20 



University of California Piiblications. [Entomology 



The final steps in the perfection of the thoracic segment are 

 not very dissimilar to those just described. The most striking 

 part of this specialization is the separation of the notuni from 

 the plenrse to make way for the attachments of the wing, which 

 results in the completion of the specialization of the pleura^. 

 The thickening which forms the interpleural suture is still 

 further strengthened to serve at its upper end for the articu- 

 lation of the wing, and the upper edges of these sclerites 

 are not only cut off sharply from the parts above, but are also 



thickened. This is 



Meto-thorax 



/^eta-thorax 



Ji-'AUjSe^. 



FIG. 4. Diagram illustrating the new theory of 

 thoracic specialization. 

 gt. sternum; em, epimeron : e.s, episternum ; p^c, 

 prii'scutnni ; .sc, scutum; scl, scutellum ; jj.sc/, post- 

 scutellvim. Stippled portions indicate thickenings; 

 dotted lines, encroachment of dorsum on the interseg- 

 mental membrane. For clearness, segments are im- 

 agined puHed apart to an impossible extent. 



required by the fact 

 that the shape of the 

 base of the wing is 

 such that the seg- 

 ment is nearly cut 

 in two for its attach- 

 ment. The notum 

 keeps its connection 

 with the pleura' 

 solely by the narrow 

 strips at either end. 

 There is a very 

 striking contrast in 

 the kind of modifica- 

 tions of the thoracic 

 segments necessary 

 to adapt them to the musculature of the wings, as contrasted 

 with that for the muscles of the legs. 



With the exception of the Odonata and Ephemerida, insects 

 have a large longitudinal dorsal muscle, the contraction of 

 which causes an arching up of the notum; since the latter is 

 attached to the inner end of the wing root, its arching produces 

 the downward stroke of the wing. To provide for the attach- 

 ment of this muscle, the front edges of the meso- and the meta- 

 thorax and of the first aljdominal segment are developed as 

 deep infoldings, almost making diaphragms across the body 

 Avhen highly developed. These infoldings are produced by the 

 chitinization of the connecting membrane as it grows inwardly, 

 and forms two additional pieces, as seen from the outside, 

 uniting closely in most cases the backs of the adjacent seg- 

 ments. The accompanying diagram (Fig. 4) gives the appear- 

 ance of a segment according to this theory. 



