1873.] 139 [Packard. 



We feel warranted then in stating that the embryological develop- 

 ment of Musca domestica is in all its stages almost identical with that 

 of Galliphora vomitoria, so elaborately worked out and copiously 

 illustrated by Weismann, in his famous work. 



When the embryo is about to slip out of its egg-membranes, includ- 

 ing the amnion, which we have observed to be as usual in the in- 

 sects, it is quite active in its motions, the body moving to and fro 

 within the shell. Undoubtedly this motion, accompanied by a twist- 

 ing motion of the body, ruptures the exchorion. 



We did not witness the process of hatching of the House Fly, but 

 have no doubt it is like that of the Meat Fly (C. vomitoria) . A larva 

 of the latter hatched under our eyes. The egg-shell split longitud- 

 inally, and in one or two seconds it pushed its way out through the 

 anterior end, and in a second or two more extricated itself from the 

 shell. The shell scarcely changed its form, and the larva left the 

 amnion within. 



The larva. First stage. (PI. 3, Fig. 3, 3 b.) In order to bring out 

 more clearly the characters of this stage, we shall compare it with 

 the freshly hatched larva of the Meat Fly (C. vomitoria), which we 

 studied at the same time in order to test our work on the House Fly. 



The larva as soon as hatched, on being compared with that of the 

 Meat Fly of corresponding age, differs from the latter in being slen- 

 derer, with the head in front rounder and narrower, while the poste- 

 rior end of the body is rounder and narrower. The sutures in M. 

 domestica are much less plainly marked, especially the three anterior 

 ones. There are but seven rows of locomotive spines, where in C. 

 vomitoria there are twelve, one for each segment (Weismann does not 

 figure the last and minute row). Moreover, the spinules are less 

 numerous than in C. vomitoria. These rows of spinules appear more 

 clearly than in the fully grown larvae in both genera. These differ- 

 ences we would regard as good generic characters, and these are, with 

 the other characters given below, the only differential characters by 

 which to distinguish the larvse. 



The head is much less free from the succeeding, or prothoracic, 

 segment than in C. vomitoria, the suture behind being less distinct. 

 The end of the body below the stigmata does not project so conspic- 

 uously as in C. vomitoria. 



There are also good generic characters in the tracheae. In M. do- 

 mestica the two main tracheae are more sinuate, and do not taper so 

 rapidly anteriorly, while the distance from the posterior commissure 



