258 ENTOMOLOGICAL SOCIETY 



The ' 4 patella," " tibia," and kt hand" of Hubbardia correspond 

 respectively to the "tibia," kt hand," and movable dactylus of 

 Thelyphonns, and to the " tibia," kt hand," and basal part of the 

 claw of Tarantula. As far as the number of joints is concerned, 

 the difference is made not by intercalation, but by the fact that 

 the claw of Thelyphonus and Tarantula has become fused with 

 the disial joint and is no longer movably articulated with it as in 

 Hubbardia. Here, however, the apparent similarity ends. In 

 Thelyphonus, the distal joint with its movable claw has a vertical 

 motion and is opposed to the produced superior corner of the fifth 

 joint. The superior angles of the second and fourth joints are 

 also strongly produced to assist in the formation of a powerful 

 armature which depends for its efficiency largely upon the co 

 operation of the two maxillaB. With the exception of the last, 

 none of the joints are, strictly speaking, opposable, a difficulty 

 which the strong processes are evidently calculated to meet. 



In Tarantula, on the contrary, the distal joint with its adnate 

 claw has a horizontal motion and functions merely as a claw ; it 

 is not opposable to the preceding joint, which sends out no proc 

 ess to meet it, but is itself accessory to the prehensory mechan 

 ism formed by the third and fourth joints which are armed along 

 both edges of their closely opposable mesial face with a series of 

 large spines. The maxillaB of Tarantula are thus adapted to work 

 separately in a manner which would be impossible in Thelypho 

 nus. The short distal joints bring the prey against the spines of 

 the long third and fourth joints which hold and impale it. The 

 third joint in Thelyphonus is not specialized in the formation of 

 the prehensory apparatus, but the second joint plays an impor 

 tant role with its powerful dentate processes. In Tarantula the 

 third joint is second only to the fourth in size and specialization, 

 while the second is reduced to a comparative rudiment. The 

 complete difference in the mechanical principles and construction 

 of the maxillae in the two groups is further emphasized by the 

 fact that thecoxaB are adnate in Thelyphonus, but free in Tarantula. 

 This condition permits in Thelyphonus the development of a 

 muscular system for increasing the power of lateral prehension 

 between the maxillaa, while the independent action of the parts 

 in Tarantula requires no such basal modification, the chief mus 

 cular strain occurring in the opposition of the fourth joint with 

 the third. 



It is accordingly evident, in spite of the general similarity of 

 the maxillaB, that these appendages offer, in the two families con 

 sidered, no details indicating a common origin from a form with 

 specialized maxillas. Although having similar habits and occu 

 pying much the same place in the economy of nature, the 

 Thelyphonidae and Tarantulidzu have evidently achieved their 

 present efficiency on two independent lines of development, and 



