote orders have abdominal styli that are considered vestigial legs. 5. Recent fossil evidence suggests winged insects were in existence about 400 million years ago. a. Ancestral flying insects may have derived from aquatic insects or insects with aquatic juveniles; the external gills on their thorax may be the derivative of wings. 6. Metamorphosis also distinguished insects by the Permian period. a. Hemimetabolous metamorphosis, chewing mouthparts and cerci group the Orthoptera, Dermaptera, Isoptera and Embioptera. b. Hemimetabolous metamorphosis and sucking mouthparts group the Thysanoptera, Hemiptera, Homoptera and perhaps the Psocoptera, Zoraptera, Mallophaga and Anoplura. c. Other orders have holometabolous metamorphosis and are the most specialized. Lecture Enrichment 1. The strange case of some insect muscles responding multiple times to a few nerve impulses was part of a puzzle to how bumblebees could fly. Based on the wing surface area, the impulses to the flight muscles gave too few wing beats to lift a big bumblebee. Photography of the wing beats gave evidence of faster wing beats than nerve impulses, thus solving the problem of the bumblebees that couldn’t [theoretically] fly! 2. Completion of the Panama Canal was possible only when mosquitoes could be controlled. Much of Africa remains unavailable to cattle ranching due to the tsetse fly carrying a bovine version of sleeping sickness. These are direct cases of zoological knowledge being directly connected to economic development. 3. Readings from the “plague years” as well as accounts of when bubonic plague entered the U.S. via San Francisco’s Chinatown a century ago can provide a human side to this legendary disease. 4. Readings from Robert Desowitz’s The Malaria Capers can “bring home” the extent of suffering from malaria, as well as the failure to achieve a malaria vaccine due to both a lack of understanding of the biology of organisms and over-reliance on quick molecular biology cures. 5. Many university libraries have reproductions of Marcello Malpighi’s early work in folio format. This can provide insight into the detailed anatomical work that Marcello was able to achieve without the use of advanced microscopes. 6. The use of genetically-engineered “B.t.” plants, introduced here, has complicated ecological implications and is not a simplistic cure-all. This will provide an opportunity to relate zoology to current world trade level events . Commentary/Lesson Plan Background: Experience with insects will vary greatly among students. International students may be able to relate experiences with larger tropical insects. A few gardeners may recognize symphylans, but most organisms and structures discussed in this chapter require visuals. The suffering caused by malaria, yellow fever, filariasis, etc. is not generally recognized by North Americans. Misconceptions: Students will assume that an insect exoskeleton is the same as a crayfish exoskeleton; the text clarifies the critical differences. In North America, contrary to public fears, the most dangerous organism students may encounter is the honeybee; more students die of anaphylactic shock from allergic reaction to bee sting than from snakebite, etc. Generally, an organism is seen as a self-sufficient unit of some independence, but most true social insects cannot survive alone and constitute a “superorganism.” DDT is generally perceived as universally bad, but was critical in preventing typhus epidemics in World War II after typhus caused more deaths than munitions in World War I, and did not enter the food chains until used widely in crop spraying. Naive students may expect all pesticide use to end with use of biological controls, but parasites and predators are only known for a small percentage of crop pests. Some students will tend to read human reasoning into insect actions that are actually based on simple reflex actions because we would need to “think out” our response while the insect has an innate response that may not even involve its “brain.” Schedule: HOUR 1 21.1. Diversity and Characteristics A. Subphylum Hexapoda B. Characteristics 21.2. Class Insecta A. Diversity B. Characteristics C. Distribution D. Adaptability E. External Form and Function F. Internal Form and Function HOUR 2 G. Metamorphosis and Growth H. Diapause I. Defense J. Behavior and Communication 21.3. Insects and Human Welfare A. Beneficial Insects B. Harmful Insects C. Control of Insects D. Classification of Insecta 21.4. Phylogeny and Adaptive Diversification A. Phylogeny B. Adaptive Diversification ADVANCED CLASS QUESTIONS: 1. Ask why insects are so successful on land and why there are so many different species of insects. Why are there so many more entomologists than there are malacologists? Answer:Insects are incredibly successful on land and have diversified into numerous species due to several key factors: 1. Adaptability: Insects are highly adaptable organisms capable of thriving in a wide range of terrestrial habitats, including forests, grasslands, deserts, and urban environments. Their ability to exploit diverse ecological niches and adapt to varying environmental conditions has contributed to their success and proliferation. 2. Reproductive Potential: Insects typically have high reproductive rates compared to many other animal groups. Many insect species produce large numbers of offspring, enabling rapid population growth and colonization of new habitats. Their short generation times and efficient reproductive strategies allow them to exploit resources quickly and occupy new ecological niches. 3. Diverse Feeding Strategies: Insects exhibit a wide range of feeding strategies, including herbivory, predation, parasitism, and scavenging. This dietary versatility allows insects to exploit diverse food sources and occupy various trophic levels within ecosystems. Their ability to utilize different food resources contributes to their ecological success and species diversification. 4. Flight: The evolution of flight has been a significant factor in the success of insects. Flight enables insects to disperse over long distances, colonize new habitats, escape predators, and locate mates and resources more efficiently. The ability to fly has facilitated the rapid geographic expansion and diversification of insect populations. 5. Ecological Interactions: Insects play essential roles in ecosystem functioning and are involved in complex ecological interactions with other organisms, including plants, fungi, and vertebrates. Many insects serve as pollinators, decomposers, herbivores, and prey, influencing ecosystem dynamics and biodiversity. Their diverse ecological roles contribute to their abundance and species richness. As for why there are more entomologists than malacologists (scientists who study mollusks), several factors may contribute to this discrepancy: 1. Species Diversity: Insects represent the most diverse group of animals on Earth, with estimates of millions of species worldwide. The sheer number and diversity of insect species provide ample opportunities for research and specialization within the field of entomology. 2. Economic and Agricultural Importance: Insects have significant economic and agricultural impacts, both as pests and beneficial organisms. Entomologists play crucial roles in studying and managing insect pests that damage crops, transmit diseases, and impact ecosystems. The economic importance of insects drives demand for entomological research and expertise. 3. Human Health Concerns: Some insects are vectors of diseases that affect human health, such as mosquitoes transmitting malaria, dengue fever, and Zika virus. Understanding the biology, behavior, and ecology of disease-carrying insects is essential for public health efforts and disease prevention, leading to a focus on insect-related research. 4. Accessibility and Visibility: Insects are ubiquitous and easily observable in terrestrial environments, making them more accessible for study compared to many other animal groups. Their diverse behaviors, ecological interactions, and economic significance also contribute to their visibility in scientific research and public awareness. Overall, the success of insects on land, their ecological diversity, and economic importance have led to a greater focus on entomology compared to other disciplines within the field of zoology, such as malacology. 2. We do not have giant ants that carry away people. Ask why? The essay by J.B.S. Haldane on “Being the Right Size” provides an excellent reading or can be used for background examples for an instructor. Answer:The absence of giant ants that carry away people can be attributed to several factors, many of which are explored in J.B.S. Haldane's essay "On Being the Right Size." Here are some key reasons: 1. Physical Constraints: In his essay, Haldane discusses the concept of scaling laws, which describe how the size of an organism affects its physiology and behavior. Ants, like all organisms, are subject to these scaling laws, which impose limitations on their size. As organisms increase in size, their surface area-to-volume ratio decreases, affecting their ability to support their own weight, exchange gases, and regulate body temperature. Giant ants would face significant physiological challenges related to their size, making them unlikely to evolve. 2. Ecological Limitations: The ecological niche occupied by ants is influenced by factors such as competition with other species, availability of resources, and environmental conditions. The evolution of giant ants capable of carrying away humans would require significant changes to their ecology, behavior, and social structure. Such changes may not be feasible or advantageous within their ecological context. 3. Energetic Costs: Haldane also discusses the energetic costs associated with different body sizes. Larger organisms require more energy to sustain themselves, move, and reproduce. The evolution of giant ants would necessitate increased energy expenditure for foraging, locomotion, and colony maintenance. It is unlikely that the benefits of being larger would outweigh the energetic costs associated with giant size. 4. Evolutionary Constraints: Evolutionary processes, including natural selection and genetic variation, shape the characteristics and behaviors of organisms over time. The evolution of giant ants capable of carrying away people would require specific genetic mutations and selective pressures favoring such traits. Given the complex social organization and specialization observed in ant colonies, the emergence of such extreme behaviors may be evolutionarily improbable. Overall, while the idea of giant ants carrying away people may capture the imagination, it is not supported by biological principles and ecological realities. Haldane's essay provides valuable insights into the relationship between size and biology, highlighting the constraints and adaptations that shape the diversity of life on Earth. 3. What features of insect anatomy prevent insects from growing to the size of humans? Some Permian insect fossils from Kansas include dragonflies with nearly a one meter (three-foot) wingspan and larger bodies than dragonflies have today. Some researchers suspect that atmospheric oxygen levels were higher in the Permian. What is the most likely reason for this larger size, and why do we not see such large insects today? Answer:The size limitations of insects compared to humans can be attributed to several factors related to their anatomy and physiology: 1. Respiratory System: Insects have a tracheal respiratory system composed of a network of tubes called tracheae that deliver oxygen directly to their tissues. This system is efficient for small body sizes but becomes less effective as organisms grow larger. Larger insects would require more efficient respiratory structures, such as lungs or a more complex tracheal system, to adequately oxygenate their tissues, which would be difficult to evolve due to anatomical constraints. 2. Exoskeleton: Insects have an exoskeleton made of chitin, a rigid material that provides support and protection but imposes physical limitations on growth. As insects grow, they must periodically molt and shed their exoskeleton to accommodate increasing body size. However, the strength and integrity of the exoskeleton decrease relative to body size, limiting the maximum size insects can attain without compromising structural integrity. 3. Muscle Efficiency: Insects have relatively small muscles compared to their body size, limiting their ability to generate and sustain large amounts of muscular force. Larger insects would require proportionally larger muscles to support their increased body mass, which would pose biomechanical challenges related to muscle efficiency, energy expenditure, and locomotion. Regarding the larger size of Permian insect fossils, such as dragonflies with nearly a one-meter wingspan, several factors may have contributed to their larger size: 1. Atmospheric Oxygen Levels: Some researchers hypothesize that atmospheric oxygen levels during the Permian period were higher than they are today. Increased oxygen levels could have facilitated larger body sizes in insects by enhancing oxygen delivery to tissues and supporting higher metabolic rates. This hypothesis is supported by evidence from fossilized plant and animal remains, as well as geochemical analyses of ancient atmospheric gases. However, it's essential to consider that the relationship between atmospheric oxygen levels and insect size is complex and not fully understood. Other factors, such as temperature, humidity, habitat availability, and ecological interactions, may also influence insect size. As for why we do not see such large insects today, several factors may contribute to their absence: 1. Oxygen Levels: While higher oxygen levels during the Permian period may have facilitated larger insect sizes, atmospheric oxygen levels have since decreased. Modern insects are adapted to current atmospheric conditions, which may not support the same degree of gigantism observed in ancient insects. 2. Ecological Pressures: Changes in environmental conditions, ecological interactions, and resource availability may have favored smaller body sizes in modern insects. Smaller insects may have advantages in terms of reproductive efficiency, predator avoidance, and resource utilization, leading to the selection against larger body sizes over evolutionary time. 3. Evolutionary Constraints: The evolution of larger body sizes in insects may be constrained by physiological, anatomical, and ecological factors. As discussed earlier, the respiratory system, exoskeleton, and muscle efficiency impose limitations on insect size, making it challenging for insects to exceed certain size thresholds without experiencing significant trade-offs in fitness and survival. In summary, while Permian insect fossils exhibit larger body sizes than modern insects, the reasons for this gigantism are complex and likely involve interactions between atmospheric oxygen levels, ecological factors, and evolutionary constraints. The absence of such large insects today may reflect changes in environmental conditions, ecological pressures, and evolutionary trajectories over geological time scales. 4. The brain hormone originating in the head triggers the prothoracic gland. How could we demonstrate that there are two separate structures involved in triggering ecdysis by tying off the head-thorax region: a) before the brain hormone was secreted, b) after brain hormone was secreted and c) after ecdysone was secreted? Answer:To demonstrate the involvement of two separate structures in triggering ecdysis (molting) in insects by tying off the head-thorax region at different stages of hormonal secretion, we would need to conduct experiments manipulating these variables. Here's how we could design and interpret each experiment: a) Before the brain hormone was secreted: 1. Experimental Design: Divide a group of insects into two groups. In one group, tie off the head-thorax region before the brain hormone is secreted. In the other group, leave the insects intact as a control group. 2. Observations: Monitor both groups of insects for signs of ecdysis, such as behavior changes, molting attempts, or physical indications of molting readiness. 3. Interpretation: If insects in the experimental group fail to molt despite being at the appropriate developmental stage, it suggests that the brain hormone is necessary for triggering ecdysis. In contrast, control insects should exhibit normal molting behavior. b) After brain hormone was secreted: 1. Experimental Design: Repeat the same experiment, but this time, tie off the head-thorax region after the brain hormone is secreted. Again, include a control group of intact insects. 2. Observations: Monitor both groups for signs of ecdysis as before. 3. Interpretation: If insects in the experimental group still molt normally despite the tie-off, it suggests that the brain hormone has already triggered the release of subsequent hormones or processes necessary for ecdysis. Control insects should exhibit normal molting behavior. c) After ecdysone was secreted: 1. Experimental Design: Tie off the head-thorax region after ecdysone, the molting hormone secreted by the prothoracic gland, is already present in the insect. 2. Observations: Monitor both groups for signs of ecdysis. 3. Interpretation: If insects in the experimental group fail to molt despite the presence of ecdysone, it suggests that ecdysone alone is not sufficient to trigger ecdysis without the brain hormone. Control insects should exhibit normal molting behavior. By conducting these experiments, we can infer the roles of the brain hormone and ecdysone in triggering ecdysis and demonstrate the necessity of both structures in the molting process. These experiments would provide valuable insights into the hormonal regulation of insect development and molting. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214