NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 29 



Weber (1928a) says, presents a case in which there can be no doubt 

 that the furca arises at a point between the basisternite, the furcaster- 

 nite, and the subcoxal laterosternite. 



In the higher insects the sternal apophyses approach each other in 

 each segment and unite upon a common basis produced by a median 



1st spn Ss spn 



B D 



Fig. 18. — Diagrams suggesting the theoretical evolution of a thoracic sternum. 



A, primitive condition in which the ventral sclerotization consists of alter- 

 nating segmental sternites (Sfn) and intersegmental intersternites (1st) ; the 

 leg basis (LB) is an undivided coxopodite. 



B, primary sternite marked by the pits (sa) of a pair of internal apophyses, 

 intersternite (1st) by the pit (spn) of a median process, or spina; leg basis 

 (A, LB) subsegmented into subcoxa (Sex) and coxa (Cx), articulated dorsally 

 and ventrally (d). 



C, area of primary sternite (A, B, Stn) divided into basisternum* (Bs) and 

 sternellum (SI) by a f ureal suture (k) forming an internal f ureal ridge between 

 bases of sternal apophyses ; the following intersternite has become a spinaster- 

 num (Ss) by union with segmental sternite; subcoxa (Sex) united ventrally 

 with sternum. 



D, typical definitive sternum, composed of primary sternite (A, B, Stn), a pair 

 of subcoxal laterosternites (Ls, Ls), and the spinasternite (Ss) ; area of pri- 

 mary sternite divided into basisternum (Bs) and sternellum (SI) by the furcal 

 suture (k), and with a narrow presternum (Ps) set ofif by a secondary presternal 

 suture (/). 



inflection of the sternal wall. In this way is formed the typical, forked 

 endosternal structure known as the furca, the evolution of which has 

 been portrayed by Weber (1928). The part of the sternum bearing 

 the furca lies between the coxae, and is usually much narrowed by 

 comparison with the region of the sternum anterior to it. 



The definitive sternal plate, whether it includes subcoxal lateroster- 

 nal elements and the following intersternite, or does not, is commonly 

 subdivided into an anterior and a posterior region. Fundamentally the 



