28 MR. A. HAMMOND ON" THE 



phenomena of development, and the study of the muscular system, 

 all combine to show that the thorax of the Diptera as illustrated 

 in this insect is almost exclusively mesothoracic. Nothing is 

 left of the metathorax except the halteres, a narrow strip possibly 

 along the posterior edge of the mesothoracic epimera, the coxae, 

 and the entosfcernum of the segment, to which must be added the 

 posterior surface of the mesophragma, formed, as in many other 

 cases, by the inversion and adherence together of the two layers 

 of integument of the postscutellum of the one segment and the 

 praescutum of the other. The only remnants of the metathoracic 

 muscles which exist are two thin slips which, originating at the 

 posterior surface of the mesophragma close to the halteres, pass 

 downward and forward, and are inserted in the entosternum of the 

 metathorax. The fact of their being so inserted proves that they 

 are metathoracic muscles. The further fact of their originating 

 on the posterior surface of the mesophragma again shows that 

 that posterior surface is metathoracic, as just stated, and that the 

 cavity of the metathorax, if cavity it can be called, is posterior to 

 this surface and continuous with that of the abdomen. Again, 

 if a further proof be sought, it may be found in the projection 

 into that cavity of the two slender apodemes of the halteres before 

 referred to. 



Lastly, I may add a few words on the light the muscular system 

 throws on the boundaries of the prothorax. "We have seen how 

 the longitudinal muscles of the mesothorax avoid the humeri. I 

 would now point out, from PI. I. figs. 9 & 11, that the vertical 

 muscles do the same, not being sufficiently advanced anteriorly to 

 reach them ; while, on the other hand, a muscle of considerable 

 size, which, passing as it does to the anterior coxa, must, I submit, 

 be regarded as prothoracic, takes its origin from the same parts 

 (see PI. I. fig. 12). [Furthermore, that the condyles form part of 

 the prothoracic segment # , I must conclude from the observation 

 that a pair of muscles connect their interior surfaces with the 

 rami which represent the entosternum f at the posterior inferior 

 margin of the segment behind the coxae ; they represent pro- 

 bably the anterior lateral processes of the prosternum of the 

 Coleoptera. 



I shall only further remark that truth is frequently only to be 



* Ante a, pp. 11 and 14. t Anted, p. 15. 



