Spiders and Their Near Relatives 



claws, and the last segment is often clavate and appears like an 

 appendage of the preceding segment, in such cases it is termed the 

 thumb. Several types of pedipalps are represented in Fig. 62. 



The head segments, those bearing the chelicerae and the 

 pedipalps, are often more or less distinct from the following 

 thoracic segment, and form what has been termed the beak, 

 rostrum, or capitulum (ca-pit'u-lum); this division, however, is 

 indistinct in some forms; and sometimes the beak is partly or 

 completely retracted into the following segments. When the 

 beak is thus retracted, the opening of the body from which the 

 mouth-parts project is known as the camerostoma (cam-e-ros'to- 

 ma). In Uropoda the base of the first pair of legs is also retracted 

 into the camerostoma (Fig. 63). 



Sometimes the basal segments of the pedipalps are united 

 and form a Up, or labium; and above the chelicerse there is in many 

 forms a thin corneous plate, termed 

 the epistoma (e-pis'to-ma). The sides 

 of the epistoma may be united to the 

 labium and thus form what is known 

 as the oral tube, for it is through it 

 that the chelicerae are protruded. 



In several families there is an 

 organ termed the tongue or hypostoma 

 (hy-pos'to-ma), which arises from the 

 inner base of the beak, and may be VvA - 6i - 



..... . . , . .... UROPODA. VENTRAL ASPECT 



divided or simple. In the ticks it is (after Banks) 



large and roughened with sharp teeth. 



Sometimes the hypostoma has a groove above, called the vomer. 



As in other arachnids, the normal number of legs is four pairs; 

 but almost invariably the newly hatched young has only three 

 pairs, and in the family Eriophyidae even the adults have only 

 two pairs. 



In the remarkable genus Ptcroptus, which is parasitic on 

 bats, the larval stage is passed within the body of the mother, 

 and the young mite when born has four pairs of legs. 



That the possession of only three pairs of legs by larval 

 mites is an adaptive characteristic is shown by the fact that the 

 embryo of certain forms, as Gamasus and Ixodes,, has four pairs 

 of legs, one pair of which is aborted before the birth of the larva, 

 and is again developed when the larva transforms to a nymph. 



83 



