150 THE CANADIAN ENTOMOLOGIST 



slightly raised; as in the Locustids and in the other Gryllids it projects ventrally 

 and articulates with the coxa. The epimeron is very narrow, almost as deep 

 as the episternum and practically uniform in width except at the ventral end, 

 where it projects farther caudad. It is entirely an internal sclerite. 



In the insects described it is possible to trace a distinct gradation in the 

 development of the propleura as ental sclerites. In the Acridiidae (Figs. 1-3) 

 the ventral end of the pleuron had undergone but little change and still stretches 

 from the anterior to the posterior end of the prothorax. The epimeron is greatly 

 reduced dorsally, much more so than the episternum. It is possible to lift the 

 edge of the pronotum and show that the episternum is a continuous sclerite 

 overgrown by the pronotum. In the Tettigidee (Figs. 4, 5) the epimeron is 

 still further reduced. In the Locustidae (Figs. 6-8) the ventral end of the 

 epimeron no longer extends to the posterior end of the segment but is gradually 

 disappearing, the last vestige of it remaining in the Conocephalinae (Fig. 6). 

 After this we find the epimeron reduced to a very narrow sclerite, best developed 

 dorsally. In Ceuthophilus (Fig. 8), except for the broad anterior lobe the pleuron 

 closely resembles that of the Gryllidse (Figs. 9 and 10). In the Locustidse and 

 Gryllidse, unlike the Acridiidae, the episternum is fused with the ventral edge of 

 the pronotum and (at least in the Gryllida?) have only a muscular attachment 

 with the inner face of the pronotum, so that the internal portion of the pleuron 

 has the appearance of a process of the small externally visible portion of the 

 episternum; only by comparison with the Acridiidae, and by a study of their 

 musculature in comparison with that of the hinder segments of the thorax can 

 their true nature be definitely ascertained. 



The Musculature of the Propleuron. (Figs. 12 and 13). 

 The reason why the propleuron has persisted in spite of the fact that its 

 position and function as an external lateral wall of the prothorax has been 

 usurped by the overgrowing tergite, probably lies in the fact that important 

 muscles of the leg arise from its inner surface. As fewer of these muscles arise 

 from the epimeron this sclerite is accordingly more reduced than the episternum. 



If we examine the mesopleuron of Grylliis (Fig. 13) we find the following 

 muscles arising from the episternum. 



40a. From the basalar sclerite, a detached portion of the episternum, 

 into the anterior edge of the trochanter. An extensor of the femur. 



42. From the dorsal edge of the episternum into the anterior edge of the 

 coxa. An extensor of the coxa. 



43. From near the middle of the sclerite into, (I) the precoxale, (2) the 

 trochantin, and (3) the anterior edge of the coxa. An extensor of the coxa. 



The pro-episterum of Rhomalea and Gryllus (Figs. 12 and 13) show homo- 

 logues of these muscles as follows: — 



25, homologue of 40a; 26, homologue of 42, and 27, hgmologue of 43. 



From the proepimeron a single muscle 25a originates. It is inserted into 

 the anterior edge of the trochanter. I have not yet found its homologue in 

 the hinder segments. 



A similar homology exists between the muscles of the propleuron and those 

 of the metapleuron. 



