10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 10 



commonly inflected to form internal ridges or platelike epistomo- 

 coxal apodemes (B, ecAp). The coxal bases themselves may be pro- 

 duced into coxal apodemes (fig. 9 B, cAp) having lateral positions 

 relative to the median epistomal apodeme (eAp) when the latter is 

 present in the same species. Only in the Palpigradi do the pedipalps 

 retain a postoral status and have no relation to the mouth (fig. 6 D, 

 Pdp). It is of interest to note that in the mandibulate arthropods 

 the mandibles have a relation to the epistome very similar to that of 

 the pedipalp coxae to the epistome in the arachnids. In the pterygote 

 insects with biting jaws, and the decapod crustaceans, for example, 

 the mandibles have an anterior articulation on the epistome (clypeus), 

 while the decapod jaw, in addition, has a long hinge line on the 

 epistomal margin. 



The telopodite of the pedipalp appendages may dififer little from 

 that of the legs, or it may be modified in various ways. Its adaptation 

 in the male spider to form a sperm-carrying organ, having nothing to 

 do with feeding, need not be considered here ; but in the Scorpionida, 

 Chelonethida, and some of the Pedipalpida the pedipalp is chelate, 

 and with these arachnids the chelae become important adjuncts to the 

 feeding function, since they serve for catching, holding, and crushing 

 the prey. To understand the nature of the pedipalp chela it will be 

 necessary to study the structure and musculature of the distal segments 

 of an ordinary walking leg. 



The simplest structure of the end segment of an arthropod limb 

 is seen in the legs of malacostracan Crustacea in which the appendage 

 terminates with a clawlike segment, called the dactylopodite, movable 

 by levator and depressor muscles arising in the segment proximal 

 to it, which is the propodite, or tarsus. The legs of Limulus have a 

 similar structure (fig. 4 F) though here the dactylopodite, or pre- 

 tarsus {Ptar), forms the movable finger of a chela. Among the 

 Arachnida a simple, clawlike end segment of the leg occurs in some 

 of the Phalangida, as in Leiohimum (B, Ptar) ; but more commonly 

 the pretarsus of the walking legs bears a pair of lateral claws, or 

 ungues (A, Un), while the median claw is reduced to a toothlike 

 dactyl {Dae) on the short base of the segment. Whatever the struc- 

 ture of the pretarsus may be, however, two tendons are always at- 

 tached to its base (A, B, Ivt, dpt), one giving insertion to a levator 

 (extensor) muscle, the other to a depressor (flexor) muscle. Even 

 though the claws be greatly reduced or entirely absent, and the pre- 

 tarsus become indistinguishable from the end of the tarsus, the two 

 pretarsal tendons and their muscles may be retained, as in the slender 

 first legs of Thelyphonidae. The levator muscle of the pretarsus in 



