12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 10 



some arachnids arises in the tarsus (fig. 4 E, Ivptar), in others it 

 takes its origin in the tibia (C). The larger and stronger depressor 

 muscle has no connection in the tarsus, but a branch of it arises in 

 the tibia, and one or several branches in the patella (C, dpptar), 

 unless a patella is absent, in which case the upper part of the muscle 

 arises in the femur (E, dpptar). 



The tarsus of an arachnid leg may be a simple segment (fig. 4 E, 

 Tar), or it may be divided into two or more subsegments, or tarso- 

 meres (C, iTar, 2Tar). No muscles are ever present between tarso- 

 meres. It is important to note, furthermore, that the tarsus itself 

 has only one muscle, a depressor, which arises in the tibia (D, E, 

 dptar) . The character of the distal musculature of the leg, therefore, 

 will serve to identify the pretarsus, the tarsus, and the tibia when 

 the identity of these segments is not otherwise clear. 



The chelate arachnid pedipalp (fig. 5 D) has only six segments, 

 of which the last is the movable finger of the chela, and might there- 

 fore appear to be a clawlike pretarsus. A study of the musculature, 

 however, shows that the finger is movable by only one muscle, and 

 that one a depressor (E, G, dptar), which thus corresponds with the 

 single muscle of the tarsus of a leg (fig. 4 D, E, dptar). In the 

 nonchelate pedipalp of one of the amblypygous Pedipalpida, Tri- 

 thyreus (fig. 5 A), it is shown by Borner (1904) that the small pre- 

 tarsus (Ptar) has the usual pretarsal musculature (Ivptar, dpptar). 

 In the Thelyphonidae of the same order the pedipalp is chelate (C) 

 and there is no distinct pretarsus, but attached within the apex of the 

 movable finger Borner finds the tendon of a depressor muscle, from 

 which fact he logically contends that the movable finger is the tarsus 

 and pretarsus combined. According to Barrows (1925) two tendons 

 are attached in the pedipalp finger in Mastigoproctus (B), one giving 

 insertion to a small muscle arising in the proximal segment {Tb), 

 the other to a muscle from the next segment {Pat). Some over- 

 hardened specimens of Mastigoproctus examined by the writer appear 

 to confirm Barrow's statement. By comparison with a leg, therefore, 

 the movable finger of the pedipalp chela is the combined tarsus and 

 pretarsus (B-G), and the proximal segment, or "hand," containing 

 the single depressor muscle of the finger, is the tibia {Th), while 

 the segment supporting the chela is the patella (Pat). Both Borner 

 and Barrows regard the basal segment of the chela as a proximal 

 segment of the tarsus, but this interpretation is clearly not in accord 

 with the musculature, since the single muscle of the finger is evi- 

 dently the tarsal muscle of a leg, and tarsal subsegments are never 

 interconnected by muscles. 



