Arthropods PDF

Page 409 19 Trilobites, Chelicerates, and Myriapods A scorpion. ©Cleveland P. Hickman, Jr. LEARNING OBJECTIVES Readers will be able to: 19.1 Characterize the arthropod body plan and describe art...

Page 409 19 Trilobites, Chelicerates, and Myriapods A scorpion. ©Cleveland P. Hickman, Jr. LEARNING OBJECTIVES Readers will be able to: 19.1 Characterize the arthropod body plan and describe arthropod phylogeny according to the mandibulate hypothesis. 19.2 Identify a trilobite and describe its habitat and lifestyle. 19.3 Identify members of the three chelicerate classes and distinguish members of the largest orders of Class Arachnida.  19.4 Compare and contrast the morphology and lifestyles of centipedes, millipedes, pauropods, and symphylans. 19.5 Predict the body form of the ancestral arthropod, taking into account the morphology of its descendants. A Suit of Armor Sometime, somewhere in the Precambrian era, a major milestone occurred in the evolution of life on earth. The soft cuticle in the segmented ancestor of animals we now call arthropods was hardened by deposition of additional protein and an inert polysaccharide called chitin. The cuticular exoskeleton offered some protection against predators and other environmental hazards, and it conferred on its possessors a formidable array of other selective advantages. For example, a hardened cuticle provided a more secure site for muscle attachment, allowed adjacent segments and joints to function as levers, and vastly improved the potential for rapid locomotion, including flight. Of course, a suit of armor could not be uniformly hard; the animal would be as immobile as the rusted Tin Woodsman in The Wizard of Oz. Stiff sections of cuticle were separated from each other by thin, flexible sections, which formed sutures and joints. The cuticular exoskeleton had enormous evolutionary potential. Jointed extensions on each segment became appendages. As the hardened cuticle evolved, or perhaps concurrently with it, many other changes took place in the bodies and life cycles of protoarthropods. Growth required a sequence of cuticular molts controlled by hormones. Coelomic compartments reduced their hydrostatic skeletal function, associated with a regression of the coelom and its replacement with an open system of sinuses (hemocoel). Motile cilia were lost. These changes and others are called “arthropodization.” They produced animals with astonishing diversity and abundance, able to colonize almost every habitat on earth. PHYLUM ARTHROPODA Subphylum Trilobita Subphylum Chelicerata Subphylum Myriapoda Page 410 19.1 PHYLUM ARTHROPODA Phylum Arthropoda (ar-thrō′pōda) (Gr. arthron, joint, + pous, podos,foot) is currently the most species-diverse phylum in the animal kingdom, composed of more than three-fourths of all known species. Approximately 1,100,000 species of arthropods are recorded, and it is likely that as many more remain to be classified. (In fact, based on surveys of insect faunas in the canopies of rainforests, many estimates of yet undescribed species are much higher.) Arthropods include spiders, scorpions, ticks, mites, crustaceans, millipedes, centipedes, insects, and other less well-known groups. In addition, there is a rich fossil record extending to the very late Precambrian period. Few arthropods exceed 60 cm in length, and most are much smaller. However, Paleozoic eurypterids reached up to 3 m and some ancient dragonfly-like insects (Protodonata) had wingspans approaching 1 m. Currently, the largest arthropod, a Japanese crab Macrocheira (Gr. makros, large, + cheir, hand), spans approximately 4 m; the smallest is a parasitic mite Demodex (Gr. dēmos, people, + dex, a wood worm), which is less than 0.1 mm long. Arthropods are usually active, energetic animals. They utilize all modes of feeding—carnivory, herbivory, and omnivory—although most are herbivorous. Many aquatic arthropods are omnivorous or depend on algae for nourishment, and most land forms live chiefly on plants. In diversity of ecological distribution, arthropods have no rivals. Although many terrestrial arthropods compete with humans for food and spread serious vertebrate diseases, they are essential for the pollination of many food plants, and they also serve as food in the ecosystem, yield drugs, and generate products such as silk, honey, beeswax, and dyes. Arthropods are more widely distributed throughout all regions of the earth’s biosphere than are members of any other eukaryotic phylum. They occur in all types of environment from the deepest ocean depths to very high elevations, and from the tropics far into both northern and southern polar regions. Different species are adapted for life in the air; on land; in fresh, brackish, and marine waters; and in or on the bodies of plants and other animals. Some species live in places where no other animal could survive. Relationships Among Arthropod Subgroups Arthropods are ecdysozoan protostomes belonging to clade Panarthropoda (see Figure 18.1). They have segmented bodies, a chitinous cuticle often containing calcium, and jointed appendages. The critical stiffening of the cuticle to form a jointed exoskeleton is sometimes called “arthropodization.” Arthropods diversified greatly, but it is relatively easy to identify particular body plans characterizing arthropod subgroups. For example, centipedes and millipedes have trunks composed of repeated similar segments, whereas spiders have two distinct body regions and lack repeated segments. Arthropoda is divided into several subphyla based on our current understanding of the relationships among subgroups. Traditionally, centipedes, millipedes, and related forms, called pauropods and symphylans, were grouped with the insects in subphylum Uniramia. Members of Uniramia all possessed uniramous appendages—those with a single branch—as opposed to biramous appendages, which have two branches (see Figure 19.1). Phylogenies constructed using molecular data did not support Uniramia as a monophyletic group. Further, as the genetic basis for the uniramian versus biramian appendage was better understood (see Section 20.3), it became increasingly unlikely that all uniramous appendages were inherited from a single common ancestor with such appendages. Page 411  Figure 19.1 Two important arthropod characters: appendages may be uniramous (honey bee leg) or biramous (lobster limbs); mouthparts may include chelicerae (spider) or mandibles (grasshopper). Note that presence or absence of gills is unrelated to appendage form. Currently, five arthropod subphyla are defined. Centipedes, millipedes, pauropods, and symphylans are placed in subphylum Myriapoda. Insects are placed in subphylum Hexapoda. Spiders, ticks, horseshoe crabs, and their relatives form subphylum Chelicerata. Lobsters, crabs, barnacles, and many others form subphylum Crustacea. We include tongue worms, members of former phylum Pentastomida, in Crustacea. The extinct trilobites are placed in subphylum Trilobita. Relationships among the subphyla are controversial. One hypothesis assumes that all arthropods possessing a particular mouthpart, called a mandible (see Figure 19.1), form a clade called Mandibulata. This clade includes members of Myriapoda, Hexapoda, and Crustacea. Arthropods that do not have mandibles possess chelicerae (see Figure 19.1), as exemplified by spiders. Thus, according to the “mandibulate hypothesis,” myriapods, hexapods, and crustaceans are more closely related to each other than are any of them to chelicerates. Critics of the mandibulate hypothesis argue that the mandibles in each group are so different from each other that they could not be homologous. Mandibles of crustaceans are multijointed with chewing or biting surfaces on the mandible bases (gnathobasic mandible), whereas those of myriapods and hexapods have a single joint with the biting surface on the distal edge (entire-limb mandible). There are also some differences in the muscles controlling the two types. Proponents of the mandibulate hypothesis respond that the 550-million-year history of the mandibulates makes possible the evolution of diverse mandibles from an ancestral type. Several recent phylogenies, including those using mitochondrial genome characters, support the “mandibulate hypothesis.” We assume that subphylum Trilobita is the sister taxon to all other arthropods, and that the earliest split within the latter group separates chelicerates from a common ancestor of myriapods, hexapods, and crustaceans. However, one phylogeny places trilobites as the sister taxon to Chelicerata, and shows the mandibulates forming a clade.1 We depict subphylum Crustacea as the sister taxon of subphylum Hexapoda (see Figure 19.2). Evidence of a close relationship between hexapods and crustaceans emerged from several phylogenetic studies using molecular characters; these studies prompted a reevaluation of the morphological characters in members of both taxa. We unite subphylum Crustacea with subphylum Hexapoda in clade Pancrustacea. The exact nature of the close relationship between these two subphyla is at issue and is discussed in Chapters 20 and 21. Page 412  Figure 19.2 Cladogram of extant arthropods showing probable relationships of the four subphyla. Only a few synapomorphies are included here. Crustaceans and hexapods are shown as sister taxa, in clade Pancrustacea. A sister taxon relationship between Pancrustacea and Myriapoda is based on shared possession of mandibles and data from molecular phylogenies. Following a general introduction to the arthropods in this chapter, we cover three subphyla: Trilobita, Chelicerata, and Myriapoda. Chapter 20 is devoted to subphylum Crustacea and Chapter 21 to subphylum Hexapoda. Why Have Arthropods Achieved Such Great Diversity and Abundance? Arthropods exhibit great diversity (number of species), wide distribution, and variety of habitats and feeding habits, and have an uncanny genetic predisposition for adaptation to changing conditions. In our discussion we briefly summarize some structural and physiological patterns that have aided their rise to dominance. 1. A versatile exoskeleton. Arthropods possess an exoskeleton that is highly protective without sacrificing flexibility or mobility. This skeleton is the cuticle, an outer covering secreted by the underlying epidermis. The cuticle consists of an inner, relatively thick procuticle and an outer, relatively thin epicuticle (see Figure 19.3). The procuticle and the epicuticle each consist of several layers (lamina). The outer epicuticle is composed of protein, often with lipids. The protein is stabilized and hardened by chemical cross-linking, called sclerotization, which increases its protective ability. In many insects the outermost layer of epicuticle is composed of waxes that reduce water loss. Figure 19.3 Structure of crustacean cuticle. The procuticle is divided into an exocuticle, which is secreted before a molt, and an endocuticle, which is secreted after molting. Both of these layers are composed of chitin bound with protein. Chitin is a tough, nitrogenous polysaccharide that is insoluble in water, alkalis, and weak acids. The procuticle not only is flexible and lightweight, but also affords protection against dehydration and other biological and physical stresses. In insects chitin forms about 50% of the procuticle, with the remainder being protein. In some crustaceans, chitin is 60% to 80% of the procuticle; additionally, most crustaceans possess some areas of the procuticle impregnated with calcium salts. The addition of calcium salts reduces flexibility, but increases strength. In the hard shells of lobsters and crabs, for example, this calcification is extreme. The cuticle may be soft and permeable or may form a veritable coat of armor. Between body segments and between the segments of appendages the cuticle is thin and flexible, creating movable joints and permitting free movements. In crustaceans and insects the cuticle forms  ingrowths (apodemes) to which muscles attach. Cuticle may also line foregut and hindgut, line and support the tracheae, and be adapted for biting mouthparts, sensory organs, copulatory organs, and ornamental purposes. It is indeed a versatile material. Page 413 The nonexpansible cuticular exoskeleton does, however, impose important restrictions on growth. To become larger, an arthropod must shed its outer covering at intervals and replace it with a larger one—a process called molting. The process of molting terminates in the actual shedding of the skin, or ecdysis. Arthropods may molt many times before reaching adulthood, and some continue to molt after that. More details of the molting process are given for crustaceans (see Section 20.1: Molting and Ecdysis) and for insects (see Section 21.4). 2. Segmentation and appendages provide for more efficient locomotion. The ancestral arthropod body plan was likely a linear series of similar segments, each with a pair of jointed appendages. However, extant groups exhibit a wide variety of segments and appendages. There has been a tendency for segments to combine or to fuse into functional groups, called tagmata (sing., tagma), which have specialized purposes. Spider bodies, for example, have two tagmata. Appendages are frequently differentiated and specialized for pronounced division of labor. Limb segments are essentially hollow levers moved by internal muscles, most of which are striated, providing rapid action. The appendages have sensory hairs (as well as bristles and spines) and may be modified and adapted for sensory functions, food handling, swift and efficient walking, and swimming. 3. Air piped directly to cells. Most terrestrial arthropods have a highly efficient tracheal system of air tubes, which delivers oxygen directly to the tissues and cells and makes a high metabolic rate possible during periods of intense activity. This system also tends to limit body size. Aquatic arthropods breathe mainly by some form of internal or external gill system. 4. Highly developed sensory organs. Sensory organs occur in great variety, from the compound (mosaic) eye to organs of touch, smell, hearing, balancing, and chemical reception. Arthropods are keenly alert to what happens in their environment. 5. Complex behavior patterns. Arthropods exceed most other invertebrates in complexity and organization of their activities. Innate  (unlearned) behavior unquestionably controls much of what they do, but learning also plays an important part in the lives of many species. 6. Use of diverse resources through metamorphosis. Many arthropods pass through metamorphic changes, including a larval form quite different from the adult in structure. Larval forms often are adapted for eating food different from that of adults and occupy a different space. The use of diverse resources seems more noteworthy because intraspecific competition will occur when population numbers are large; regardless of the resources used, one resource will become limiting. 19.2 SUBPHYLUM TRILOBITA Trilobites probably arose before the Cambrian period. They have been extinct for 245 million years, but were abundant during the Cambrian and Ordovician periods. Despite their antiquity, they were highly specialized arthropods. Their name refers to the trilobed shape of the body in cross section, caused by a pair of longitudinal grooves. They were dorsoventrally flattened bottom dwellers and probably scavengers (see Figure 19.4A). Most of them could roll up like pill bugs (isopods), and they ranged from 2 to 67 cm in length. Page 414  Figure 19.4 Fossils of early arthropods. A, Trilobite fossils, dorsal view. These animals were abundant in the mid-Cambrian period. B, Eurypterid fossil from the Silurian period, Eurypteris remipes. Eurypterids flourished in Europe and North America from the Ordovician to the Permian periods. Their exoskeleton contained chitin, strengthened in some areas by calcium carbonate. There were three tagmata in the body: cephalon (head), trunk, and pygidium. The cephalon was one piece but showed signs of ancestral segmentation; the trunk had a variable number of segments; and segments of the pygidium, at the posterior end, were fused into a plate. The cephalon bore a pair of antennae, compound eyes, a mouth, and four pairs of leglike appendages. There were no true mouthparts; instead, food could be crushed with the basal portions of the legs and passed forward to the mouth. Trilobites had a convex covering, called a hypostome, over the base of the legs nearest the mouth, the mouth, and base of the antennae. The form of the hypostome varied, with some types thought to characterize predators and others characterizing scavengers and particle-feeders. Each body segment except the last also bore a pair of biramous appendages. One of the branches had a fringe of filaments that may have served as gills. Characteristics of Phylum Arthropoda 1. Jointed appendages; ancestrally, one pair to each segment, but number often reduced; appendages often modified for specialized functions 2. Living in marine, freshwater, and terrestrial habitats; many capable of flight 3. Free-living and parasitic taxa 4. Bilateral symmetry; segmented body divided into functional groups called tagmata: head and trunk; head, thorax, and abdomen or cephalothorax and abdomen; definite head 5. Triploblastic body 6. Reduced coelom in adult; most of body cavity consisting of hemocoel (sinuses, or spaces, in the tissues) filled with blood 7. Cuticular exoskeleton; containing protein, lipid, chitin, and often calcium carbonate secreted by underlying epidermis and shed (molted) at intervals; chitin occurs less pervasively in some other groups 8. Complete digestive system; mouthparts modified from ancestral appendages and adapted for different methods of feeding; alimentary canal shows great specialization by having, in various arthropods, chitinous teeth, compartments, and gastric ossicles 9. Complex muscular system, with exoskeleton for attachment, striated muscles for rapid actions, smooth muscles for visceral organs; no cilia 10. Nervous system similar to that of annelids, with dorsal brain connected by a ring around the gullet to a double nerve chain of ventral ganglia; fusion of ganglia in some species 11. Well-developed sensory organs; behavioral patterns much more complex than those of most invertebrates, with wider occurrence of social organization 12. Parthenogenesis in some taxa 13. Sexes usually separate, with paired reproductive organs and ducts; usually internal fertilization; oviparous, ovoviviparous, or viviparous; often with metamorphosis  14. Paired excretory glands called coxal, antennal, or maxillary glands present in some; others with excretory organs called Malpighian tubules 15. Respiration by body surface, gills, tracheae (air tubes), or book lungs 16. Open circulatory system, with dorsal contractile heart, arteries, and hemocoel (blood sinuses) Key Theme 19.1 EVOLUTION Speciation and Extinction in Trilobites Trilobite fossils are identified to species by their morphology. Calmoniid trilobites are an extremely diverse group. Members of the calmoniid genus Metacryphaeus were abundant in the Devonian and so well preserved in Bolivian strata that the evolution and extinction of taxa can be followed. An excellent fossil record and good information on biogeography and relative sea levels permits the study of evolutionary diversification in relation to abiotic factors. It appears that calmoniid speciation occurred after populations were isolated by falling sea level. Later, rising sea level permitted dispersal of new species. Extinction rates were low for much of the time, but the entire genus disappeared when extinction outpaced speciation. Speciation slowed when sea level was high, presumably because geographic isolation of populations did not occur. 19.3 SUBPHYLUM CHELICERATA Chelicerate arthropods are known from the Ordovician period over 445 million years ago and include eurypterids (extinct), horseshoe crabs, spiders, ticks and mites, scorpions, sea spiders, and other less well-known groups such as sun scorpions and whip scorpions. Their bodies are composed of two tagmata: a cephalothorax, or prosoma, and an abdomen, or opisthosoma. They are characterized by six pairs of cephalothoracic appendages that include a pair of chelicerae (mouthparts), a pair of pedipalps, and four pairs of walking legs. They have no antennae. Most chelicerates suck liquid food from their prey. There are three chelicerate classes (see Figure 19.5). Page 415  Figure 19.5 Cladogram of chelicerates, showing one proposed relationship within the chelicerate clade. Source: Modified from R. C. Brusca and G. J. Brusca, Invertebrates, Sinauer Associates, Inc., Sunderland, MA, 1990. Class Merostomata Class Merostomata contains eurypterids, all now extinct, and xiphosurids, or horseshoe crabs, which are sometimes called “living fossils” because extant forms resemble Ordovician fossils. Subclass Eurypterida The eurypterids, or giant water scorpions (see Figure 19.4B), were the largest of all fossil arthropods, some reaching a length of 3 m. Their fossils occur in rocks from the Ordovician to the Permian periods. They had many features resembling those of marine horseshoe crabs (see Figure 19.6) as well as those of scorpions. Their heads had six fused segments and bore both simple and compound eyes as well as chelicerae and pedipalps. They also had four pairs of walking legs, and their abdomen had 12 segments and a spikelike telson.  Figure 19.6 A, Dorsal view of horseshoe crab Limulus (class Merostomata). They grow to 0.5 m in length. B, Ventral view of female. Eurypterids were the dominant predators of their time and some had anterior appendages modified into large, crushing claws. It is possible that development of dermal armor in early fishes (see Section 23.5: The Earliest Vertebrates) resulted from selection pressure of eurypterid predation. Subclass Xiphosurida: Horseshoe Crabs Xiphosurids are an ancient marine group that dates from the Ordovician period. Our common horseshoe crab Limulus (L. limus, sidelong, askew) (see Figure 19.6) goes back practically unchanged to the Triassic period. Only three genera (four species) survive today: Limulus, which lives in shallow water along the North American Atlantic coast, including the Gulf coast down through Texas and Mexico; Carcinoscorpius (Gr. karkinos, crab, + skorpiōn, scorpion), along the southern shore of Japan; and Tachypleus (Gr. tachys, swift, + pleutēs, sailor), in the East Indies and along the coast of southern Asia. They usually live in shallow water. Xiphosurids have an unsegmented, horseshoe-shaped carapace (hard dorsal shield) and a broad abdomen, which has a long telson, or tailpiece. Their cephalothorax bears a pair of chelicerae, one pair of pedipalps, and four pairs of walking legs, whereas their abdomen has six pairs of broad, thin appendages that are fused in the median line (see Figure 19.6). On five abdominal appendages, book gills (flat, leaflike gills) occur under the gill opercula. There are two lateral, rudimentary eyes and two simple eyes on the carapace. A horseshoe crab swims by means of its abdominal plates and can walk with its walking legs. It feeds at night on worms and small molluscs, which it seizes with its chelicerae and walking legs.  Page 416 During the mating season horseshoe crabs come to shore by the thousands at high tide to mate. A female burrows into sand where she lays eggs, with one or more smaller males following closely to add sperm to the nest before the female covers it with sand. American Limulus mate and lay eggs during high tides of full and new moons in spring and summer. Eggs are warmed by the sun and protected from waves until young larvae hatch and return to the sea, carried by another high tide. Larvae are segmented and are often called “trilobite larvae” because they resemble trilobites. Class Pycnogonida: Sea Spiders About 1000 species of sea spiders occupy marine habitats ranging from shallow, coastal waters to deep-ocean basins. Some sea spiders are only a few millimeters long, but others are much larger, with legspans up to nearly 0.75 m. They have small, thin bodies and usually four pairs of long, thin walking legs. In addition, they have a feature unique among arthropods: segments are duplicated in some groups, so that they possess five or six pairs of legs instead of the four pairs normally characteristic of chelicerates. Males of many species bear a subsidiary pair of legs (ovigers) (see Figure 19.7) on which they carry developing eggs, and ovigers are often absent in females. Many species also are equipped with chelicerae and palps. Chelicerae are sometimes called chelifores in this group.  Figure 19.7 A, Pycnogonid, Nymphon sp. In this genus all anterior appendages (chelicerae, palps, and ovigers) are present in both sexes, although ovigers are often not present in females of other genera. B, Pycnogonum hancockii, a pycnogonid with relatively short legs. Females of this genus have neither chelicerae nor ovigers and males have ovigers. The small head (cephalon) has a raised projection with two pairs of simple eyes positioned to provide nearly 360 degree vision. The mouth is at the tip of a long proboscis, which sucks juices from cnidarians and soft-bodied animals. Their circulatory system is limited to a simple dorsal heart, and excretory and respiratory systems are absent. The long, thin body and legs provide a large surface area, in proportion to body volume, that is evidently sufficient for diffusion of gases and wastes. Because of the small size of the body, the digestive system and gonads have branches that extend into the legs. Sea spiders occur in all oceans, but they are most abundant in polar waters. Pycnogonum (see Figure 19.7B) is a common intertidal genus on both Atlantic and Pacific coasts of the United States; it has relatively short, heavy legs. Nymphon (see Figure 19.7A) is the largest genus of pycnogonids, with over 200 species. It occurs from subtidal depths to 6800 m in all oceans except the Black and Baltic seas. Some research suggests that pycnogonids belonged to an early diverging arthropod lineage outside any of the subphyla, but morphological and molecular evidence strongly supports the placement of pycnogonids in the Chelicerata (see Section 19.5). Class Arachnida Arachnids (Gr. arachnē, spider) exhibit enormous anatomical variation. In addition to spiders, the group includes scorpions, pseudoscorpions, whip scorpions, ticks, mites, daddy longlegs (harvestmen), and others. There are many differences among these taxa with respect to form and appendages. They are mostly free-living and are most common in warm, dry regions. Arachnids have become extremely diverse: more than 80,000 species have been described to date. They were among the first arthropods to move into terrestrial habitats. For example, scorpions are among Silurian fossils, and by the end of the Paleozoic era, mites and spiders had appeared. All arachnids have two tagmata: a cephalothorax (head and thorax) and an abdomen, which may or may not be segmented. The abdomen houses the reproductive organs and respiratory organs such as tracheae and book lungs. The cephalothorax usually bears a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs (see Figure 19.8). Most arachnids are predaceous and have fangs, claws, venom glands, or stingers; fangs are modified chelicerae, whereas claws (chelae) are modified pedipalps. They usually have a strong sucking pharynx with which they ingest the fluids and soft tissues from the bodies of their prey. Among their interesting adaptations are spinning glands of spiders.  Figure 19.8 External anatomy of a jumping spider, with anterior view of head (at right). Page 417 Most arachnids are harmless to humans and actually do much good by destroying injurious insects. Arachnids typically feed by releasing digestive enzymes over or into their prey and then sucking the predigested liquid. A few, such as black widow and brown recluse spiders, can give dangerous bites. Stings of scorpions may be quite painful, and those of a few species can be fatal. Some ticks and mites are carriers of serious diseases as well as causes of annoyance and painful irritations. Certain mites damage a number of important food and ornamental plants by sucking their juices. Several smaller orders are not included in our discussion. Order Araneae: Spiders Spiders are a large group of arachnids comprising about 40,000 species distributed throughout the world and known from the Silurian period approximately 420 million years ago. The spider body is compact: a cephalothorax (prosoma) and abdomen (opisthosoma), both unsegmented and joined by a slender pedicel. A few spiders have a segmented abdomen, which is considered an ancestral character. Anterior appendages include a pair of chelicerae (see Figure 19.8), which have terminal fangs through which run ducts from venom glands, and a pair of leglike pedipalps, which have sensory function and are also used by males to transfer sperm. The basal parts of pedipalps may be used to manipulate food (see Figure 19.8). Four pairs of walking legs terminate in claws. All spiders are predaceous, feeding largely on insects, which they effectively dispatch with venom from their fangs. Some spiders chase prey; others ambush them; and many trap them in a net of silk. After a spider seizes prey with its chelicerae and injects venom, it liquefies the prey’s tissues with digestive fluid and sucks the resulting broth into its stomach. Spiders with teeth at the bases of chelicerae crush or chew prey, aiding digestion by enzymes from their mouth. Spiders respire using book lungs or a system of internal tubes called tracheae. Some spiders have tracheae and book lungs. Book lungs open to the outside through a slit in the exoskeleton. Internal to the slit are many thin- walled chambers filled with blood (see Figure 19.9). Between a pair of chambers is a cavity where air enters the body. Oxygen moves across the thin chamber walls to enter the blood, whereas carbon dioxide leaves the blood. A stack of thin-walled chambers resembles the pages of a book; hence the name. Tracheae begin at an opening in the exoskeleton called a spiracle. From the spiracle, tubes extend inward, branching into progressively smaller tubules. The smallest tubules interface with the blood, allowing oxygen to move from the tracheae into the blood, and carbon dioxide to move from the blood into the tracheae. The tracheae are similar to those in insects (see Section 21.3: Gas Exchange) but are much less extensive and have evolved independently in both spiders and insects. Figure 19.9 Spider, internal anatomy. Spiders and insects have also independently evolved a unique excretory system of Malpighian tubules (see Figure 19.9), which work in conjunction with specialized resorptive cells in the intestinal epithelium. Potassium and other solutes and waste materials are secreted into the tubules, which drain the fluid, or “urine,” into the intestine (see Figure 30.8). Resorptive cells recapture most potassium and water, leaving behind such wastes as uric acid. This recycling of water and potassium allows species living in dry environments to conserve body fluids by producing a nearly dry mixture of urine and feces. Many spiders also have coxal glands, which are modified nephridia that open at the coxa, or base, of the first and third walking legs. Spiders usually have eight simple eyes, each with a lens, optic rods, and a retina (see Figure 19.8). They are used chiefly to perceive moving objects, but some, such as those of hunting and jumping spiders, may form images. Since a spider’s vision is often poor, its awareness of its environment depends largely on cuticular mechanoreceptors, such as sensory setae (sensilla). Fine setae covering the legs can detect vibrations in the web, struggling prey, or even air movements. Web-Spinning Habits An ability to spin silk is central to a spider’s life, as it is in some other arachnids such as tetranychid spider mites. Two or three pairs of spinnerets containing hundreds of microscopic tubes run to special abdominal silk glands (see Figure 19.9). A scleroprotein secretion emitted as a liquid from the spinnerets hardens to form a silk thread. Threads of spider silk are stronger than steel threads of the same diameter and second in strength only to fused quartz fibers. Recent work has shown that this strength is due to the nanostructure of the thread. The thread of a brown recluse spider is actually a ribbon-like cable composed of about 2500 parallel nanofibrils. Page 418 Many species of spiders spin silk webs. The kind of web varies among species. Some webs are simple and consist merely of a few strands of silk radiating out from a spider’s burrow or place of retreat. Others spin beautiful, geometrical orb webs. However, spiders use silk threads for many additional purposes: nest lining, sperm webs or egg sacs, bridge lines, draglines, warning threads, molting threads, attachment discs, nursery webs, and the wrapping of prey items (see Figure 19.10). Not all spiders spin webs for traps. Some spiders throw a sticky bolus of silk to capture their prey or throw a net to entangle prey. Others, such as wolf spiders, jumping spiders (see Figure 19.8), and fisher spiders (see Figure 19.11), simply chase and catch their prey. These spiders likely lost the ability to produce silk for prey capture. Figure 19.10 Grasshopper, snared and helpless in the web of a golden garden spider (Argiope aurantia), is wrapped in silk while still alive. If the spider is not hungry, the prize is saved for a later meal.  Figure 19.11 An Okefenokee fishing spider, Dolomedes okefenokensis, has captured a fish. This handsome spider pulls its paralyzed victim from the water, pumps in digestive enzymes, then sucks out the predigested contents. Reproduction A courtship ritual precedes mating. Before mating, a male spins a small web, deposits a drop of sperm on it, and then picks up the sperm to be stored in special cavities of his pedipalps. When he mates, he inserts his pedipalps into the female genital opening to place the sperm in his mate’s seminal receptacles. A female lays her eggs in a silken net, which she may carry or attach to a web or plant. This cocoon may contain hundreds of eggs, which hatch in approximately two weeks. Young usually remain in the egg sac for a few weeks and molt once before leaving it. The number of molts may vary, but typically ranges between 4 and 12 before adulthood is reached. Are Spiders Really Dangerous? It is amazing that such small and innocuous creatures have generated so much unreasonable fear in human minds. Spiders are timid creatures that, rather than being dangerous enemies to humans, are actually allies in the continuing battle with insects and other arthropod pests. Venom produced to kill prey is usually harmless to humans. The most venomous spiders bite only when threatened or when defending their eggs or young. Even American tarantulas (see Figure 19.12), despite their fearsome size, are not dangerous. They rarely bite, and their bite is about as serious as a bee sting.  Figure 19.12 A tarantula in the family Theraphosidae. There are, however, two genera in the United States that can give severe or even fatal bites: Latrodectus (L. latro, robber, + dectes, biter; black widow, five species) and Loxosceles (Gr. loxos, crooked, + skelos, leg; brown recluse, 13 species). Black widows are moderate to small in size and shiny black, usually with a bright orange or red spot, commonly in the shape of an hourglass, on the underside of their abdomen (see Figure 19.13A). Their venom is neurotoxic, acting on the nervous system. About four or five of every 1000 reported bites are fatal. Page 419 Figure 19.13 A, Black widow spider, Latrodectus mactans. Note the red “hourglass” on the ventral side of her abdomen. B, Brown recluse, Loxosceles reclusa, is a small spider whose venom is hemolytic and dangerous. There is a small violin-shaped marking on its cephalothorax. C, A camouflaged crab spider, Misumenoides formosipes, awaits its insect prey. Its coloration permits it to blend in among the flowers as it awaits an insect in search of pollen or nectar. D, A female regal jumping spider, Phidippus regius. This species has excellent vision and stalks an insect until it is close enough to leap with unerring precision, fixing its chelicerae into its prey. Brown recluse spiders are brown and bear a violin-shaped dorsal stripe on their cephalothorax (see Figure 19.13B). Their venom is hemolytic rather than neurotoxic, producing death of tissues and skin surrounding the bite. Their bite can be mild to serious and there are a few unconfirmed reports of deaths of small children and older individuals. Some spiders in other parts of the world are also dangerous, for example, funnelweb spiders Atrax spp. in Australia. Most dangerous of all are spiders in the South and Central American genus Phoneutria. They are large (10 to 12 cm leg span) and quite aggressive. Their venom is among the most pharmacologically toxic of spider venoms, and their bites cause intense pain, neurotoxic effects, sweating, acute allergic reaction, and nonsexual enlargement of the penis. However, most spiders simply use their venom in prey capture (Figures 19.13 C and D). Key Theme 19.2 ADAPTATION AND PHYSIOLOGY Spider Diversity and Abundance W. S. Bristowe (The World of Spiders. 1971. Rev. ed. London, Collins) estimated that at certain seasons a field in Sussex, England (that had been undisturbed for several years) had a population of 2 million spiders to the acre. He concluded that so many spiders could not successfully compete except for the many specialized adaptations they had evolved. These include adaptations to cold and heat, wet and dry conditions, and light and darkness. Some spiders capture large insects, some only small ones; web-builders snare mostly flying insects, whereas hunters seek those that live on the ground. Some lay eggs in the spring, others in the late summer. Some feed by day, others by night, and some have developed flavors that are distasteful to birds or to certain predatory insects. As it is with spiders, so has it been with other arthropods; their adaptations are many and diverse and contribute in no small way to their long success. Order Scorpiones: Scorpions Scorpions comprise about 1400 species worldwide and, like spiders, are known from the Silurian period. Although scorpions are more common in tropical and subtropical regions, some occur in temperate zones. Scorpions are generally secretive, hiding in burrows or under objects by day and feeding at night. They feed largely on insects and spiders, which they seize with their pedipalps and shred with their chelicerae. Sand-dwelling scorpions locate prey by sensing surface waves generated by the movements of insects on or in the sand. These waves are detected by compound slit sensilla located on the last segment of the legs. A scorpion can locate a burrowing cockroach 50 cm away and reach it in three or four quick movements. Scorpion tagmata are a rather short cephalothorax, which bears chelicerae, pedipalps, legs, a pair of large median eyes, and usually two to five pairs of small lateral eyes; a preabdomen (or mesosoma) of seven segments; and a long slender postabdomen (or metasoma) of five segments, which ends in a stinging apparatus (see Figure 19.14A). Their chelicerae are small, their pedipalps are large and chelate (pincerlike), and the four pairs of walking legs are long and eight-jointed. Figure 19.14 A, An emperor scorpion (order Scorpiones), Paninus imperator, with young, which stay with the mother until their first molt. B, A camel spider. C, Harvestmen, Mitopus sp. (order Opiliones). Harvestmen run rapidly on their stiltlike legs. They are especially noticeable during the harvesting season; hence the common name. Page 420 On the ventral side of the abdomen are curious comblike pectines, which serve as tactile organs for exploring the ground and for sex recognition. The stinger on the last segment consists of a bulbous base and a curved barb that injects venom. Venom of most species is not harmful to humans but may produce a painful swelling. However, the sting of certain species of Androctonus in Africa and Centruroides (Gr. kenteō, to prick, + oura, tail, + oides, form) in Mexico can be fatal unless antivenom is administered. In general, larger species tend to be less venomous than smaller species and rely on their greater strength to overpower prey. Scorpions perform a complex mating dance, the male holding the female’s chelae as he steps back and forth. He kneads her chelicerae with his own and, in some species, stings her on her pedipalp or on the edge of her cephalothorax. The stinging action is slow and deliberate, and the stinger remains in the female’s body for several minutes. Both individuals remain motionless during that time. Finally, the male deposits a spermatophore and pulls the female over it until the sperm mass is taken into the female orifice. Scorpions are truly viviparous; females brood their young within their reproductive tract. After several months to a year of development anywhere from 1 to over 100 young are produced, depending on the species. The young, only a few millimeters long, crawl onto their mother’s back until after their first molt (see Figure 19.14A). They mature in 1 to 8 years and may live for as long as 15 years. Order Solpugida: Sun or Camel Spiders Solpugids, also called solfugids, and by such common names as sun, camel, or wind spiders, are nonvenomous arachnids that shred prey with their large chelicerae (see Figure 19.14B). They range in size from 1 cm to nearly 15 cm. They are common in the tropical and subtropical deserts in America, the Middle East, Asia, and Africa. Order Opiliones: Harvestmen Harvestmen (see Figure 19.14C), often called “daddy longlegs,” are common throughout the world and comprise about 5000 species. They are easily distinguished from spiders: their abdomen and cephalothorax are rounded and broadly joined, without the constriction of a pedicel, their abdomen shows external segmentation; and they have only two eyes, mounted on a tubercle on their cephalothorax. They have four pairs of long, spindly legs that end in tiny claws. They can cast off one or more of these legs without apparent ill effect if they are grasped by a predator (or human hand). The ends of their chelicerae are pincerlike, and, while carnivorous, they are often scavengers as well. Harvestmen are not venomous and are harmless to humans. Odoriferous glands that open on the cephalothorax deter some predators with their noxious secretions. Other than some mites, opilionids are unique among arachnids in having a penis for direct sperm transfer; all are oviparous. Traditionally allied with Acari, some studies indicate that Opiliones forms a clade with scorpions and two smaller orders. They are the sister group of scorpions. Order Acari: Ticks and Mites Members of order Acari are without doubt the most medically and economically important group of arachnids. They far exceed other orders in numbers of individuals and species. Although about 40,000 species have been described, some authorities estimate that 500,000 to 1 million species exist. Hundreds of individuals of several species of mites live in a few square meters of leaf mold in forests. They occur throughout the world in both terrestrial and aquatic habitats, even extending into such inhospitable regions as deserts, polar areas, and hot springs. Many acarines are parasitic during one or more stages of their life cycle. Most mites are 1 mm or less in length. Ticks, which are only one suborder of Acari, range from a few millimeters to occasionally 3 cm. A tick may become enormously distended with blood after feeding on its host. Acarines differ from all other arachnids in having complete fusion of the cephalothorax and abdomen, with no sign of external division or segmentation (see Figure 19.15). They carry their mouthparts on a little anterior projection, the capitulum, which consists mainly of the feeding appendages surrounding the mouth. On each side of their mouth is a chelicera, which functions in piercing, tearing, or gripping food. The form of the chelicerae varies greatly in different families. Lateral to the chelicerae is a pair of segmented pedipalps, which also vary greatly in form and function related to feeding. Ventrally the bases of the pedipalps fuse to form a hypostome, whereas a rostrum, or tectum, extends dorsally over their mouth. Adult mites and ticks usually have four pairs of legs, although there may be only one to three in some specialized forms.  Figure 19.15 A, The Western blacklegged tick, Ixodes pacificus, (order Acari) is a vector for the bacterium Borrelia burgdorferi which causes Lyme disease. B, Velvet mite, Dinothrombium sp. from South Africa. Adults are free-living predators, while certain immature instars are often ectoparasites. Page 421 Most acarines transfer sperm directly, but many species use a spermatophore. A larva with six legs hatches from the egg, and one or more eight-legged nymphal stages follow before the adult stage is reached. Many species of mites are entirely free-living. Dermatophagoides farinae (Gr. dermatos, skin, + phag, to eat, + eidos, likeness of form) (see Figure 19.16) and related species are denizens of house dust all over the world, sometimes causing allergies and dermatoses. There are some marine mites, but most aquatic species live in freshwater. They have long, hairlike setae on their legs for swimming, and their larvae may be parasitic on aquatic invertebrates. Figure 19.16 Color enhanced scanning electron micrograph of a house dust mite. Such abundant organisms must be important ecologically, but many acarines have more direct effects on our food supply and health. Spider mites (family Tetranychidae) are serious agricultural pests on fruit trees, cotton, clover, and many other plants. They suck the contents of plant cells, producing a mottled appearance on the leaves (see Figure 19.17), and construct a protective web from silk glands opening near the base of the chelicerae. Larvae of genus Trombicula are called chiggers or redbugs. They feed on the dermal tissues of terrestrial vertebrates, including humans, and may cause an irritating dermatitis, but they do not burrow or remain attached to the host. Some species of chiggers transmit a disease called Asiatic scrub typhus. Hair follicle mites, Demodex (see Figure 19.18), are apparently nonpathogenic in humans; they infect most of us although we are unaware of them. In some cases they may produce a mild dermatitis. Other species of Demodex and other genera of mites cause mange in domestic animals. Human itch mites, Sarcoptes scabiei (see Figure 19.19), cause intense itching as they burrow beneath the skin. The Varroa mite was implicated in the dramatic loss of honey bees that began in the United States in 2006 (see the discussion of colony collapse disorder, Section 21.6). The mite carries “deformed wing virus” or DWV. Global reemergence of this virus is linked to viral evolution, new hosts for the mite, and human movement of infected bees. Figure 19.17 Damage to Chamaedorea sp. Palm caused by mites of the family etranychidae (order Acari). Over 130 species of this family occur in North America, and some are serious agricultural pests. Mites pierce plant cells and suck out contents, giving leaves the mottled appearance shown here. Figure 19.18 Colored scanning electron micrograph of Demodex folliculorum, a human follicle mite.  Figure 19.19 Sarcoptes scabiei, a human itch mite. Key Theme 19.3 HUMAN CONNECTIONS Why Chigger Bites Itch The inflamed welt and intense itching that follows a chigger bite is not the result of the chigger burrowing into the skin, as is popularly believed. Rather, a chigger bites through the skin with its chelicerae and injects a salivary secretion containing powerful enzymes that liquefy skin cells. Human skin responds defensively by forming a hardened tube that the larva uses as a drinking straw and through which it gorges itself with host cells and fluid. Scratching usually removes the chigger but leaves the tube, which is a source of irritation for several days. In addition to disease conditions that they themselves cause, ticks are among the world’s premier disease vectors, ranking second only to mosquitos. They surpass other arthropods in carrying a great variety of infectious agents, including apicomplexans, rickettsial, viral, bacterial, and fungal organisms. Species of Ixodes carry the most common arthropod-borne infection in the United States, Lyme disease (see Key Theme 19.4). Species of Dermacentor and other ticks transmit Rocky Mountain spotted fever, a poorly named disease because most cases occur in the eastern United States. Dermacentor also transmits tularemia and agents of several other diseases. Texas cattle fever, also called red-water fever, is caused by an apicomplexan parasite transmitted by cattle ticks, Rhipicephalus annulatus. The brown dog tick is a disease-vector for domestic animals and humans (see Figure 19.20). Many more examples could be cited. Page 422  Figure 19.20 Rhipicephalus sanguine, the brown dog tick, occurs throughout the world. Key Theme 19.4 HUMAN CONNECTIONS Lyme Disease An epidemic of arthritis occurred in the 1970s in the town of Lyme, Connecticut. Subsequently called Lyme disease, it is caused by a bacterium and carried by ticks of the genus Ixodes. There are now thousands of cases a year in Europe and North America, and other cases have been reported from Japan, Australia, and South Africa. Over 200,000 cases were confirmed in the United States from 2008 to 2015, according to the Centers for Disease Control and Prevention. Many people bitten by infected ticks recover spontaneously or do not get the disease. Others, if not treated at an early stage with appropriate antibiotics, develop a chronic, disabling disease. Control measures for ticks or Lyme disease have not been effective. 19.4 SUBPHYLUM MYRIAPODA The term “myriapod,” meaning “many footed,” describes members of four classes in subphylum Myriapoda that have evolved a pattern of two tagmata— head and trunk—with paired appendages on most or all trunk segments. Myriapods include Chilopoda (centipedes), Diplopoda, (millipedes), Pauropoda (pauropods), and Symphyla (symphylans) (see Figure 19.21). Page 423  Figure 19.21 Cladogram showing hypothetical relationships of myriapods. Organs of Tömösvary are unique sensory organs opening at the bases of the antennae, and repugnatorial glands, located on certain segments or legs, secrete an obnoxious substance for defense. The gnathochilarium is formed in diplopods and pauropods by fusion of the first maxillae, and the collum is the collarlike tergite of the first trunk segment. Myriapods use tracheae to carry respiratory gases directly to and from all body cells in a manner similar to that of onychophorans (see Section 18.7) and some arachnids, but tracheal systems have likely evolved independently in each group. Excretion is usually by Malpighian tubules, but these have evolved independently of Malpighian tubules found in Chelicerata. Class Chilopoda Chilopoda (ki-lop′o-da) (Gr. cheilos, margin, lip, + pous, podos, foot), or centipedes, are land forms with somewhat flattened bodies. Centipedes prefer moist places such as under logs, bark, and stones. They are very agile carnivores, living on cockroaches and other insects, and on earthworms. They kill their prey with their venom claws and then chew it with their mandibles. The largest centipede in the world, Scolopendra gigantea, is nearly 30 cm in length. Common house centipedes Scutigera (L. scutum, shield, + gera, bearing), which have 15 pairs of legs, are much smaller and often seen scurrying around bathrooms and damp cellars, where they catch insects. Most species of centipedes are harmless to humans, although many tropical centipedes are dangerous. There are about 3000 species worldwide. Centipede bodies may contain from a few to 177 segments (see Figure 19.22). Each segment, except the one behind the head and the last two in the body, bears a pair of jointed legs. Appendages of the first body segment are modified to form venom claws. The last pair of legs is longer than the others and serves a sensory function. Figure 19.22 A, The centipede Scolopendra heros sits atop a lichen-covered rock. It occurs in the central United States from Texas to Colorado and may be more than 15 cm (6 inches) long. Like all centipedes, it is a carnivore and uses the paired venom claws on the first segment to kill prey. B, Head of centipede. The head appendages are similar to those of an insect (see Figure 19.22B). There are a pair of antennae, a pair of mandibles, and one or two pairs of maxillae. A pair of eyes on the dorsal side of the head consists of groups of ocelli. The digestive system is a straight tube into which salivary glands empty at the anterior end. Two pairs of Malpighian tubules empty into the hind part of the intestine. There is an elongated heart with a pair of arteries to each segment. The heart has a series of ostia to provide for return of blood to the heart from the hemocoel. Respiration is by means of a tracheal system of branched air tubes that come from a pair of spiracles in each segment. The nervous system is typically arthropodan, and there is also a visceral nervous system. Sexes are separate, with unpaired gonads and paired ducts. Some centipedes lay eggs and others are viviparous. The young are similar in form to adults and do not undergo metamorphosis. Class Diplopoda Diplopoda (Gr. diploo, double, two, + pous, podos, foot) are commonly called millipedes, which literally means “thousand feet” (see Figure 19.23A). Millipedes are not as active as centipedes: they walk with a slow, graceful motion, not wriggling as centipedes do. They prefer dark, moist places under logs or stones. Most millipedes are herbivorous, feeding on decayed plant matter, although sometimes they eat living plants. Millipedes are slow- moving animals and may roll into a coil when disturbed. Many millipedes also protect themselves from predation by secreting toxic or repellent fluids, sometimes containing hydrogen cyanide, from special glands (repugnatorial glands) positioned along the sides of the body. Common examples of this class are Spirobolus and Julus, both of which have wide distribution. There are more than 10,000 species of millipedes worldwide. Figure 19.23 A, A pair of mating millipedes from Thailand. B, Head of millipede. The cylindrical body of a millipede is formed by 25 to more than 100 segments. Their short thorax consists of four segments, each bearing one pair of legs. Each abdominal segment has two pairs of legs, leading to the impression of a thousand feet. The millipede exoskeleton is reinforced with calcium carbonate. Page 424 Their head bears two clumps of simple eyes and a pair each of antennae, mandibles, and maxillae (see Figure 19.23B). The general body structures are similar to those of centipedes. Two pairs of spiracles on each abdominal segment open into air chambers that connect to tracheal air tubes. There are two genital apertures toward the anterior end. In most millipedes the appendages of the seventh segment are specialized as copulatory organs. After millipedes copulate, females lay eggs in a nest and guard them carefully. Interestingly, larval forms have only one pair of legs to each segment. Class Pauropoda Pauropoda (Gr. pauros, small, + pous, podos, foot) are a group of minute (2 mm or less), soft-bodied myriapods, numbering almost 500 species. Although widely distributed, pauropods are the least well-known myriapods. They live in moist soil, leaf litter, or decaying vegetation and under bark and debris. Pauropods have a small head with branched antennae and no true eyes (see Figure 19.24A). Their 12 trunk segments usually bear nine pairs of legs (none on the first or the last two segments). They have only one tergal (dorsal) plate covering every two segments. Figure 19.24 A, Pauropods are minute, whitish myriapods with three-branched antennae and nine pairs of legs. They live in leaf litter and under stones. They are eyeless but have sense organs that resemble eyes. B, Scutigerella, a symphylan, is a minute whitish myriapod that is sometimes an agricultural and greenhouse pest. Tracheae, spiracles, and circulatory system are lacking. Pauropods are probably most closely related to diplopods. Class Symphyla Symphyla (Gr. sym, together, + phylon, tribe) are small (2 to 10 mm) and have centipede-like bodies (see Figure 19.24B). They live in humus, leaf mold, and debris. Scutigerella (L. dim. of Scutigera) are often pests on vegetables and flowers, particularly in greenhouses. They are soft bodied, with 14 segments, 12 of which bear legs and one a pair of spinnerets. The antennae are long and unbranched. Symphylans are eyeless but have sensory pits at the bases of the antennae. Their tracheal system is limited to a pair of spiracles on their head and tracheal tubes to anterior segments only. Only 160 species are described. Key Theme 19.5 ADAPTATION AND PHYSIOLOGY Symphylan Mating The mating behavior of Scutigerella (see Figure 19.24B) is unusual. Males place a spermatophore at the end of a stalk. When a female finds it, she takes it into her mouth, storing sperm in special buccal pouches. Then she removes eggs from her gonopore with her mouth and attaches them to moss or lichen, or to walls of crevices, smearing them during handling with some of the semen, thereby fertilizing them. Nymphs hatch from eggs, adding pairs of legs as they grow toward adulthood. Page 425 19.5 PHYLOGENY AND ADAPTIVE DIVERSIFICATION Phylogeny Extant arthropods are divided among four subphyla. Relationships among subphyla are subject to debate, but the taxon Pancrustacea, which contains hexapods and crustaceans, is well supported. Which subphylum is the sister taxon to Pancrustacea? According to the mandibulate hypothesis, Myriapoda is grouped with Pancrustacea. Biologists assume that the ancestral arthropod had a segmented body with one pair of appendages per segment. During evolution, adjacent segments fused to make body regions (tagmata). How many segments contributed to a head in each group of arthropods? Hox gene studies indicate that the first five segments, at least, fused to form the head tagma in all four extant subphyla. It is surprising to find the same pattern of fusion in chelicerates as in other subphyla because a head is not immediately obvious in a chelicerate. Spider bodies have two tagmata: prosoma, or cephalothorax, and opisthosoma, or abdomen. Is the head part of the prosoma? Hox gene comparisons indicate that the entire prosoma corresponds to the head of other arthropods. Studies of the heads of pycnogonids were used to detect the phylogenetic position of these odd animals. Sea spiders have spindly bodies and unusual chelicerae. There was speculation that pycnogonids were not chelicerates at all, but instead were the sister taxon of all other arthropods. In the earliest fossil arthropods, appendages emerge from the first head segment, but in spiders and horseshoe crabs, chelicerae and the nerves that control them originate from the second segment during early development. Initial studies of nerve patterns in larval sea spiders indicated that their chelicerae arose, and were controlled, from the first segment. If this result had been confirmed, pycnogonids would be considered the sister taxon to all other arthropods. However, subsequent studies using Hox gene expression to define segment boundaries did not support this result. Sea spiders remain within subphylum Chelicerata. They and all living arthropods have head appendages that arise from the region of the head that corresponds to the second segment. Another controversial area of arthropod biology where genetic studies have proved helpful lies in the evolution and antiquity of uniramous and biramous appendages. Hexapods and myriapods have uniramous appendages, but trilobites and some crustaceans have biramous appendages. If the ancestral appendage were biramous, then the switch to uniramous appendages might have occurred in one lineage whose descendants now carry this trait. Such reasoning led biologists to group hexapods with myriapods, but phylogenies using molecular characters repeatedly placed hexapods with crustaceans. Is it likely that the uniramous limb evolved more than once? This question would be more easily answered if the genetic basis of limb structure were understood. Studies of the genetic determination of limb branching show that modulation of expression of one gene (Distal-less, or Dll) determines the number of limb branches (see Section 20.3). Gene expression can be modified within a lineage, so the number of limb branches present is unlikely to be homologous. The number of appendages per segment is another variable character within Arthropoda. The ancestral arthropod is assumed to have had one pair per segment. Millipedes, in class Diplopoda, have two pairs of appendages on most trunk segments. Did the millipede pattern originate through the repeated fusion of two ancestral segments? Perhaps it did, but expression of the Distal-less gene might also have a role here. Larval millipedes have only one pair of appendages per segment. Adaptive Diversification Arthropods demonstrate multiple evolutionary trends toward pronounced tagmatization by differentiation or fusion of segments, giving rise to such combinations of tagmata as head and trunk; head, thorax, and abdomen; or cephalothorax (fused head and thorax) and abdomen. The ancestral arthropod condition is to have similar appendages on each segment. More derived forms have appendages specialized for specific functions, or some segments that lack appendages entirely. Much of the amazing diversity in arthropods seems to have evolved because of modification and specialization of their cuticular exoskeleton and their jointed appendages, yielding a wide variety of locomotor and feeding adaptations. Taxonomy of Phylum Anthropoda Subphylum Trilobita (trī′lō-bi′ta) (Gr. tri, three, + lobos, lobe): trilobites. All extinct forms; Cambrian to Carboniferous; body divided by two longitudinal furrows into three lobes; distinct head, trunk, and abdomen; biramous (two-branched) appendages. Subphylum Chelicerata (ke-lis′e-ra′ta) (Gr. chēlē, claw, + keras, horn, + ata, group suffix): eurypterids, horseshoe crabs, spiders, ticks. First pair of appendages modified to form chelicerae; pair of pedipalps and four pairs of legs; no antennae; no mandibles; cephalothorax and abdomen usually unsegmented. Subphylum Myriapoda (mir-ē-a′pōda) (Gr. myrias, a myriad, + pous, podus, foot); myriapods. All appendages uniramous; head appendages consisting of one pair of antennae, one pair of mandibles, and one or two pairs of maxillae. Subphylum Crustacea (crus-ta′she-a) (L. crusta, shell, + acea, group suffix): crustaceans. Mostly aquatic, with gills; cephalothorax usually with dorsal carapace; biramous appendages, modified for various functions; head appendages consisting of two pairs of antennae, one pair of mandibles, and two pairs of maxillae; development primitively with nauplius stage (see Section 20.3: Taxonomy of Crustacea box). Subphylum Hexapoda (hex′-a pōda) (Gr. hex, six, + pous, podus, foot): hexapods. Body with distinct head, thorax, and abdomen; pair of antennae; mouthparts modified for different food habits; head of six fused segments; thorax of three segments; abdomen with variable number, usually 11 somites; thorax with two pairs of wings (sometimes one pair or none) and three pairs of jointed legs; separate sexes; usually oviparous; gradual or abrupt metamorphosis.  Page 426 Taxonomy of Subphylum Chelicerata Class Merostomata (mer′ō-sto′ma-ta) (Gr. mēros, thigh, + stoma, mouth, + ata, group suffix): aquatic chelicerates. Cephalothorax and abdomen; compound lateral eyes; appendages with gills; sharp telson; subclasses Eurypterida (all extinct) and Xiphosurida, horseshoe crabs. Example: Limulus. Class Pycnogonida (pik′no-gon′i-da) (Gr. pyknos, compact, + gony, knee, angle): sea spiders. Small (3 to 4 mm), but some reach 500 mm; body chiefly cephalothorax; tiny abdomen; usually four pairs of long walking legs (some with five or six pairs); mouth on long proboscis; four simple eyes; no respiratory or excretory system. Example: Pycnogonum. Class Arachnida (ar-ack′ni-da) (Gr. arachnē, spider): scorpions, spiders, mites, ticks, harvestmen. Four pairs of legs; segmented or unsegmented abdomen with or without appendages and generally distinct from cephalothorax; respiration by tracheae or book lungs; excretion by Malpighian tubules and/or coxal glands; dorsal bilobed brain connected to ventral ganglionic mass with nerves, simple eyes; chiefly oviparous; no true metamorphosis. Examples: Argiope, Centruroides. Taxonomy of Subphylum Myriapoda Class Diplopoda (dip-lō′ pōda) (Gr. diploos, double, + pous, podos, foot): millipedes. Body almost cylindrical; head with short antennae and simple eyes; body with variable number of segments; short legs, usually two pairs of legs to a segment; oviparous. Examples: Julus, Spirobolus. Class Chilopoda (kī-lō′-pōda) (Gr. cheilos, lip, + pous, podos, foot): centipedes. Dorsoventrally flattened body; variable number of segments, each with one pair of legs; one pair of long antennae; oviparous. Examples: Cermatia, Lithobius, Geophilus. Class Pauropoda (pau-ro+pōda) (Gr. pauros, small, + pous, podos, foot): pauropods. Minute (1 to 1.5 mm); cylindrical body consisting of double segments and bearing 9 or 10 pairs of legs; no eyes. Examples: Pauropus, Allopauropus. Class Symphyla (sym′fy-la) (Gr. syn, together, + phylē, tribe): garden centipedes. Slender (1 to 8 mm) with long, filiform antennae; body consisting of 15 to 22 segments with 10 to 12 pairs of legs; no eyes. Example: Scutigerella. SUMMARY Sections Key Concepts Sections Key Concepts 19.1 Arthropoda is the largest, most abundant and Phylum diverse phylum of animals. Arthropods occur in Arthropo da virtually all habitats capable of supporting life. Arthropods are segmented, coelomate, ecdysozoan protostomes with well-developed organ systems. Most show marked tagmatization. Perhaps more than any other single factor, prevalence of arthropods is explained by adaptations made possible by their cuticular exoskeleton and small size. Other important elements are jointed appendages, tracheal respiration, efficient sensory organs, complex behavior, and metamorphosis. Extinct trilobites are likely to be the oldest arthropod subphylum. According to the mandibulate hypothesis, Crustacea and Hexapoda are sister taxa forming the group Pancrustacea. Pancrustacea is the sister taxon of Myriapoda, together forming the group Mandibulata. Chelicerata is the sister taxon to Mandibulata. Chelicerate mouthparts differ significantly from those in Mandibulata. 19.2 Trilobites, now extinct, were a dominant Paleozoic Subphylu subphylum. m Trilobita There were three tagmata in the body: cephalon, trunk, and pygidium. 19.3 Subphylu m Sections Key Concepts Chelicera ta Members of subphylum Chelicerata have no antennae, and their main feeding appendages are chelicerae. They have a pair of pedipalps (which may be similar to the walking legs) and four pairs of walking legs. The great majority of living chelicerates are in class Arachnida. This group comprises spiders (order Araneae), scorpions (order Scorpiones), harvestmen (order Opiliones), ticks and mites (order Acari), and others. Class Merostomata includes the extinct eurypterids and the ancient, although still extant, horseshoe crabs. Class Pycnogonida comprises the sea spiders, which are odd little animals with a large suctorial proboscis and vestigial abdomen. Tagmata of most spiders (cephalothorax and abdomen) show no external segmentation and are joined by a waistlike pedicel. Spiders are predaceous, and their chelicerae are provided with venom glands for paralyzing or killing prey. They breathe by book lungs, tracheae, or both. Most spiders spin silk, which they use for a variety of purposes, including webs for trapping prey. Distinctive characters of scorpions are their large, clawlike pedipalps and their clearly segmented abdomen, which bears a terminal stinging apparatus. Harvestmen have small, ovoid bodies with very long, slender legs. Their abdomen is segmented and broadly joined to their cephalothorax. Sections Key Concepts In ticks and mites, the cephalothorax and abdomen are completely fused, and mouthparts are borne on an anterior capitulum. Like spiders, some mites can spin silk. Ticks are the most numerous arachnids; some are important carriers of disease such as Lyme disease and Rocky Mountain Spotted Fever, and others are serious plant pests. Page 427 19.4 Members of subphylum Myriapoda have a head Subphylu followed by a series of trunk segments. m Myriapod The most familiar myriapods are predatory a centipedes and herbivorous millipedes. Pauropods and symphylans are soil-dwellers.  Sections Key Concepts 19.5 The ancestral arthropod is hypothesized to have had Phylogen a segmented body with one pair of appendages per y and Adaptive segment. Diversific Tagmatization, the evolutionary fusion of segments ation to make body regions called tagma, has led to bodies with two or three tagma. Work with the Distal-less gene led to the discovery that uniramous and biramous appendages are modulated by expression of a single gene and not likely to be a strong phylogenetic character. Arthropods have adapted to life in almost every part of the world—on land, in freshwater, and in the sea —and feed in a myriad of ways. Their flexible body plan and hardened cuticle make possible this diversity. REVIEW QUESTIONS 1. What are important distinguishing features of arthropods? 2. Name the subphyla of arthropods, and give a few examples of each. 3. Briefly discuss the contribution of the cuticle to the success of arthropods, and name some other factors that have contributed to their success. 4. How are tagmata formed? 5. What is a trilobite? 6. What appendages characterize chelicerates? 7. Briefly describe the distinguishing morphological features of each of the following: eurypterids, horseshoe crabs, pycnogonids. 8. Why are horseshoe crabs in the same subphylum as spiders? 9. What are the tagmata of arachnids, and which tagmata bear appendages?  10. Describe the mechanism of each of the following with respect to spiders: feeding, excretion, sensory reception, web-spinning, reproduction. 11. Is it true that spider silk is stronger than steel? 12. What are the most important spiders in the United States that are dangerous to humans? How does their venom work? 13. How are spiders beneficial to humans? 14. Distinguish each of the following orders from the others: Araneae, Scorpiones, Opiliones, Acari. 15. Discuss the economic and medical importance of members of order Acari to human well-being. 16. What kind of arthropods are chiggers? 17. How do centipedes capture and subdue prey? 18. Which myriapods occur in moist soil or leaf litter? For Further Thought Filter-feeding is a very common way to collect food from water, but this method is rarely used on land. Why might webspinning spiders be considered filter feeders? 1 Legg, D. A., M. D. Sutton, and G. D. Edgecombe. 2013. Arthropod fossil data increase congruence of morphological and molecular phylogenies. Nature Communications 4:2485.