Chap. 30 ARTHROPODS INSECTS, SPIDERS, AND ALLIES 623 



foraging for water skaters and other insects. Spiders are predators that seize 

 and crush their prey between the chelicerae or jaws and suck the juices. They 

 are generally solitary with no hint of any such group organization as that of 

 the social insects. In the instincts that guide female spiders in the architecture 

 of their webs and the trapping of their prey, they are unsurpassed among 

 invertebrates. 



General Structure. Spiders are examples of the narrow-waisted arachnids, 

 a contrast to the thick waisted harvestmen (Figs. 30.29, 30.30), They have 

 neither antennae nor true mandibles. In front of the mouth are the two special 

 jaws or chelicerae, each with a sharp fang through which a poison gland opens, 

 and behind these is a pair of pedipalps. In the female each of the latter ends 

 in a claw, often used in manipulating the silk. In the male the enlarged tip of 

 each pedipalp is the organ by which sperms are transferred to the female. The 

 four pairs of legs vary in size and function; some of them are important in 



King Crab 



Scorpion Whip Scorpion 

 \ ^. ./ 



Pseudoscorpion 



XIPH08TJRA 



SCOKPIONIDA 



PEDIPALPI 



PSEUDO- 

 BCOBPIONIDA 



Sunspider Spider 



( ) 



Harvestman 



Tick 



BOLPUGIDA 



ABANEAE 



PHALANGIDA 



ACARINA 



Fig. 30.29. Relatives in the Class Arachnoidea. King crab, a relative of the 

 fossil trilobites; scorpions, the oldest of land arachnids, with fossils going back 

 400 million years; pseudoscorpions, the largest a quarter of an inch long and 

 without the poisonous tail gland of the true scorpions; sunspiders of the American 

 southwest, an inch long or more; spiders; harvestmen, long-legged, frequently in 

 companies; Ucks that push their heads through the skin and gorge themselves with 

 blood. (Courtesy, Storer: General Zoology, ed. 2. New York, McGraw-Hill Book 

 Co., 1951.) 



