416 THE WINGS OF INSECTS 



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

 Rapid cooling is imperative, for in melted glycerine- 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 tracheas can be followed only with 

 difficulty. 



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

 tracheation of the wings of nymphs and of pupag will be original investiga- 

 tions. 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. 



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 illus- 

 trate specialization by reduction; the latter specialization by addition. 

 During the winter, when it is difficult 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 devel- 

 oped. 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 identified easily, and it will serve as a starting point for the determi- 

 nation of the homologies of the other principal tracheae. 



In most orders of insects the longitudinal veins can be distinguished 

 from the cross-veins by the fact that the cross-veins are not preceded by 

 tracheae. 



In some of the many-veined insects, as Odonata, the cross-veins, as well 

 as the longitudinal veins, are preceded by tracheas; there being, in these 

 insects, a great multiplication of tracheas. 



On the other hand, in the Trichoptera, Diptera, and most Hymenoptera, 

 a great reduction of the tracheal system has taken place. It is not well, 

 therefore, for the student to begin his studies of this subject with members 

 of either of these orders. 



In those orders where a specialization of wing-veins by addition has 

 taken place, the accessory longitudinal veins are preceded by tracheas. 



Finally, it should be remembered that it is not safe to base conclusions 

 upon the study of a single insect; a large series, representing as many 

 genera and families as is practicable, should be investigated. 



