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THE TRACHEATION OF THE WINGS OF NYMPHS AND OF 



PUP^. 



It has been found that, in the course of the development 

 of the wings of the more generalized insects, the tracheae 

 which traverse the principal veins are developed before the 

 veins appear, and that later the veins are developed about 

 these tracheae. It is evident, therefore, that much light 

 •can be thrown upon questions regarding the homologies of 

 wing- veins by studies of the tracheae which precede them ; 

 and the following suggestions are given to aid students who 

 wish to make such studies. 



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 

 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 rami- 

 fications 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 



