The External Anatomy of Spiders 



two delicate keels, of which the lower is usually finely and regu- 

 larly toothed (Fig. 81, k). The claw is traversed by the duct 

 of the poison gland, which opens near the tip of the convex side 

 (Fig. 81, o). The position of this opening is such that it is not 

 closed by the pressure of the claw against the victim, and it 

 allows the venom to flow into the wound made by the claw. 



The claw of the chelicera is freely movable, but only in one 

 plane. In the tarantulas, the claws of the two chelicerae move 

 parallel to each other in vertical planes; while in the true spiders 

 they move obliquely inward and backward toward each other. 



The articular sclerite. — At the base of the claw and on the 

 side toward the furrow, there is a small sclerite (Fig. 81, a); 

 this is the articular sclerite of the chelicera. This sclerite may be 

 a vestige of an intermediate segment of the chelicera (See Bernard, 

 '96, p. 322). 



Chelate chelicerce. — In most cases the chelicera of spiders 

 are of the uncate type (See p. 12); but in a few spiders the terminal 

 portion of the inner margin of the basal segment is prolonged 

 so as to oppose the tip of the claw and thus form a chela (Fig. 84). 



The Rostrum. — The rostrum or upper lip is a single, median, 

 appendage-like part of the head, which resembles in form and 

 position the labrum of an insect; and it is quite probable that 

 the two are homologous. The rostrum is situated below or behind 

 the chelicerae and between the second pair of appendages, the 

 pedipalps. 



By removing the chelicerae, the rostrum can be seen lying 

 between and upon the endites of the pedipalps (Fig. 85, r). On 

 the dorsal surface of the rostrum there is a longitudinal, median 

 keel, which is clothed with a band of hairs. 



The Epipharynx. — On the ventral surface of the rostrum 

 there is a plate which may be termed by analogy the epipharynx. 

 This plate is strongly chitinized, and is marked by many trans- 

 verse striae, which lead to a central longitudinal slit, opening 

 into a longitudinal tube within the rostrum (Fig. 86,. 



As the tube in the rostrum extends back to the beginning of 

 the oesophagus, the function of these striae in the epipharynx 

 is, evidently, to collect the fluid pressed from the spider's victim 

 and to conduct this fluid to the tube, from which it can flow to 

 the oesophagus. 



The Labium (la'bi-um).— The ventral wall of the head is 



102 



