68 MICROSCOPIC ILLUSTRATIONS. 



small quantity of air or gas probably evolved during tbe 

 change ; and which, by insinuating itself between the 

 case and the insect, may facilitate the process. After 

 remaining a few minutes in this state, and making occa- 

 sional etforts to disentangle itself, it bursts forth from its 

 watery dwelling, and wings its flight into the aerial 

 regions, either leaving the entire skin behind it in the 

 water, or carrying part of it away. 



The exuviae, or skins, which are shed previous to the 

 creature assuming the perfect form, are very fine trans- 

 parent objects for the single microscope, with a lens of 

 the tenth of an inch focus. In this state they afford much 

 useful information, as to the structure, formation, &c. of 

 which the paddles, tail, &c. furnish no inconsiderable 

 portion. If it be thought desirable to expedite the casting 

 of the exuviae, this may be effected by removing the 

 insect from the water in which it is found into spring 

 water. This change is most probably produced by the 

 water mechanically altering the dimensions of the skin 

 or case which inclosed the insect, and thereby suffering 

 it to escape with great ease ; or, perhaps, from the same 

 cause which produces this effect in the common spider 

 when deprived of sustenance. These skins should all be 

 mounted in sliders, in the order in which the insect sheds 

 them, that, by comparing them together under the 

 microscope, we may ascertain with precision the progress 

 made during each change. 



When the crysalis is divested of its envelope,.it remains 

 apparently inert on some neighbouring plant for a few 



