NO. I 



INSECT THORAX — SNODGRASS 



93 



downward on their dorsal articulations with the unguifer process at 

 the end of the tarsus (fig. 36 A, D, k) . The claws are extended by 

 the elasticity of their basal connections when the flexor muscles 

 relax, or by the weight of the insect on the supporting surface. 



MORPHOLOGY OF THE LEG 



Entomologists can not resist the temptation of endeavoring to trace 

 the evolution of the insect leg from a biramous appendage such as is 

 supposed to have been the ancestral form of all arthropod limbs. 

 The plan of this hypothetical generalized appendage, as conceived 

 principally by the carcinologists (fig. 42 A), includes a basal stalk, or 



Propodite ____YJ __ ,/ Hx Tarsus 



ABC 



Fig. 42. — Diagram suggesting homologies of segments in arthropod limbs. 



A, generalized crustacean limb; B, leg of a chilopod (centipede) ; C, leg of an 

 insect. 



protopodite, and two distal branches, an exopoditc and an endopoditc. 

 The stalk is divided into three segments (formerly only two were 

 recognized), a plcuropoditc, a coxopodite, and a hasipodite. The 

 exopodite has a number of small subdivisions, fortunately not named 

 individually ; the endopodite consists of five segments, an ischiopo- 

 dite, a meropodite, a carpopodite, a propodite, and a terminal dactylop- 

 oditc. The basal two segments of the stalk may bear external appen- 

 dages called cpipodites; lobes on the inner margins of the segments 

 are cnditcs. 



The only fact that can be construed as evidence of a biramous 

 origin of insect appendages is the presence of styli on the coxae of 

 the middle and hind legs of Machilis and related genera (fig. 12, Sty). 

 No insect appendage develops in the embryo with a biramous struc- 



