16 University of California PiihUcations. [Extomology 



rudimentary wings as arising "at the point where the suture 

 between the tergum and pleura later develops." Stated in 

 other words, these three views are as follows: According to 

 Calvert's view both articular membranes are independent of 

 the dorso-pleural suture; according to the common conception 

 tlie lower articular membrane is formed at this suture; while 

 the idea expressed by Comstock and Needham woud seem to 

 make the suture the source of both articular membranes. 



There is the possibility that these differences in theory and 

 the apparent differences in fact are the result of actual differ- 

 ences in origin; that is, wings may not be in all cases strictly 

 homologous structures. But the degree of uniformity, or 

 rather the lack of distinctive characteristics for the separation 

 of wings into groups, indicates very strongly that we have to 

 do with a single monogenetic organ and the differences must 

 be considered secondary. 



Morphological evidence, as far as I have l)een able to accu- 

 mulate it, has failed to yield any very decisive conclusions in 

 regard to this problem, but does concur with the results 

 arrived at from somewhat theoretical considerations regarding 

 the part played by the wing in the evolution of the thoracic 

 segment. The structure of insects has been quite elaborately 

 investigated from some standpoints, but the order and method 

 of specialization of the parts of the thorax have quite escaped 

 the consideration of investigators. 



I will only briefly outline what appear to l)e the salient 

 points in the process and those that have a bearing on the 

 problem in hand. The accepted nomenclature, that of Audouin 

 ('24), who made the first comprehensive study of the structure 

 of the thorax, is based on the idea that a segment consists of 

 a number of separate pieces, called sclerites, grown together 

 into a more or less solid ring. Audouin recognized one sclerite 

 lielonging to the ventral region, the sternum; two belonging 

 to each side, the episternum and epinieron; and four dorsal 

 pieces arranged in a series from before backward, the pra?- 

 scutum, scutum, scutellum, and postscutellum. Besides these 

 were the legs and wings and their accessory basal pieces. The 

 tergal pieces, like the pleural, are separated from each other 

 l)y transverse lines at right angles to the axis of the body 

 and to the lines that divide the tergal pieces from the pleural, 

 and these latter from the sternum. 



