Ii8 



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

 lymph, 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 glyce- 

 rine-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 trachese, 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 trachese 

 soon become filled with the jelly, and are then practically 

 invisible. 



The preparation of specimens. — Collect living nymphs 

 or pupcX, place them in formol (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 

 cause the jelly to melt enough to envelop the wing. 



Cool the mount quickly on ice, a marble slab, or some other 

 cold object. Rapid cooling is imperative, for in melted glyc- 



