24 SOME ASPECTS OF MORPHOLOGY 



larvae, are familiar to entomologists. Most Thysanura exhibit a 

 biungulate condition, the two claws being secondary formations 

 situated on the base of the pretarsus. The claw of the original 

 dactylus is retained in Japyx in a greatly reduced condition, 

 while in Campodea it has either disappeared or is represented by 

 a plumose arolium. In Lepisma, on the other hand, the feet are 

 three-clawed owing to the retention of the claw of the dactylus 

 between the paired or secondary claws. A simple, dactylus-like 

 claw is retained in most Myriapoda, some Protura and among 

 many insect larvae. In the great majority of insects the original 

 dactylus is represented only by the base of the pretarsus, which is 

 partially invaginated within the tarsus. Among both insects and 

 myriapods only a single muscle is associated with the pretarsus. 

 This muscle, the flexor (depressor) of the claws, arises from the 

 tibia, or from the tibia and femur, and, traversing the length of the 

 tarsus, is inserted on a long tendon which is attached to the 

 tractor plate of the pretarsus. The extensor (levator) muscle is 

 never developed in these two classes, whereas in Crustacea both 

 flexor and extensor muscles are evident, as well as in most of the 

 Arachnida. 



The tarsus, in its simplest form, is an undivided segment, well 

 seen in the Symphyla, Japyx, Protura and many larvae. There is 

 little doubt that it is the homologue of the propodus of the 

 biramous limb. Its division into segments, so characteristic of 

 the Pterygota, is evidently a secondary condition possibly acquired 

 to ensure greater flexibility. In whatever condition the tarsus is 

 present, it never possesses intrinsic muscles : it is movable solely 

 by means of either a flexor muscle, or by both flexor and extensor 

 muscles, attached to its proximal border. The origins of these 

 muscles are variable ; in the Thysanura and the Symphyla they 

 are located in the tibia, or in the femur and the trochanter, whereas 

 in the Pterygota they commonly originate in the tibia. 



The tihia or carpus is similarly movable by flexor and extensor 

 muscles which arise within the femur to be inserted in association 

 with the margin of the base of the tibia. The femur or merus 

 varies greatly in its degree of development. Thus in polypodous 

 forms, like the Symphyla and other Myriapoda, it is commonly 



