322 ^'^^ ANIMAL KINGDOM 



narrow segments forming a long tail. These lack appendages. The telson 

 is modified as a powertul sting. 



Although no arachnids have compound eyes, most of them have 

 ocelli resembling those of the insects (Fig. 16.21). Ocelli are also found 

 in king crabs and eurypterids. 



Ihe class Arachnida is divided into eleven or more orders. Only 

 two forms are discussed here, the scorpions, above, and the spiders, order 

 Araneae, below. The other scorpion-like and spider-like orders are listed 

 in the appendix. 



1 47. Argiope, an Orb Spider 



Of the many species of spiders only a few build geometrical webs. 

 These are the orb spiders, about one inch long. Both the golden (Argiope 

 surantia) and the banded (A. trifaciata) orb spiders are common in gar- 

 dens and marshes (Fig. 16.29). 



The prosoma bears four anterior ocelli that look forward, upward 

 and to the side. Below them are the chelicerae, no longer chelate but 

 modified as poison fangs. The second pair of appendages are small 

 pedipalps, tactile in function and used to manipulate prey. The remain- 

 ing four pairs are typical walking legs, each composed of seven segments. 

 The prosoma is joined to an opisthosoma by a slender waist. The 

 opisthosoma is a large soft bag formed embryologically by the fusion of 

 ten segments. A pair of ventral slits opens to the one pair of book lungs 

 and posteriorly are three pairs of spinnerets and one pair of small anal 

 papillae. The anus is terminal, just anterior to the spinnerets is a 

 single median opening, the spiracle. 



The mouth, just behind the chelicerae, opens into a narrow esoph- 

 agus that leads to a sucking stomach (Fig. 16.30). This is followed by a 

 midgut. After traversing the waist the midgut expands dorsally and 

 continues posteriorly to join the hindgut, a short rectum with a dorsal 

 storage sac leading to the anus. The midgut has the usual pouches. The 

 first pair extend forward in the prosoma and send branches into the 

 bases of the legs. In the opisthosoma are several more highly branched 

 pouches. As in other arthropods these not only secrete enzymes but also 

 absorb nutrients. 



A pair of Malpighian tubules are located at the junction of the mid- 

 gut and hindgut. The dorsal heart lies in a separate part of the hemocoel 

 as in the crustaceans. The nervous system is condensed into a brain and a 

 large subesophageal ganglion with connectives and nerves to all parts of 

 the body. 



Argiope has two respiratory systems. The book lungs are continu- 

 ally flushed with blood that is oxygenated as it passes through. Since the 

 blood is not known to contain any respiratory pigment, however, not 

 much oxygen can be carried. The single spiracle opens into a tracheal 

 system. The tubes are small and do not branch as much as in the insects, 

 but they are structurally identical and have the same spirally thickened, 

 chitinous lining. 



A pair of poison glands fills the dorsal part of the prosoma, opening 



