THE WINGS OF INSECTS 415 



delicate insects only a few hours 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 tracheae, 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 so 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 penetrate the tracheae. In this way most 

 beautiful objects can be prepared, which will show the minutest ramifica- 

 tions of the tracheae. 



Not only can the tracheae that precede the wing veins be studied in this 

 manner, but, if the wings be taken at the right stage, the forming veins will 

 appear as pale bands when viewed by transmitted light. This is due to the 

 fact that at this time the veins are merely cavities, filled with hTtiph, and 

 are more translucent than the spaces between them, which are occupied 

 by tissue. 



Unfortunately, however, this distinction is only temporary in most 

 specimens. As a rule, the entire wing becomes transparent in a few hours 

 after it is mounted in the glycerine-jelly. It is necessary, therefore, to make 

 drawings or photo-micrographs promptly, in order to keep a record of the 

 courses of the veins. 



On the other hand, the tracheae, as a rule, stand out more sharply 

 twenty-four hours after mounting, because of the clearing effect of the 

 glycerine- jelly upon the tissue of the wing. But the making of drawings 

 or photo-micrographs of the tracheae should not be delayed long; for the 

 trachete soon become filled with the jelly, and are then practically invisible. 



The preparation of specimens. — Collect living n\Tnphs or pupae, place 

 them in fonnol (4*^^), and leave them for a time, as indicated above. The 

 formol will make the wings of the insects more translucent ; but it will not 

 remove dark colors from chitin. It is well, therefore, to select, at first, the 

 paler species for study. 



When ready to mount a wing, spread a drop of melted glycerine-jelly 

 on a slide and allow it to cool. 



Dissect off the wing to be studied, taking with it just enough of the 

 thorax to include the basal attachments of the tracheae. The dissection 

 may be made under water; but the wing should be removed from the water 

 promptly, so that the tracheae may not become filled with water. 



Place the wing upon the solidified glycerine-jelly on the slide; and lower 

 upon it a heated cover-glass, which will caitse the jelly to melt enough to 

 envelop the wing. 



