NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 21 



where shown (1928), there are no true subcoxal segments in the 

 mouth part appendages. From the evidence at hand, therefore, it ap- 

 pears most in accord with the known facts to conclude that the subcoxa, 

 •wherever it occurs as a basal leg segment, has been produced by a 

 secondary subdmision in the primitive limb basis, or coxopodite. 



THE THORACIC STERNA 



Sternal plates are by no means so constant a feature in the scleroti- 

 zation of arthropod segments as are the tergal plates. They may be 

 present or absent within the same major group, and, where present. 



Ist-.^ 



Fig. 15. — Sternites and leg bases of two consecutive segments of Lithobius 



(Chilopoda). 



Cx, coxa ; d, ventral subcoxo-coxal articulation ; 1st, intersternite ; Sex, sub- 

 coxa ; Stn, segmental sternite ; iTr, first trochanter ; 2Tr, second trochanter. 



they are often highly variable both in form and extent of development 

 between closely related groups and in the dififerent body regions of 

 almost any species. 



In adult insects the sternal mechanism of the thorax differs in three 

 important respects from that of the abdomen, and the functional dif- 

 ferences in the two body regions are reflected as three distinctive struc- 

 tural features in the sternal parts. 



The first distinction to be noted in the sternal structure, as between 

 the thorax and the abdomen, pertains to the segmental relations of the 

 intersegmental sternites. In the abdomen of adult insects the inter- 

 segmental sclerotizations of both the dorsum and the venter are con- 

 tinuous with the segmental sclerotizations following, and the trans- 

 verse inflections in the cuticula of the primary intersegmental regions, 



