CH-18 Smaller Ecdysozoans – Integrated Principles of Zoology : Chapter Notes

This document contains Chapters 18 to 19 18 SMALLER ECDYSOZOANS PHYLUM NEMATODA: ROUNDWORMS PHYLUM NEMATOMORPHA PHYLUM LORICIFERA PHYLUM KINORHYNCHA PHYLUM PRIAPULIDA CLADE PANARTHROPODA PHYLUM ONYCHOPHORA PHYLUM TARDIGRADA CHAPTER OUTLINE 18.1. Protostome Diversity and Diversity of Superphylum Ecdysozoa (Figure 18.1) A. Many protostomes posses a cuticle, a non-living outer layer secreted by the epidermis. 1. This cuticle restricts growth and must be shed or molted via ecdysis. 2. Members of Ecdysozoa molt this cuticle as they grow. 3. Regulation of molting is achieved by the hormone ecdysone. 4. It’s this biological pathway that led scientists to group the ecdysozoans in the same taxa. B. Ecdysozoans do not share the same body plan. 1. Members of Nematoda, Nematomorpha, and Kinorhyncha have pseudocoelomate bodies. 2. Members of Priapulida presumed to be pseudocoelomate. a. The pseudocoelom is used as a hydrostatic skeleton in nematodes, kinorhynchs, and priapulids. b. In Loricifera, species vary between pseudocoelomate and acoelomate. 3. Member of Panarthropoda have coelomate bodies but their coeloms are reduced in size compared those of annelids. The Panarthropoda contains three phyla: Onychophora, Tardigrada, and Arthropoda. Arthropoda is the largest phylum in terms of number of described species. 18.2 Phylum Nematoda: Roundworms A. Diversity 1. About 25,000 species are described; perhaps a half million exist. 2. They live in virtually all habitats in all biomes; topsoil may contain billions per acre. 3. Parasitic nematodes exist in nearly all animal and plant species; they are economically important. 4. Free-living nematodes feed on bacteria, yeasts, fungal hyphae and algae. 5. Predatory nematodes eat rotifers, tardigrades, small annelids and other nematodes. 6. Nematodes are also important as food for mites, insects, larvae and fungi. 7. Caenorhabditis elegans is important model for studies of genomics, cell development and differentiation. B. Form and Function (Figures 18.2 − 18.4) 1. Exhibit eutely: a set number of body cells. 2. Cylindrical shape; all but one species lack motile eilia or flagella. 3. Excretion: nematodes lack protonephridia; have > 1 large glands or similar structures for excretion. 4. Integumentary system a. Integument consists of outer, thick, noncellular cuticle is secreted by the underlying hypodermis and shed between each of four juvenile growth stages b. The cuticle has layers of crisscrossing collagen, providing elasticity but constraining expansion. c. The hypodermis is syncytial with nuclei located in four hypodermal cords projecting inward. d. Dorsal and ventral hypodermal cords bear nerves and lateral cords bear excretory canals. 5. Skeletomuscular system a. Longitudinal muscles lie beneath the cuticle; there are no circular muscles. b. Muscles run in four bands, marked off by the four hypodermal cords. c. Each muscle cell has contractile fibrillar portion and noncontractile sarcoplasmic portion. d. The sarcoplasmic portion extends into the pseudocoel and stores glycogen. e. Unlike other animals, the cell body or muscle arm extends to the ventral or dorsal nerve. f. Compression and stretching of the cuticle returns the body to resting position when muscles relax. 6. Digestion, nutrition, and metabolism. a. Disgestive tract consists of mouth, pharynx, non-muscular intestine, short rectum and anus. b. The muscular pharynx, containing a triradiate lumen, sucks food in. c. The intestine is one cell thick; food moves as new food enters and the body moves. d. Defecation occurs via pseudocoelomic pressure expelling waste through anal opening. e. Some parasitic adults use anaerobic metabolism because they lack Krebs cycle and cytochrome system; in contrast, both entirely free-living nematodes and free-living stages of parasitic nematodes that are obligate aerobes utilize both the Krebs cycle and cytochrome systems. 7. Nervous system a. A ring of nerve tissue and ganglia around the pharynx lead to dorsal and ventral nerve cords. b. Sensory papillae are at the head (amphids) and/or tail (phasmids). 8. Reproduction a. Most nematodes are dioecious with males smaller than females; the male has copulatory spicules to hold the female vulva open against hydrostatic pressure. b. . Fertilization is internal and eggs are stored in the uterus until deposited. C. Representative Nematode Parasites 1. Some nematodes are important pathogens of humans (Table 18.1); most are tropical. 2. Ascaris lumbricoides: The Large Roundworm of Humans (Figure 18.5) a. Ascaris lumbricoides is one of the most common nematode human parasite; infecting >1.27 billion people worldwide ,and up to 25% of people in some areas of the southeastern U.S. c. A congener roundworm, A. megalocephala ,is found in the intestines of horses, A. suum is found in pig intestines. d. A female Ascaris may lay 200,000 eggs a day, which pass out in the host’s feces. e. Embryos can develop into infective juveniles in two weeks. f. Viable eggs remain after fecal matter has disappeared; eggs survive long periods in soil. g. Infection rates tend to be higher in children, and males tend to be more heavily infected than females. h. Mode of infection 1. When host swallows embryonated eggs, juveniles hatch and burrow through intestinal wall into circulatory system. 2. Carried to lungs, they break into the alveoli and are carried up to the tracheae. 3. Coughed up and swallowed, they mature in intestine 2 months after they were swallowed. 4. Adults feed on intestinal contents and may block or perforate the intestines. 3. Hookworms (Figures 18.6, 18.7) a. The anterior end of these small (9–11 mm) worms has a hook-like curve. b. Necator americanus is the most common hookworm. c. Hookworms are dioeceous. d. Large plates in their mouths cut into intestinal mucosa; then they suck the host’s blood. e. They pump through more blood than they digest; heavy infections cause anemia. f. Eggs pass in feces and juveniles hatch in soil where they live on bacteria. g. If human skin comes in contact with soil, infective juveniles burrow through skin to blood. h. Similar to Ascaris, they travel in blood to the lungs, are coughed up to be swallowed, and mature in the intestine. 4. Trichina Worm (Figure 18.8) a. Trichinella spiralis is tiny but causes a potentially lethal trichinosis. b. Adult worms burrow into the intestinal mucosa and females directly produce juvenile worms. c. Juveniles penetrate blood vessels and circulate throughout the body to all tissues and spaces. d. They penetrate skeletal muscle cells, redirecting gene expression of the musculature so it loses its striations and becomes a nurse cell to the parasite. e. When poorly cooked meat containing encysted juveniles is eaten, worms are liberated and mature in the intestine. f. Infect humans, hogs, rats, cats and dogs; hogs can become infected eating uncooked scraps of infected meat or rats. g. There are four other sibling species with variable distribution, freezing resistance, etc. h. Heavy infections cause death; about 12 cases are discovered annually. 5. Pinworms (Figure 18.9) a. It is the most common nematode parasite in the U.S. but causes little disease. b. Adults live in the large intestine and cecum. c. Females, about 12 mm long, migrate to the anal region at night and lay eggs, causing itching. d. Scratching the anal region contaminates hands and bedclothes. e. Eggs develop rapidly and become infective within six hours at body temperature. f. When swallowed, they hatch in the duodenum and mature in the large intestine. g. Members of this order have haploid males from unfertilized eggs; females are diploid and come from fertilized eggs (haplodiploidy). 6. Filarial Worms (Figures 18.10–18.12) a. Eight species of filarial nematodes infect humans; some cause major and serious diseases. b. Wucheria bancrofti and Brugia malayi live in the lymphatic system. c. The worms cause inflammation and blockage of the lymphatics. d. Females release live young, tiny microfilariae, into blood and lymph. e. Mosquitoes ingest the microfilariae when they feed; the worms develop to the infective stage and move into the mosquito bite wound when it feeds. f. Elephantiasis is caused by repeated exposure; swelling and growth of connective tissue causes enormous swelling of body parts. g. River blindness or onchocerciasis is carried by black flies and infects 37 million people in tropics. h. Dog heartworm, Dirofilaria immitis, is carried by mosquitoes and is the most common U.S. filarial worm. 18.3. Phylum Nematomorpha A. Diversity 1. “Horsehair worms” so-called because they resemble horse tail hairs. 2. Adult structures resemble those seen in nematodes: cuticle, epidermal cords, only longitudinal muscles, and a nervous system. 3. However, early larval forms resemble priapulids; they are currently placed as the sister taxon to nematodes. 4. About 320 species are known; they occur worldwide. 5. Adults are free-living in moist habitats; juveniles are parasites of arthropods. B. Form and Function 1. Can be up to 1 m long but only 0.5 to 3.0 mm in diameter. 2. Anterior end is rounded; the posterior end has two or three caudal lobes. 3. Body wall resembles that of nematodes but lateral hypodermal cords are absent. (Figure 18.13) 4. Ventral nerve cord is connected to the ventral hypodermal cord by nervous lamellae. 5. The digestive system is vestigial; larvae absorb food from arthropod hosts. 6. Adults utilize stored nutrients, although recent studies reveal that they can absorb organic molecules through their vestigial gut and body wall. 7. Circulatory, respiratory and excretory systems are lacking. 8. Life cycles of nematomorphs are poorly known. 9. Gordius (named for a king who tied an intricate knot), juveniles may encyst on vegetation and are likely eaten by an arthropod like a grasshopper. a. Gordiid larval stages have hooks that may be used to bore into a host. b. It may use the hooks to infect the integument or gut. c. In some cases, the gordiid may infect via drinking water. d. Larvae encyst within the host. e. Marine nemotomorphs infect hermit crabs and other crabs. f. After emerging from hemocoel of arthropod host, juveniles molt once and emerge as adults. g. After months in arthropod host, mature worm emerges into nearby water or rainfall. h. Somehow, the parasite stimulates terrestrial insects to seek water. 10. Nematomorphs are dioecious. 11. In both sexes, gonads empty into a cloaca through gonoducts. 12. Females discharge eggs into water in long strings. 18.4. Phylum Loricifera A. Diversity 1. Loriciferans were recently discovered (1983) in spaces between grains of marine gravel. 2. There are 11 currently described species and 80 undescribed species. 3. They are apparently widely distributed. 4. Most species have been found in coarse sediments at depths of 300–450 m. 5. One species was collected at 8000 m. B. Form and Function 1. The body has five regions: mouth cone, head or introvert, neck, thorax, and abdomen. 2. Nine circlets of scalids are on the introvert; scalids are similar to those of kinorhynchs. 2. The entire forepart can be retracted into the circular Lorica. (Figure 18.14) 3. Their diet is unknown, but they may feed on bacteria. 4. The brain fills the head and nerves innervate scalids. 5. Loriciferans are dioecious with dimorphic males and females. 6. Copulation occurs, but the life cycle is not well known. 7. The body cavity is pseudocoel. 8. In one species, a Higgins larva molts into a second stage, which then molts into an adult. 18.5. Phylum Kinorhyncha A. Diversity 1. Kinorhynchs are usually under 1 mm long; about 179 species are known. 2. They are found worldwide, from intertidal areas to 6000 m deep. 3. Most live in mud; some have been found in algae, sponges and other invertebrates. B. Form and Function (Figure 18.15) 1. The body is divided into head, neck, and trunk regions. 2. Eleven segments on the trunk that are marked externally by spines and cuticular plates. 3. The spines (scalids) function in locomotion, chemoreception and mechanoreception. 4. The retractile head has a circle of spines and a retractile proboscis (introvert). 5. The body wall is made of a cuticle and syncytial epidermis. 6. Circular, longitudinal and diagonal muscles are anchored in each segment. 7. It cannot swim, but anchors in its silt or mud burrow with spines. 8. Digestive system consists of mouth (at tip of proboscis), pharynx, esophagus, stomach-intestine and anus. 9. The pseudocoel is filled with amebocytes and fluid. 10. Excretory system composed of multinucleated solenocyte protonephridium at each side of the gut. 11. Nervous system composed of a brain encircling the pharynx and a ventral nerve cord. 12. Sensory organs include eyespots and sensory bristles. 13. Sexes are separate with paired gonads and gonoducts; no asexual reproduction has been found. 14. Development includes a series of six juvenile stages and a nonmolting adult. 18.6. Phylum Priapulida A. Diversity 1. 16 species of these marine worms occur in colder waters. 2. They occur from intertidal zones to deep ocean floors, several thousand meters deep. 3. Some are tube dwellers and feed on detritus. B. Form and Function (Figure 18.16) 1. Priapulids have cylindrical bodies under 15 cm long. 2. They burrow by body contractions and orient their mouth at the surface. 3. The retractable introvert has papillae and rows of curved spines to sample and capture prey. 4. The trunk has 30–100 superficial rings covered with tubercles and spines. 5. Anus and urogenital pores are at the posterior end of the trunk. 6. Caudal appendages are hollow stems probably respiratory and chemoreceptive in function. 7. A chitinous cuticle covers the body and is molted regularly. 8. Digestive system contains a pharynx, straight intestine, and rectum. 9. The nerve ring around the pharynx connects to the ventral nerve cord. 10. The body cavity contains amebocytes and in one species, a respiratory pigment: hemerythrin. 11. Sexes are separate. 12. Paired urogenital organs have gonads and clusters of solenocytes connected to a protonephridial tubule that carries both gametes and excretory products outside. 13. Embryology is poorly known. 18.7 Clade Panarthropoda A. This clade contains Arthropoda and two allied phyla, Onychophora and Tardigrada. 1. Characteristics a. Coelom reduced. New cavity called a hemocoel forms from fusion of t main coelomic cavity with the blastocoel. b. Coelom develops by schizocoely in onychophorans and arthropods, enterocoely in tardigrades. c. Blood from open circulatory system enters hemocoel and surrounds internal organs. d. Have a muscular heart but limited muscular blood vessels. B. Phylum Onychophora 1. History and Diversity a. About 70 species of these “velvet worms” exist; they range from 1.4 to 15 cm in length. b. They are limited to moist, leafy rain forest habitat in tropical and subtropical regions. c. They have changed little over 500 million years; fossil Aysheaia is similar to modern forms. d. Share many traits with both annelids and arthropods and were considered a “missing link.” e. They were probably far more common in earlier geological times than they are today. f. Most velvet worms are predaceous and some live in termite nests. 2. Form and Function a. External Features 1) Body has no external segmentation except for paired appendages. (Figure 18.17) 2) Soft skin; the cuticle contains protein and chitin but does not harden as in arthropods. 3) The body is covered with tiny tubercles bearing sensory bristles. 4) Minute scales on the tubercles give it an iridescent and velvety appearance. 5) The head has one pair of large antennae and an annelid-like eye at the base. 6) The ventral mouth has a pair of claw-like mandibles flanked by a pair of oral papillae. 7) 14 to 43 pairs of unjointed legs are short and stubby and clawed. 8) Legs are more vental than annelid parapodia and move by waves of body contractions. b. Internal Features (Figure 18.18) 1) Similar to annelids, the body wall is muscular. 2) The hemocoel is incompletely divided into compartments or sinuses as in arthropods. 3) Slime glands on each side of the body cavity open on oral papillae. 4) The mouth leads to a straight digestive tract. 5) Each segment contains a pair of nephridia, each containing a vesicle, ciliated funnel and duct. 6) Absorptive cells in the midgut excrete crystalline uric acid. 7) Pericardial cells function as nephrocytes to store excretory products taken from the blood. 8) A tracheal system provides respiration to all parts of the body; the tracheae opens outside at spiracles. 9) Because they cannot close their spiracles, they are restricted to moist habitats. 10) Tracheal system likely evolved independently from that of arthropods. 11) Open circulatory system, with a dorsal tubular heart and a pair of ostia in each segment. 12) A pair of cerebral ganglia joins with widely separated nerve cords. 13) Ladder-like nervous system with commisures connecting the paired ventral nerve cords. 14) Possess sensory pigment cup ocelli, taste spines, tactile papillae and hygroscopic receptors. 3. Reproduction a. With the exception of one parthenogentic species, onychophorans are dioecious with paired reproductive organs. b. Males deposit spermatophores on the female’s back. c. White blood cells dissolve the skin and the sperm migrate to the ovaries. d. Onychophorans can be oviparous, ovoviviparous or viviparous. C. Phylum Tardigrada (Figures 21.19–21.22) 1. Diversity a. Tardigrades are very small, less than a millimeter long. b. Most species known live in a water film around mosses and lichens; a few are marine. 2. Form and Function (Figures 18.19, 18.20) a. The elongated, cylindrical or oval body is unsegmented. b. The head is the anterior part of the trunk. c. The trunk bears four pairs of short, stubby, unjointed legs, each with four to eight claws. d. The body has a non-chitinous cuticle that is molted four or more times during their lifetime. e. The mouth leads to a muscular pharynx that is adapted for sucking. f. Two stylets are protruded to pierce the integument of nematodes or the cellulose walls of plant cells and allow them to suck juices. g. Three glands, perhaps Malpighian tubules, empty into the digestive system. h. Most of the body cavity is hemocoel; the true coelom is the gonadal cavity. i. There are no circulatory or respiratory systems; gas exchange occurs at the body surface. j. Only longitudinal muscles; it uses hydrostatic pressure as a skeleton and moves awkwardly. k. The brain is relatively large, covering the dorsal surface of the pharynx. l. The brain connects to a double ventral nerve cord that forms a chain of four ganglia. 3. Reproduction (Figures 18.21, 18.22) a. Sexes separate; in parthenogenetic freshwater and moss-dwelling species, males are unknown. b. Egg-laying, like defecation, occurs at molting; the eggs may be highly ornate. c. Males gather around an old cuticle and shed sperm on it; others may use internal fertilization at molting. d. Cleavage is complete and forms a stereogastrula. e. Five of the six coelomic pouches disaggregate to form structures; last pair forms the gonocoel. 4. Cryptobiosis a. Terrestrial tardigrades can suspend metabolism to survive harsh conditions. b. Tardigrades can dehydrate from 85% water to only 3% water; in this state they can resist extreme temperatures, ionizing radiation, oxygen deficiency, etc. for years! c. When water is available, they become metabolically active again. 18.8 Phylogeny and Adaptive Diversification A. Phylogeny 1. Evolutionary relationships among ecdysozoans are not well-understood. 2. Members of this clade do not share a common cleavage pattern. a. Nematodes and nemotomorphs cleavage is unique; it’s not spiral or radial. b. Cleavage in kinorhynchs, lorificiferans, and tardigrades has yet to be studied. c. Priapulids cleavage is nearly radial. 3. Phyla Nematoda and Nematomorpha are sister taxa since they share a collagenous cuticle. 4. Phyla Kinorhyncha and Priapulida are sister taxa because they share a two-layered pharynx. 5. Clade Panarthropoda unites three phyla: Onychophora (velvet worms); Tardigrada (water bears); and Arthropoda 6. Phylum Onychophora is sister taxon to clade comprising arthropods and tardigrades. 7. Onychophorans share a number of characteristics with annelids: metamerically arranged nephridia, muscular body wall, pigment cup ocelli, and ciliated reproductive ducts. 8. Onychophorans also share features with arthropods: poseession of cuticle, tubular heart, hemocoel with open circulatory system, presence of tracheae (possibly not homologous) and large brain. 9. Genetic sequence analysis supports placement of Onychophora in clade Panarthropoda. 10. Tardigrades have some similarities to rotifers, particularly in their reproduction and their cryptobiotic tendencies. 11. Tardigrades and arthropods also share arthropod-type setae and muscles inserted on the cuticle. B. Adaptive Diversification 1. Nematodes show the most impressive adaption. a. They are the most abundant and have been found in almost every habitat available. b. Their body structure is plastic enough to allow adaptation. c. Their life cycle ranges from simple to complex. d. Nematodes have been found to survive in suboptimal conditions. Lecture Enrichment 1. Note that the number of species known, especially in these groups, may be more dependent on our ability to collect the organisms than on their actual rarity. Often new techniques, such as deep-sea dredging or use of Berlese sampling for soil organisms, create a surge in new species previously unknown. 2. The text refers to the onychophoran as a “missing link”—a term rightly placed in parentheses. A “missing link” no longer accurately describes the onychophoran since: 1) It has been discovered; and 2) It itself is not the link, but merely a less-changed organism that has nevertheless seen as much evolutionary time as the groups (annelids and arthropods) it’s ancestor “linked.” 3. The discussion of terrestrial tardigrades that bring their metabolic chemistry to a total stop and lose nearly all their water content returns students to the questions of “what is life” discussed in the first chapter. Commentary/Lesson Plan Background: Even students who live near or visit the coast and with diving experience will not recognize these remote animals. Preserved specimens or visuals will be important. Misconceptions: Terms can be misleading (i.e., the term “proboscis” needs to be carefully defined across these groups as it is uniquely different from a butterfly proboscis, etc.) Life will generally be assumed to always involve active metabolism; the tardigrades described here will of necessity require a redefinition of life as the structural integrity necessary to allow the metabolism of life to resume. Schedule: HOUR 1 18.1. Diversity 18.2. Phylum Nematoda A. Diversity B. Form and Function C. Representative Nematodes 18.3. Phylum Nematomorpha A. Diversity B. Form and Function 18.4. Phylum Loricifera A. Diversity B. Form and Function 18.5. Phylum Kinorhyncha A. Diversity B. Form and Function 18.6. Phylum Priapulida A. Diversity B. Form and Function HOUR 2 18.7 Clade Panarthropoda A. Diversity B. Phylum Onychophora C. Phylum Tardigrada 18.8 Phylogeny and Adaptive Diversification A. Phylogeny B. Adaptive Diversification ADVANCED CLASS QUESTIONS: 1. What selective advantages would accrue to an organism where the male lives as a small attachment to the body of the much larger female? If this is an advantage, why do most other sexual organisms not also develop this strategy? Answer: Selective Advantages of Male Attachment to Female: 1. Increased Access to Mating Opportunities: • By attaching to the body of the larger female, the male ensures constant proximity, increasing mating opportunities. 2. Protection from Predators: • Living attached to the female provides protection from predators, as the female is often better equipped to defend against predators. 3. Nutritional Benefits: • Some species may allow the attached male to access nutrients from the female's body, ensuring better survival and reproductive success. 4. Reduced Competition: • Attachment reduces competition between males for access to females, as the attached male can monopolize the female. Why Not Adopted by Most Sexual Organisms: 1. Species-Specific Adaptation: • This strategy is highly specific to certain species where it provides significant reproductive advantages. 2. Ecological Constraints: • Attachment may not be feasible or advantageous in all ecological contexts or reproductive strategies. 3. Evolutionary Trade-offs: • Evolutionary trade-offs between different reproductive strategies may favor other mating behaviors in most sexual organisms. 2. Why would tardigrades be one of the major organisms found in Antarctica? Answer: Adaptations of Tardigrades to Antarctic Environment: 1. Extreme Cold Tolerance: • Tardigrades have remarkable adaptations to survive in extreme cold temperatures, including the ability to withstand freezing and desiccation. 2. Cryptobiosis: • Tardigrades can enter a state of cryptobiosis, where metabolic activities are suspended, allowing them to survive harsh environmental conditions, including freezing temperatures. 3. Dehydration Resistance: • Tardigrades have mechanisms to survive desiccation, allowing them to endure the low humidity conditions prevalent in Antarctica. 4. Slow Metabolism: • Tardigrades have a slow metabolism, allowing them to conserve energy and survive in environments with limited resources. 5. High Environmental Tolerance: • Tardigrades can withstand a wide range of environmental conditions, making them well-suited for survival in the extreme cold and harsh conditions of Antarctica. 6. Presence of Water: • Despite the cold temperatures, water is available in liquid form in some parts of Antarctica, providing a habitat where tardigrades can thrive, especially in mosses and lichens. 3. The text refers to the suspended animation of tardigrades and rotifers as cryptobiosis (“hidden life”) rather than anabiosis (“return to life”) or anhydrobiosis (“life without water”) or resurrection (“rising from dead”). Pasteur solidly disproved spontaneous generation, and science is hesitant to propose any return to life arising from non-life, with the exception of the long process of chemical evolution that led to the first protocell. What problems do each of these terms pose in describing the case of the tardigrades and rotifers? Answer: Issues with Terminology: 1. Anabiosis ("Return to Life"): • Problem: The term "anabiosis" suggests a return to life from a state of suspended animation. • Issue: Tardigrades and rotifers do not return to life in the traditional sense; they resume metabolic activity after a period of cryptobiosis but are not truly dead during this state. 2. Anhydrobiosis ("Life Without Water"): • Problem: The term "anhydrobiosis" implies life without water, which is not entirely accurate for tardigrades and rotifers. • Issue: While these organisms can survive extreme dehydration, they are not entirely devoid of water during cryptobiosis. 3. Resurrection ("Rising from Dead"): • Problem: The term "resurrection" implies a return to life from death. • Issue: Tardigrades and rotifers do not experience death during cryptobiosis; they enter a state of suspended animation, from which they can revive. 4. Cryptobiosis ("Hidden Life"): • Appropriateness: The term "cryptobiosis" accurately describes the state of suspended animation observed in tardigrades and rotifers. • Explanation: Cryptobiosis reflects the hidden or dormant state these organisms enter, during which metabolic activities are suspended, allowing them to survive harsh environmental conditions. CHAPTER 19 TRILOBITES, CHELICERATES, AND MYRIAPODS PHYLUM ARTHROPODA SUBPHYLUM TRILOBITA SUBPHYLUM CHELICERATA SUBPHYLUM MYRIAPODA CHAPTER OUTLINE 19.1. Characteristics A. Anthropodization 1. The soft cuticle of the ancestors of arthropods was stiffened by deposition of protein and inert polysaccharide chitin. 2. Joints had to provide flexibility and a sequence of molts was necessary to allow for growth. 3. Molting required hormonal control. 4. The hydrostatic skeleton function was lost, the coelom regressed and open sinuses replaced them. 5. Motile cilia were lost. 6. If this were not a monophyletic group, these features would all have to arise independently. B. Phylum Arthropoda 1. This contains over three-fourths of all known species. 2. Approximately 1,100,000 species of arthropods have been recorded. 3. The phylum has a rich fossil history dating to the late Precambrian. 4. They are eucoelomate protostomes with well-developed organ systems. 5. Similar to annelids, they have distinct segments; however, comparative molecular analyses indicate that annelids and arthropods evolved from different ancestors. 6. Sizes range from the Japanese crab (four meters in leg span) to the 0.1 mm long follicle mite. 7. Their abundance and wide ecological distribution makes them the most diverse animal group. 8. Some arthropods are serious disease agents and compete with humans for food; others are beneficial. 9. All modes of feeding occur among arthropods but most are herbivorous. 10. Relationships Among Arthropod Subgroups (see Figure 18.1 and Figure 19.1 and 19.2) a. Arthropoda is divided into subphyla based on current understanding of the relationships between subgroups. b. These groupings among subphyla are based on molecular data. c. Centipedes, millipedes, pauropods, and symphylans are placed in subphylum Myriapoda. d. Insects are placed in subphylum Hexapoda. e. Spiders, ticks, horseshoe crabs and their relatives form subphylum Chelicerata. f. Lobsters, crabs, barnacles, and others form subphylum Crustacea. g. Also included in Crustacea are tongue worms (formerly of phylum Pentastomida). h. Extinct trilobites are placed in subphylum Trilobita. i. These relationships are controversial. j. A “mandibulate hypothesis” places myriapods, hexapods, and crustaceans more closely related because of a shared mouthpart called a mandible. k. Molecular evidence of a close relationship between hexapods and crustaceans led us to unite subphylum Crustacea with subphylum Hexapoda in clade Pancrustacea. 19.2. Why Have Arthropods Achieved Such Great Diversity and Abundance? A. The diversity of species, wide distribution, variety of habitats and feeding habits, and adaptations are due to a constellation of structures and physiological patterns. 1. Versatile Exoskeleton (Figure 19.3) a. The cuticle is highly protective but is jointed, providing mobility. b. It consists of an inner thick procuticle and an outer thin epicuticle. c. Procuticle has an exocuticle secreted before a molt and an endocuticle secreted after molting. d. Both layers of procuticle contain chitin bound with protein. e. Thus the procuticle is lightweight, flexible, and provides protection against dehydration. f. Chitin varies from composing 40% of the procuticle in insects to as much as 80% in crustaceans. g. Impregnation with calcium salts makes the procuticle very hard in lobsters and crabs. h. The cuticle is laminated and further hardened by tanning, a chemical process. i. As the cuticle is thin between segments, it allows movement at the joints. j. Cuticle also folds inward to line the foregut, hindgut and trachea. k. Ecdysis (molting) is the process of shedding an outer covering and growing a new, larger one. l. Arthropods typically molt four to seven times; the weight is a limit to ultimate body size. 2. Segmentation and Appendages for Efficient Locomotion a. The primitive pattern is a linear series of similar somites with jointed appendages. b. Many somites may be fused or combined into specialized groups called tagmata. c. Appendages are often highly specialized for division of labor. d. Limb segments are hollow levers with internal striated muscles. e. Appendages may function in sensing, food handling, walking, or swimming. 3. Air Piped Directly to Cells a. Terrestrial arthropods use an efficient tracheal system that delivers oxygen directly to cells. b. Aquatic arthropods respire via various forms of efficient gills. 4. Highly Developed Sensory Organs a. Eyes vary from simple light sensitive ocelli to a compound mosaic eye. b. Other senses accomplish touch, smell, hearing, balancing and chemical reception. 5. Complex Behavior Patterns a. Arthropods surpass most other invertebrates in complex and organized activities. b. Most behavior is innate or unlearned but some is learned. 6. Use of Diverse Resources Through Metamorphosis a. Many arthropods have metamorphic changes that result in different larval and adult stages. b. Larvae and adults eat different foods and occupy different habitats and avoid competition. 19.3. Subphylum Trilobita A. History of an Ancient Group (Figure 19.4) 1. Trilobites arose before the Cambrian, flourished, and then became extinct 200 million years ago. 2. They have a trilobed body shape due to a pair of longitudinal grooves. 3. They were bottom dwellers and probably were scavengers. 4. Ranging from 2 to 67 centimeters long, they could roll up like pill bugs. 5. Their exoskeleton contained chitin strengthened by calcium carbonate. 6. The body was divided into an anterior fusion of segments called a cephalon, a central trunk that varied in somites, and a posterior fused plate called a pygidium. 7. The cephalon bore antennae, compound eyes, a mouth, and jointed appendages. 8. Each body somite except the last bore a pair of biramous appendages. 9. One of the branches of the biramous appendage was fringed and may have been a gill. B. Trilobite Identification 1. Species identified by morphology. 2. Evolutionary diversification of Calmoniid trilobites associated with sea level flucturation, with rapid speciation when sea levels were low (via geographical isolation), and slow when sea levels were high. 19.4. Subphylum Chelicerata (Figure 19.5) A. Characteristics 1. Bodies composed of two tagmata: a cephalothorax (prosoma) and abdomen (opithsoma). 2. Have six pairs of cephalothoracic appendages including chelicerae, pedipalps and four pair of legs. 3. Lack mandibles and antennae. 4. Most suck liquid food from prey. B. Class Merostomata: Subclass Euryptida (Figure 19.5) 1. Eurypterids (giant water scorpions) were largest fossil arthropods (3 meters long). (Figure 19.4B) 2. Their fossils occur in rocks from the Ordovician to the Permian periods. 3. They resemble both marine horseshoe crabs and terrestrial scorpions. 4. Their head had six fused segments, six pairs of appendages, and both simple and compound eyes. 5. The abdomen had 12 segments and a spike-like telson. 6. Their head had six fused segments and bore simple and compound eyes as well as chelicerae, pedipalps, and four pairs of walking legs. C. Class Merostomata: Subclass Xiphosurida, Horseshoe Crabs (Figure 19.6) 1. The modern horseshoe crab is nearly unchanged from ancestors in the Triassic period. 2. Five species in three genera survive. 3. Most live in shallow water. 4. Structures a. An unsegmented shield or carapace covers the body in front of a broad abdomen and a telson. b. The cephalothorax has five pairs of walking legs and a pair of chelicerae. c. The abdomen bears six pairs of broad, thin, appendages fused in the median line. d. Book gills are exposed on some of the abdominal appendages. e. The carapace has two compound and two simple eyes. 5. They walk with their walking legs and swim with abdominal plates. 6. They feed at night on worms and small molluscs. 7. During the mating season, they come to shore at a very high tide to mate. 8. Females burrow into sand to lay eggs; males follow to add sperm before she covers the eggs. 9. The young larvae hatch and return to the sea at the next very high tide. 10. Larvae are segmented and resemble trilobites. D. Class Pycnogonida: Sea Spiders (Figure 19.7) 1. Sea spiders vary from a few millimeters to larger sizes; all have small, thin bodies. 2. There are about 1,000 species of sea spiders. 3. Some species duplicate the somites, and they may have five or six pairs of legs. 4. Some males may have a subsidiary pair of legs (ovigers) to carry developing eggs. (Figure 19.7) 5. Many also have chelicerae and palps. 6. The mouth, at the tip of a proboscis, sucks juices from cnidarians and soft-bodied animals. 7. Most have four simple eyes. 8. There is a simple dorsal heart but excretory and respiratory systems are lacking. 9. The digestive system sends branches into the legs; most gonads are also in the legs. 10. Sea spiders occur in all oceans but are most common in polar waters. 11. Some suggest that pycnogonids belonged to an early-diverging arthropod lineage. E. Class Arachnida (Figure 19.8) 1. There is a great diversity among scorpions, mites, ticks, daddy longlegs, and others. 2. Most are free living and more common in warm, dry regions. 3. Structures a. Arachnids are divided into two tagmata: a cephalothorax and an abdomen. b. The cephalothorax bears a pair of chelicerae, a pair of pedipalps and four pairs of walking legs. c. Antenna and mandibles are lacking. d. Most are predaceous and have claws, fangs, poison glands, or stingers. e. Sucking mouthparts ingest the fluids and soft tissues from bodies of their prey. f. Spiders have spinning glands. g. A few spiders may have a segmented abdomen, a primitive character. h. Pedipalps of males are modified, sometimes elaborately, for sperm transfer. 4. Over 80,000 species have been described. 5. Scorpions appeared on land in the Silurian; mites and spiders were found by the end of the Paleozoic Era. 6. Most are harmless to humans and provide essential control of injurious insects. 7. Some spiders are venomous and can cause pain or death in humans; ticks may carry human diseases; mites can be crop pests. 8. Order Araneae: Spiders (Figures 19.8, 19.9) a. About 40,000 species of spiders are known. b. The body consists of an unsegmented cephalothorax and abdomen joined by a slender pedicel. c. The anterior appendages are a pair of chelicerae with terminal fangs. d. A pair of pedipalps have sensory functions and are used by males to transfer sperm. e. The basal parts of the pedipalps are used to handle food. f. Four pairs of walking legs terminate in claws. g. All spiders are predaceous, mostly on insects, which are dispatched by venom and fangs. h. The injected venom liquefies and digests the tissues; this is sucked into the spider’s stomach. i. Spiders breathe by book lungs and/or tracheae. 1) Book lungs are unique to spiders; parallel air pockets extend into a blood-filled chamber. 2) Air enters the chamber through a slit in the body wall. 3) The tracheae system is less extensive than in insects; it carries air directly to tissues. 4) The tracheal systems of arthropods represent a case of evolutionary convergence. j. Spiders and insects have Malpighian tubules for an excretory system. 1) Potassium, other solutes and waste molecules are secreted into the tubules. 2) Rectal glands reabsorb the potassium and water, leaving wastes and uric acid for excretion. 3) This conserves water and allows the organisms to live in dry environments. 4) Many spiders have coxal glands that are modified nephridia at the base of some legs. k. Sensory Systems 1) Most spiders have eight simple eyes, each with a lens, optic rods and a retina. 2) They detect movement and may form images. 3) Sensory setae detect air currents, web vibrations and other stimuli. 4) A spider’s vision is usually poor and its awareness of its environment depends largely on cuticular mechanoreceptors such as sensory setae. l. Web-Spinning Habits (Figures 19.9 − 19.11) 1) Spinning silk is a critical ability for spiders and some other arachnids. 2) Two or three pairs of spinnerets contain microscopic tubes that run to silk glands. 3) A liquid scleroprotein secretion hardens as it is extruded from the spinnerets. 4) Silk threads are very strong and will stretch considerably before breaking. 5) Silk is used for orb webs, lining burrows, forming egg sacs and wrapping prey. m. Reproduction 1) Before mating, the male stores his sperm in his pedipalps. 2) Mating involves inserting the pedipalps into the female genital openings. 3) A courtship ritual is often required before the female will allow mating. 4) Eggs may develop in a cocoon in the web or may be carried by the female. 5) The young hatch in about two weeks and may molt before leaving the egg cocoon. n. Are spiders really dangerous? (Figures 19.12, 19.13) 1) Most people fear spiders without good reason. 2) Spiders are allies of humans in our battle with insects. 3) American tarantulas rarely bite, and the bite is not dangerous. 4) Species of black widow spiders are dangerous; the venom is neurotoxic. 5) The brown recluse spider has hemolytic venom that destroys tissue around the bite. 6) Some Australian and South American spiders are the most dangerous and aggressive. 9. Order Scorpionida: Scorpions (Figure 19.14) a. Scorpions are more common in tropical and subtropical zones but do occur in temperate areas. b. There are about 1,400 species worldwide. c. They are nocturnal and feed largely on insects and spiders. d. Sand-dwelling scorpions locate prey by detecting surface waves with their leg sensillae. e. The cephalothorax has the appendages, a pair of medial eyes and 2–5 lateral eyes. f. The preabdomen has seven segments. g. The postabdomen has the long, slender tail of five segments that ends in a stinging apparatus. h. Under the abdomen are comblike pectines that explore the ground and help in sex recognition. i. The stinger on the last segment has venom that varies from mildly painful to dangerous. j. Scorpions are ovoviviparous or viviparous and produce from six to 90 young. k. Scorpions perform complex mating dances, and in some species the male stings the female on her pedipalp or on the edge of her cephalothorax. 10. Order Solpugida: Sun or Camel Spiders (Figure 19.14B) a. Common in subtropical deserts in America, Asia, Africa, and the Middle East. b. Size ranges from 1 – 15 cm long. c. Nonvenomous; use large chelicerae to shred prey. 11. Order Opiliones: Harvestmen (Figure 19.14C) a. Harvestmen or daddy longlegs are common, particularly in tropical regions. b. There are about 5,000 species worldwide. c. Unlike spiders, their abdomen and cephalothorax join broadly without a narrow pedicel. d. They can lose most of their eight long legs without ill effect. e. Their chelicerae are pincerlike and they feed more as scavengers than do spiders. 12. Acari: Ticks and Mites (Figure 19.15 − 19.20) a. Acari are medically and economically the most important arachnids. b. About 30,000 species have been described, many more are estimated to exist. c. They are both aquatic and terrestrial, and inhabit deserts, polar areas and hot springs. d. Most mites are less than 1 millimeter long; ticks may range up to 2 cm. e. Acarines have complete fusion of cephalothorax and abdomen with no sign of external segmentation. f. Mouthparts are on the tip of the anterior capitulum. g. The chelicerae to each side help pierce, tear or grip food. h. Other mouthparts include pedipalps with a fused base, the hypostome, a rostrum and tectum. i. Adult mites and ticks possess four pairs of legs. j. Acarines may transfer sperm directly or by spermatophores. k. The egg hatches, releasing a six-legged larva; eight-legged nymphal stages follow. l. House dust mites are free-living and often cause allergies. m. Spider mites are one of many important agricultural pest mites that suck out plant nutrients. n. Chiggers are larval Trombicula mites; they feed on dermal tissues and cause skin irritation. o. Hair follicle mite Demodex is harmless but other species cause mange in domestic animals. p. The human itch mite causes intense itching. q. Tick species of Ixodes carry Lyme disease. r. Tick species of Dermacentor transmit Rocky Mountain spotted fever. s. The cattle tick transmits Texas cattle fever. 19.5 Subphylum Myriapoda (Figure 19.21) A. Characteristics 1. Myriopods include Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda (pauropods), and Symphyla (symphylans). 2. Myriopods use trachea to carry respiratory gases to and from all body cells. 3. Excretion is usually by Malpighian tubules. B. Class Chilopoda (Figures 19.21. 19.22) 1. Natural History a. Centipedes are found under logs, bark and stones. b. They are carnivorous, eating earthworms, cockroaches and other insects. c. The house centipede has 15 pairs of long legs and is common in bathrooms and damp cellars. d. Most are harmless to humans but a few large, tropical centipedes are dangerous. e. There are about 3,000 species worldwide. 2. Characteristics a. Centipedes are terrestrial and have flattened bodies with up to 177 segments. b. Each segment except the one behind the head and the last two, bears a pair of jointed legs, the last pair of which serves a sensory function. c. Appendages of the first body segment form poison claws. d. The head has one pair of antennae, a pair of mandibles and one or two pairs of maxillae. e. Eyes on either side of the head consist of groups of ocelli. f. Salivary glands empty into the anterior end of the straight digestive tract. g. Two pairs of Malpighian tubules empty into the hind intestine. h. The elongated heart has a pair of arteries in each somite; ostia provide return flow of hemolymph. i. A pair of spiracles in each somite allows air to diffuse through branched air tubes of the tracheae. j. The arthropod nervous system includes a portion that serves as a visceral nervous system. 3. Reproduction a. Sexes are separate with unpaired gonads and paired ducts. b. Some lay eggs and others are viviparous. c. Young resemble adults and do not undergo metamorphosis. C. Class Diplopoda (Figure 19.23) 1. Natural History a. Millipedes are less active than centipedes; walk with a graceful rather than wriggling motion. b. Most eat decayed plants but a few eat living plant tissue. c. Most are slow moving and roll into a coil for defense. Some secrete toxic or repellant fluids from special repugnatorial glands on sides of body. e. There are more than 10,000 species of millipedes worldwide. 2. Characteristics a. Their cylindrical bodies have from 25 to more than 100 segments. b. Their short thorax consists of four segments, each bearing one pair of legs. c. The head has two clusters of simple eyes and a pair each of antennae, mandibles and maxillae. d. Each abdominal somite has two pairs of spiracles opening into air chambers and tracheal air tubes. e. The two genital apertures are toward the anterior end. 3. Reproduction a. The appendages of the seventh segment are specialized for copulatory organs. b. After copulation, the female lays eggs in a nest and guards them. c. Larvae have only one pair of legs to each segment. D. Class Pauropoda 1. Life History a. Pauropods live in moist soil, leaf litter, decaying vegetation, or under bark and debris. b. They are the least well known of myriapods. 2. Characteristics (Figure 19.24A) a. Pauropods are soft-bodied, small (2 mm or less) myriapods. b. Almost 500 species are known. c. The head lacks true eyes, has branched antennae, and a pair of sense organs. d. The 12 trunk segments bear nine pairs of legs but none on the last two segments. e. A tergal plate covers each of the two segments. f. They lack tracheae, spiracles, and a circulatory system. g. Pauropods are probably most closely related to diplopods. E. Class Symphyla 1. Life History and Reproduction a. They live in humus, leaf mold and debris. b. The male Scutigerella places a spermatophore at the end of a stalk. c. The female stores the sperm in special pouches; she removes eggs and smears them with sperm before attaching them to moss or lichen. d. Young hatch with only six or seven pairs of legs. 2. Characteristics (Figure 19.24B) a. Symphylans are small (2–10 mm) with centipede-like bodies. b. Are soft-bodied with 14 segments; 12 segments bear legs and one bears a pair of spinnerets. c. Antennae are long and unbranched. d. About 160 species are known. e. They are eyeless with sensory pits at the base of the antennae. f. The tracheal system connects to a pair of spiracles on the head and tracheal tubes to the anterior only. 19.6. Phylogeny and Adaptive Diversification A. Phylogeny 1. Relationships between subphyla are debated. 2. However, the taxon of Pancrustacea, which includes hexapods and crustaceans, is well-supported. 3. Phylogenies using molecular data rarely support grouping Myriapoda with Pancrustacea. 4. There is support for placement of Myriapoda as the sister taxon for Cheliceratea. 5. Biologists assume ancestral arthropod had a segmented body with one pair of legs per segment. 6. Evolution caused adjacent segments to fuse and produced body regions. 7. Hox gene studies indicate that the first five segments fused to form the head tagma in all four extant subphyla. 8. In spiders, Hox gene studies indicate that the entire prosoma corresponds to the head of other arthropods. 9. Sea spiders remain within subphylum Chelicerata because Hox gene studies have found that their head appendage arose from the region of the head that corresponds to the second segment. 10. Genetic studies have aided our understanding of the evolution of uniramous and biramous appendages. a. Molecular evidence repeatedly places hexapods with crustaceans even though hexapods have uniramous appendages and crustaceans have biramous appendages. b. This leads to the question: Did uniramous appendage development evolve more than once? 11. Numbers of appendages per segment is another variable character among arthropods that lends itself to more testing. B. Adaptive Diversification 1. In contrast to annelids, arthropods have pronounced tagmatization by fusion of somites. 2. Those with primitive characters have appendages on each somite; derived forms are specialized. 3. Modification of exoskeleton and appendages allowed variation in feeding and movement. 4. While adaptations made possible by their cuticular exoskeleton have fostered high diversity, their small size is likely another important feature. C. Classification Subphylum Trilobita Subphylum Chelicerata Class Merostomata Class Pycnogonida Class Arachnida Subphylum Myriapoda Class Diplopoda Class Chilopoda Class Pauropoda Class Symphyla Subphylum Crustacea (Chapter 20) Subphylum Hexapoda (Chapter 21) Lecture Enrichment 1. Review again the similarities between the protostomes and deuterostomes—presence of a coelom, tube-within-a-tube body plan, mesoderm, etc.; audiovisuals are of great help in illustrating embryological development. 2. Discuss the presence of sensory receptors in the head in some annelids, their absence in the earthworm, and what this implies about the environments in which these worms live. Commentary/Lesson Plan Background: Thanks to the abundance, small size, and adaptations of arthropods, most students cannot help but have some experience with them, particularly insects and spiders. Nevertheless, the astounding diversity of the groups will again require substantial visuals. Students from rural areas can relate the economic impact of crop pest species. Most students will be unaware of the massive beneficial aspects of bees for pollination, spiders for insect control, and arthropods overall role in food chains, economic benefits that accrue to hundreds of millions of dollars. And field biologists studying fish, lizards, mammals and birds encounter arthropods as important components in stomach content surveys, etc. Misconceptions: The misconception that recent organisms will always be more complex can be corrected in this section by the general trends seen in reduction of body segments through fusion of somites (tagmosis), specialization, and reduction in the case of modern parasitic forms. Schedule: HOUR 1 19.1. Characteristics A. Anthropodization B. Phylum Arthropoda 19.2. Great Diversity and Abundance of Arthropods A. Diversity of Species 19.3. Subphylum Trilobita A. Histroy of an Ancient Group 19.4. Subphylum Chelicerata A. Characteristics B. Class Merostomata: Subclass Euryptida C. Class Merostomata: Subclass Xiphosurida, Horseshoe Crabs D. Class Pycnogonida: Sea Spiders E. Class Arachnida 19.5 Subphylum Myriapoda A. Characteristics B. Class Chilopoda C. Class Diplopoda D. Class Pauropoda E. Class Symphyla 19.6. Phylogeny and Adaptive Diversification A. Phylogeny B. Adaptive Diversification C. Classification ADVANCED CLASS QUESTIONS: 1. Generally, a parasite evolves to do less-and-less harm to its host, since killing its host also ends its own life. How do parasitoids that kill the host avoid this evolutionary fate? Answer: 1. Life Cycle Strategy: • Parasitoids: Parasitoids are organisms that lay their eggs on or inside another organism (host), and the larvae develop by feeding on the host's tissues, eventually killing it. • Evolutionary Strategy: Unlike typical parasites, which aim to maintain their host alive for an extended period, parasitoids have evolved to kill their hosts as part of their life cycle. 2. Multiple Hosts: • Reproduction: By killing the host, the parasitoid ensures the development and survival of its offspring. • Evolutionary Advantage: Parasitoids typically have multiple hosts in their life cycle, allowing them to exploit a broader range of resources and reducing the risk of their own extinction if one host species becomes extinct. 3. Rapid Development: • Quick Development: Parasitoid larvae develop rapidly within the host's body, minimizing the time during which the host is alive and potentially harmful to the parasitoid offspring. 2. The text correctly notes that the earliest land invertebrates, based on fossil evidence, were scorpions. However, modern scorpions feed by attacking other invertebrates. What is the logical problem here, and what additional research or evidence is necessary to solve this dilemma? Answer: 1. Logical Problem: • Predatory Behavior: Modern scorpions are primarily predatory, feeding on other invertebrates. • Herbivorous Ancestors: The existence of herbivorous ancestors contradicts the predatory behavior observed in modern scorpions. 2. Additional Research Needed: • Fossil Evidence: Further fossil evidence is required to determine the diet and feeding behavior of early scorpion ancestors. • Stable Isotope Analysis: Analyzing stable isotopes in fossilized scorpion remains can provide insights into their dietary preferences and trophic levels. • Comparative Morphology: Comparative morphology studies can reveal anatomical adaptations related to feeding behavior, providing clues about the diet of early scorpions. 3. Why are giant spiders the size of humans or elephants not possible? Answer: 1. Physiological Limitations: • Exoskeleton: Spiders have an exoskeleton made of chitin, which limits their size because it must be shed periodically to accommodate growth. • Respiratory System: Larger spiders would have difficulty respiring efficiently through tracheal systems due to the surface area-to-volume ratio. 2. Ecological Constraints: • Prey Availability: Large spiders would require a substantial amount of prey to sustain themselves, and such prey may not be readily available in sufficient quantities. • Predation Risk: Larger size may make spiders more vulnerable to predation themselves. 4. If spiders primarily feed on insects, what relationship would you expect between the evolution of spider diversity and insect diversity? Answer: 1. Co-evolution with Insects: • Predator-Prey Relationship: Spiders and insects have a predator-prey relationship, with spiders primarily feeding on insects. • Diversity Relationship: As insect diversity increases, the diversity of spider species that prey on them is also expected to increase. 2. Impact of Prey Availability: • Resource Availability: Insect diversity provides a broader range of prey resources for spiders, allowing for niche specialization and the evolution of a diverse array of spider species. 5. How does the evolutionary “invention” of a harder cuticular exoskeleton cascade into a series of additional arthropod features? Answer: 1. Hard Cuticle Evolution: • Protective Exoskeleton: The evolution of a harder cuticular exoskeleton provided protection and support, allowing arthropods to exploit new ecological niches. • Consequences: • Segmentation: Hard exoskeleton facilitated the development of segmentation, allowing for greater flexibility and mobility. • Jointed Appendages: The presence of a rigid exoskeleton necessitated jointed appendages for movement. • Specialized Functions: Differentiation of body segments and appendages led to specialized functions, such as feeding, locomotion, and sensory perception. 6. How do the phylogenies based on morphology and rRNA sequences compare? How might these contradictory phylogenies be resolved? Answer: 1. Comparison: • Morphology-Based Phylogeny: Historically, arthropod phylogeny has been based on morphological characteristics. • rRNA-Based Phylogeny: Recent studies have used molecular data, such as rRNA sequences, for phylogenetic analyses. 2. Contradictory Phylogenies: • Discrepancies: Phylogenies based on morphology and rRNA sequences sometimes produce conflicting results. • Resolution: • Integration of Data: Combining morphological and molecular data can provide a more comprehensive understanding of arthropod phylogeny. • Increased Taxon Sampling: Including a broader range of taxa in phylogenetic analyses can improve accuracy and resolve discrepancies. • Improved Analytical Methods: Refinement of analytical methods for integrating morphological and molecular data can lead to more robust phylogenetic reconstructions. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214