14 TheArthropods: Blueprint for Success Arthropods are among the most misunderstood of alt the animals. Studying chapters 14 and 15, free of blemished preconceptions, will helpyou understand how studying members of this phylum has enthralledprofessional and amateur zoologists for hundredsof years. Chapter Outline 14.1 Evolutionary Perspective 14.1 EVOLUTIONARY PERSPECTIVE Classification and Relationships to Other Animals 14.2 Metamerism and Tagmatization LEARNING OUTCOMES 14.3 The Exoskeleton 14.4 The Hemocoel 1. Describe characteristics of members of the phylum Arthropoda. 14.5 Metamorphosis 2. Explain the phylogenetic relationships of arthropods to other phyla. 14.6 Subphylum Trilobitomorpha 14.7 Subphylum Chelicerata Class Merostomata To be misrepresented and misunderstood-what an awful way to live one's life. Class Arachnida The image of a spider conjures up fears that we have learned, not from encounters Class Pycnogonida (Subphylum with spiders, but from relatives and friends who learned their fears in a similar fash­ Cheliceriformes?) 14.8 Subphylum Myriapoda ion. Virtually no one became fearful of spiders from being bitten-spider bites are Class Diplopoda rare and seldom serious. While spiders do have fangs, they are adapted for preying Class Chilopoda on insects and other arthropods, and most spider fangs are unable to pierce human Classes Pauropoda and Symphy/a 14.9 Further Phylogenetic Considerations skin. The rare spider bite usually occurs by unknowingly placing a hand into some spider abode. Then, if the spider can, it may bite to defend itself or its egg sac. Bites, sores, and rashes may be diagnosed as "spider bites," but often mistakenly so. Other arthropods-like fleas, ticks, and mosquitoes-can bite and leave sores and rashes that are misdiagnosed. Bacterial infections and reactions to plant toxins may also be diagnosed as "spider bites." To misunderstand and misrepresent-that's also an unfortunate way to live one's life. There are a few spiders around the world that are venomous to humans, and two of these will be discussed later in this chapter. There are over 30,000 spe­ cies of spiders that provide the service of insect control. They are among the most numerous of all predators of insects, although their value in this regard has been very difficult to quantify. They are a valuable food resource of other predators in terrestrial food webs. Very importantly, they are teaching us novel ways to use, and even to produce, the silk that makes up their very impressive webs. Finally, once we set aside our preconceived and falsely placed fears, we can begin to appreciate the intricate structure and beauty of these remarkable creatures (figure 14.1). Spiders are one of the many groups of animals belonging to the phylum Arthropoda (ar"thrah-po' dab) (Gr. arthro, joint + podos, foot). Crayfish, lobsters, mites, scorpions, and insects are also arthropods. Zoologists have described about 1 million species of arthropods and estimate that millions more are undescribed. In this chapter and chapter 15, you will discover the many ways in which some arthro­ pods are considered among the most successful of all animals. Characteristics of the phylum Arthropoda include: 1. Metamerism modified by the specialization of body regions for specific functions (tagmatization) 256 CHAPTER POURTEEN These three phyla form the ecdysozoan clade Panarthropoda (see figure 13.17.). The onychophorans and tardigrades, and their position within the Panarthropoda, will be discussed at the end of chapter 15. There has been an explosion of new information regarding the evolutionary relationships within the phylum Arthropoda that is causing zoologists to reexamine current and older hypotheses regarding arthropod phylogeny. Liv­ ing arthropods are divided into four subphyla: Chelicerata, Myriapoda, Hexapoda, and Crustacea. All members of a fifth subphylum, Trilobitomorpha, are extinct (table 14.1). Ideas regarding the evolutiona1y relationships among these sub­ phyla are discussed at the end of chapter 15. This chapter examines Trilobitomorpha, Chelicerata, and Myriapoda, and chapter 15 covers Crustacea and Hexapoda. SECTION REVIEW 14.1 The phylum A1thropoda includes crayfish, lobsters, spiders, insects, and others. Arthropods are characterized by metam­ erism with tagmatization, a chitinous exoskeleton, and paired jointed appendages. As ecdysozoans, arthropods are most closely related to the nematodes, nematomorphs, and other animals that shed a cuticle during growth. Five subphyla of FIGURE 14.1 arthropods have been described. Class Arachnida, Order Araneae. Members of the family Araneidae, the orb weavers, produce some of the most beautiful Why is metamerism, which is common to the Annelida and intricate spider webs. Many species are relatively large, and Arthropoda, no longer considered to be evidence like this garden spider-Aigiope. A web is not a permanent construction. When webs become wet with rain or dew, or when of close evolutionary ties between the two phyla? they age, they lose their stickiness. The entire web, or at least the spiraled portion, is then eaten and replaced. E'J'Al\-lEHISM 2. Chitinous exoskeleton that provides support and protection and is modified to form sensory structures 3. Paired, jointed appendages LEARNING OUTCOME 4. Growth accompanied by ccdysis or molting 1. Compare arthropod metamerism with annelid 5. Ventral ne1vous system metamerism. 6. Coelom reduced to cavities surrounding gonads and sometimes excretory organs Four aspects of arthropod biology have contributed to their 7. Open circulatory system in which blood is released into success. One of these is metamerism. Metamerism of arthro­ tissue spaces (hemocoel) derived from the blastocoel pods is most evident externally because the arthropod body 8. Complete digestive tract is often composed of a series of similar segments, each bear­ 9. Metamorphosis often present; reduces competition ing a pair of appendages. Internally, however, septa do not between immature and adult stages divide the body cavity of an arthropod, and most organ sys­ tems are not metamerically arranged. The reason for the loss of internal metamerism is speculative; however, the presence Classification and Relationships of metamerically arranged hydrostatic compartments would be of little value in the support or locomotion of animals to Other Animals enclosed by an external skeleton (discussed under "The Arthropoda is a monophyletic taxon that is a part of the proto­ Exoskeleton"). some clade Ecdysozoa. Arthropods are thus related to the Nema­ As discussed in chapter 12, metamerism permits the toda, Nematomorpha, Kinorhyncha, and others (figure 14.2 and specialization of regions of the body for specific functions. see chapter 13). Synapomorphies for this clade include a cuticle, This regional specialization is called tagmatization. In arthro­ loss of epidermal cilia, and shedding the cuticle in a process pods, body regions, called tagmata (sing., tagma), are spe­ called ecdysis. Two smaller phyla within the Ecdysozoa, Ony­ cialized for feeding and sensory perception, locomotion, and chophora, and Tardigrada, are sister groups with the Atthropoda. visceral functions. The Arthropods: Blueprint for Success 257 Basal Protists. Lophotrochozoa Ecdysozoa Phyla FIGURE 14.2 E olutlonacy Relationships of the Arthropods to Other Animals. This figure shows on' inLerprelation nr lh,e relationsl1ips or the ArLhropocl,1 rn oth··r mc::mhers ol' th �1nimal I illgd m. Th rel.1Lltmslllps tkpi ·te<l here ar-' I ased ou evidence rwm develop1m;ntal ancl mole ·uh1r biology. Arthrop id� are pl�1 ·et! wiLhi11 th·' Il:l.;dysozoa along wi1h rhe Nematoda. Nematornorph;t. ny h • pit rn, and others (see pa�es .n 1i-.,.:um. The r,li)'lum includes over L mill.ion spe iei:,. This sc<)rpion (B1tlf.111ssp.> bdongs to the �uhphylum Chelicern1a--ol1e of 1hc four arthropod subphyla containing living species. This photo was taken in the Sarah desert. SECTION REVIEW 14.2 portions of the gut tract. It is nonliving and is secreted by a single layer of epidermal cells (figure 14.3). The epidermal Metamerism of arthropods is most evident externally where layer is sometimes called the hypodermis because, unlike body segmentation is most obvious and has been modified other epidermal tissues, it is covered on the outside by an through tagmatization. Internal metamerism has been reduced. exoskeleton, rather than being directly exposed to air or How is the metamerism of arthropods different from water. that observed in the Annelida (see chapter 12)? The exoskeleton has two layers. The epicuticle is the outermost layer. Made of a waxy lipoprotein, it is imperme­ able to water and a barrier to microorganisms and pesticides. l I ) The bulk of the exoskeleton is below the epicuticle and is called the procuticle. (In crustaceans, the procuticle is some­ LEARNING OUTCOMES times called the endocuticle.) The procuticle is composed of 1. Describe the structure of the atthropod exoskeleton or chitin, a tough, leathe1y polysaccharide, and several kinds cuticle. of proteins. The procuticle hardens through a process called 2. Assess the influence the exoskeleton has had on the sclerotization and sometimes by impregnation with calcium evolution of the arthropods. carbonate. Sclerotization is a tanning process in which lay­ ers of protein are chemically cross-linked with one another­ An external, jointed skeleton, called an exoskeleton or hardening and darkening the exoskeleton. In insects and most cuticle, encloses arthropods. The exoskeleton is