THE PUPA OR CHRYSALIS. 117 



from the chrysalis are very various. In the case of 

 Butterflies and Moths, which do not make a cocoon 

 in which to pass the chrysalis state, the manner of 

 coming forth is very simple. The skin of the chry- 

 salis splits up the back so soon as the imago attains 

 its full size, and the rent being increased by the 

 effort of the imprisoned insect, the escape is easy. 

 It is much the same with Dragon-flies, though the 

 chrysalis is of a different kind. 



In several other classes of insects, however, the 

 means by which the escape is effected are ex- 

 ceedingly curious. Many kinds of flies burst open 

 the top of their little egg-shaped prison by a very 

 singular application of the principle of atmospheric 

 pressure. They possess the means of inflating a 

 membrane on the top of the head till it assumes 

 the appearance of a bladder, much larger than the 

 head itself. The pressure exerted by this means on 

 the upper part of the shell forces it open, and then, 

 the only purpose of the membrane in question 

 having been carried into effect, it is immediately 

 shed, like the dried skin of a cicatrized wound, and 

 no vestige of such an appendage remains. Other 

 flies, by somewhat analogous means, force out the 

 opposite end of the pupa case. 



The pupa of the pretty Beetle Cetonia Aurata, 



