FROM EMBRYOLOGICAL DATA. 19 



The fact, however, that flies, when hatched from their hard envelope, are often 

 observed to carry after them a thin membranous film, has induced me to inquire 

 more minutely into this case. Having secured a large number of so-called pupae 

 of the meat-fly, Musca vomitoria, I tried to separate their outer coat as soon as it 

 began to harden, and it was probable that the process of further separation of the 

 future perfect insect from its primitive envelope had commenced. I succeeded 

 without much difficulty in tearing off the hard case without damaging in the least 

 the soft animal within, when I found that there was really a true chrysalis * 

 formed under the larva-skin, differing only from ordinary pupse in the softness of 

 its envelope, which is a simple, transparent, white membrane, presenting in a ru- 

 dimentary condition all the peculiarities and characteristics which distinguish the 

 perfect fly, but in an imperfect state of development, and very similar to the con- 

 dition of the young butterfly when it is ready to cast for the last time its larva-skin 

 before it passes into the state of a real pupa. 



This transparent chrysalis of the fly shows, like the young pupa of the butterfly, 

 all the parts of the head ; rudiments of wings, in the form of short vesicles ; three 

 pairs of legs, tubular, unarticulate, bent under the thorax ; and distinct joints of 

 the body, already contracted and combined in such a manner as to define in their 

 general outline the head, chest, and abdomen. But that this is a real chrysalis 

 within the larva-skin, and that the difference between the Diptera and the other 

 insects which undergo complete metamorphoses consists simply in the circum- 

 stance that the skin of the larva is retained to protect the soft pupa, may be posi- 

 tively concluded from the fact that, within this transparent pupa, we may see, as 

 soon as it is completely separated from the larva-skin, the next stage of development 

 more or less advanced, and the perfect insect appearing within that thin covering. 

 The transparency of the pupa-skin very much facilitates the investigation ; as it is 

 easy to see through it all the parts of the perfect insect fully developed at a very 

 early period of the hardening of the larval skin. A week had scarcely passed after 

 my larvae had ceased to be movable, when, on removing the outer larval covering, I 

 could not only distinguish the perfect pupa, as I have described it above, but within 

 it the well defined parts of the perfect insect, with all their minute characters, could 

 also be satisfactorily distinguished. The legs, which in the pupa were simple, 

 tubular, and unarticulated, were now seen within, with all their joints, hairs, and 

 hooks. The vesicles first representing wings now contained perfect wings, with 

 their nervules and hairs. The eyes with all their facets were well defined. The 

 antennae also, and all the parts of the mouth, had lost their larval appearance, and 

 assumed the character which they exhibit in the perfect insect. The surface of 

 the rings of the body presented, throughout, the hairy covering and the scales 

 which characterize them, though all these parts were still white, transparent, and 

 entirely soft. 



As a striking analogy between the pupa of the fly and that of the mosquito, I 

 may add, that the pupa presented on the sides of the interior part of the thorax 



* See also Dr. Harris's Report on the Insects of Massachusetts injurious to Vegetation, pp. 14 and 403. 



