44 THE AMERICAN NATURALIST. (VoL. XXXII. 
the medium in which a wing is mounted for microscopic study. 
they are usually invisible. It is not strange, therefore, that 
they have been studied so little. But in the course of our 
investigations we have devised a method of study of the wings 
of immature insects which renders the observation of the 
tracheze in them a simple matter. 
If a living pupa or nymph be placed in formol (4%) the 
tissues of the wings will be rendered translucent in a short 
time. In the case of very delicate insects only a few hours 
Fic. 1.— Part of a wing of a pupa of Corydalis cornuta. 
are required for this, but with larger ones with more opaque 
wings it is necessary to leave them in the formol for several 
days, or even for several weeks. While the formol renders the 
tissues translucent, it does not soon penetrate the trachex, 
which are, therefore, left filled with air, and appear as dark 
lines when the wing is examined with transmitted light. Just 
after molting some wings are translucent, but there are few S0 
clear that a short stay in formol will not make them clearer. 
In order to study wings prepared in this way, they are. 
removed from the body and mounted in glycerine jelly, care 
being taken to cool the mount quickly so that the jelly will not 
