119 



erine-jelly the tracheae soon become filled, and the smaller 

 ones are then invisible. 



It is imperative, also, that the wings be handled with care. 

 Being sac-like structures, the tracheae are almost free within 

 them, and a slight pinch with forceps in the middle of the 

 wing may throw all of its tracheae out of place. It is better 

 to lift the wing by its thoracic attachments or upon a section 

 lifter. 



Not every nymphal wing is fitted for this study. Just before 

 molting, and especially just before the last molting, the wing 

 becomes so crumpled within its old sheath that the course of 

 its tracheae can be followed only with difificulty. 



The method of study. — So far as is practicable, the 

 studies of the tracheation of the wings of nymphs and of 

 pupae will be original investigations. For this reason, no 

 particular species is suggested for study. The student will 

 select the most available material, and will endeavor to make 

 an addition to our knowledge of this subject. The series of 

 articles in the American JVa finalist, already referred to (see 

 page 86, note), may be used as a work of reference. A few 

 suggestions, drawn from the studies upon whicli those arti- 

 cles were based, are given here : — 



Among the more available subjects for the beginner in 

 this line of work, are the pupae of moths and the nymphs of 

 Orthoptera. The former illustrate specialization by reduc- 

 tion ; the latter specialization by addition. During the win- 

 ter, when it is difificult to collect Orthoptera, the nymphs of 

 stone-flies (Plecoptera) may be used instead. These can be 

 -found under stones in the beds of streams. 



In many insects the costal trachea is wanting or is but 

 slightly developed. It does not follow, therefore, that the 

 trachea nearest the costal margin of the wing is the costal 

 trachea. In most cases, the radial trachea can be identi- 

 fied easily, and it will serve as a starting point for the 



