NO. 2 THORACIC INIECH ANISM OF A GRASSHOPPER SNODGRASS 3I 



sion anterior to the apophyses. A prefurcal division sometimes does 

 occur (fig. 17, /), but it is clearly of a secondary nature and is vari- 

 ously produced. 



The furcal suture is subject to much diversity in form, being some- 

 times produced forward and branched laterally, or curved posteriorly, 

 thus giving a variety of structure to its apodemal ridge, and often 

 obscuring the primary line of the sternal division. 



The form and size of the sternal plates are frequently altered by a 

 variation in the extent of the ventral sclerotization in the different 

 thoracic segments. In the Blattidae it is evident that a partial de- 

 sclerotization of the sternal cuticula has produced the unusual shapes 

 and relationships of the sternal sclerites of the thorax (fig. 19). The 

 prosternum most nearly preserves the typical form (B). It comprises 

 two median plates (A, Bsi, Sh) separated by a transverse fold (k) 

 across the sternal region, from which arise the prosternal apophyses 

 (^'^^i). A comparison with the assumed generalized structure of a 

 thoracic sternum (B) will easily suggest that the transverse fold is the 

 furcal suture (k), and that the pattern of the prosternal plates (C) 

 has been produced by suppression of sclerotization in the lateral fields 

 of the sternal area. In the mesothorax of Blatta (A) the ventral sclero- 

 tization is reduced to a pair of basisternal plates (BS2), and a Y-shaped 

 f urea-bearing sclerite (Si^). the two separated by an ample mem- 

 branous area. In the latter are remnants of the sternal fold (k) from 

 which arise the sternal apophyses (SAo) at the ends of the sternellar 

 arms. The diagram D shows more clearly the relation of the mesoster- 

 nal structure in the roach to the fundamental sternal structure (B), 

 and again suggests that the peculiar features of the thoracic sterna of 

 the roach are results merely of a reduction in the extent of the sclero- 

 tized areas. The metasternum of Blatta (A) is essentially the same as 

 the mesosternum, but the sternal fold appears to be suppressed and the 

 apophyses {SA^) arise from the sternellum {SI3). 



In some insects a thoracic sternum may be divided into two parts by 

 a suture that is quite independent of the furcal suture. A clear case 

 of this is seen in the thorax of the large embiid Cylindrachacta (fig. 

 17), where a suture (/) cuts the mesosternum into an anterior plate 

 {iSo) and a posterior plate {2S0). The second plate is marked by the 

 usual furcal suture (k) and bears the furcal arms ; it is a true furca- 

 sternite. The metasternum has the usual structure, though the sternel- 

 lum is reduced to a narrow band behind the furcal suture (A'). 



Most entomologists have believed that the sternum of a thoracic 

 segment of an insect is " composed of " two principal plates, and the 



