294 THE DEVELOPMENT OF THE NYMPH. 



' There is nothing more easy than to get a number of fly 

 pupae, and, after boilmg them, to remove the pupa-case. In 

 this way we can watch the progress of events. At the end of 

 two or three days the very short legs project at the anterior 

 extremity, and on the next day the wings appear ; in another 

 day the extremity of the proboscis, and afterwards the whole 

 organ becomes apparent. The head first shows itself in those 

 nymphs in which the legs have almost reached the posterior 

 extremity of the abdomen. 



' I have recognised that it is not that the parts grow day 

 by day, as the appearances would lead us to believe, but that 

 they exist preformed, and the mechanism of their evolution is 

 very simple. 



' I have alread}' spoken of a cavity seen at the an- 

 terior extremity of the ovoid, which contains the exuviated 

 hooks and darts (mouth armature) of the larva. I have 

 observed a similar cavity at this stage in all insects which 

 pass through the ovoid condition, and there is a little horn 

 bearing a stigma at two opposite points of its margin. I 

 conclude that these stigmata belong to the corselet of the 

 fly, and the parts which appear day by day are really en- 

 sheathed in the cavity at the anterior extremity of the ovoid 

 nymph. 



' In order to prove that this is really the case, I press 

 upon an insect in the ovoid stage in which the extremities of 

 the feet only are seen, and succeed at length in suddenly 

 producing a nymph. I achieve by pressure a result which 

 requires several days for its accomplishment in the natural 

 state of things. 



' The most essential parts of the nymph and the imago, the 

 head, wings and legs, are therefore lodged in the body-cavity 

 of the worm before its first transformation, each enclosed in its 

 envelope; for when they appear each is so enclosed. It is as 

 if all the parts were invaginated, like the fingers of a glove 

 withdrawn into the hand.' 



No doubt Reaumur believed, from the analogy of what he 

 knew to be the case in the Lepidoptera, that the limbs — wings, 



