OF WASHINGTON. 97 



14 and 25, Ph). The posterior edge usually becomes deflexed 

 and folded forward forming a reduplication and consequent 

 free posterior margin overlapping the following segment or the 

 membrane going to it (figs. 7 and 14, Rd, and fig. 18, g). 

 Each notum becomes divided into more or less definite areas 

 through the formation of regional elevations and depressions 

 and through the development of ridges on the ventral surface. 

 Yet there are never true sutures formed in either the meso- 

 notum or the metanotum nor any real divisions of the original 

 plate into the traditional prsescutum, scutum, scutellum, 

 and postscutellum. In the pronotum of the Acridiidae, 

 however, there are three generally distinct transverse 

 sutures defining four transverse subsclerites. But these 

 sutures simply mark internal ridges which accommodate the 

 leg muscles just as the internal pleural ridges of the mesothorax 

 and metathorax accommodate the leg muscles of these seg- 

 ments. In the prothorax the notum has grown downward on 

 the sides until it has all but crowded out the pleural plates. It 

 is, hence, evident that the apparent division of the acridiid 

 pronotum into the above mentioned regions is simply an 

 acknowledgment on its part of the pleural function devolving 

 upon it through its usurpation of the original pleural areas. 

 Therefore, we may conclude that, in the Orthoptera at least, the 

 prsescutum, scutum, scutellum, and postscutellum are not primi- 

 tive divisions of the notum. 



In the base of the wing are developed articular sclerites, 

 three of which are constant in all insects (figs. 15, 18, and 

 19, X, y, r). The first and third are articulated to special proc- 

 esses of the adult notum (figs. 6, 7, 14, 15, 18, 19, 24, and 25, 

 a, h). The first (a) is at the anterior lateral angle of the 

 notum, the third (&) occupies a varying position on the side, 

 while between the two is the deep lateral emargination (c). 

 From the posterior notal angles project processes that carry 

 the axillary cord of the wing {Ax) which forms the posterior 

 margin of the axillary or axillary and anal membrane of the 

 wing. 



Thus it is true that in all adult insects the wing arises from 

 the entire lateral margin of the notum. But this does not al- 

 ways appear to be the case, for immediately behind the part 

 bearing the axillary cords there is in many insects a large and 

 distinct sclerite, especially in the metathorax. This is well de- 

 veloped in Ephemerida, Plecoptera, Isoptera, Euplexoptera, 

 Neuroptera, and Coleoptera. It is not present in any Orthop- 

 tera, and an examination of nymphal specimens of Ephemerida 

 and Plecoptera shows that in these orders it is an adult charac- 



