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University of California Publications. [Entomology 



structure and function, and the very evident dependence of the 

 function upon the region specialized, lend considerable pre- 

 sumptive evidence in favor of the idea here advanced, that the 

 specialization is in fact dependent on the function. 



There are many facts bearing on this problem, to which it 

 will not be necessary to refer in detail, by means of which the 

 course of the development of the segment may be traced from 

 the simplest type of structure, such as occurs in the lowest 



articulata, or in the apodous larva? 

 of insects, or only slightly more com- 

 plex in the abdomen of many adult 

 forms to the most complicated. 

 There are evidently two stages in 

 the development of the thoracic seg- 

 ment: first, the one embracing those 

 changes that follow and are depend- 

 ent on the development of the leg; 

 and, secondly, the one embracing 

 those which are quite as evidently 

 to be assigned to the specialization 

 of the thorax for flight. 



Before discussing the nature of 

 these modifications it will be useful 

 to note the nature of the suture sep- 

 arating the thoracic sclerites. Con- 

 trary to the usual conception, these 

 sclerites are not to be considered as 

 separate pieces in the same sense 

 that the segments themselves are separate. The division of 

 the segments is brought about by a specialization of the cells 

 of the connecting membrane, whereby they do not take part 

 in the modification that occurs in the other cells of the skin 

 resulting in a harder and thicker cuticle, but become instead 

 capable of producing thinner and more flexible cuticle. The 

 purpose of this thinning is to afford a freer motion between 

 the harder parts. Another example of the same kind of 

 specialization is found in the separation of the dorsal from the 

 ventral portion of the abdominal segment, and it is developed 

 for a similar purpose, that is, for motion. There is consid- 

 erable modification for motion, and therefore of the segmental 

 type, at the base of both the legs and the wings, but it should 

 be clearlv understood that the sutures between the thoracic 



FIG. S. The thoracic segment 

 according to Audouin. 

 St, sternum; em, epimeron; 

 en, episternum; psc, prasscu- 

 tum ; sc, seutnm ; sci, scutellum ; 

 pscl, postscutellum. 



