CHAPTER II 

 THE TRACHEATION OF THE WINGS OF INSECTS 



A Study of the developing wings of nymphs and of pupse has shown that 

 in the more generahzed orders of insects the principal, longitudinal wing- 

 veins are formed about tracheae, which extend into the wing early in its 

 development and before the beginning of the fonnation of the wing-veins. 



As these trachea are very constant in number and position, they furnish 

 the most available data for determining the homologies of the wing-veins 

 that are later developed about them. 



An account of the earlier publications on the relation of the tracheation 

 of the wings to the wing-venation is given in the preceding chapter. A 

 comparative study of the tracheation of the wings of the several orders of 

 insects for the purpose of determining the homologies of the wing-veins was 

 first made by Comstock and Needham ('gS-'gg). That work of this kind 

 had not been undertaken previously was doubtless due to difficulties that 

 stood in its way. 



The tracheae of the wings of pupae and of nymphs are often very delicate ; 

 and, when filled with the mediiun in which a wing is mounted for micro- 

 scopic study, they are usually invisible. It is not strange, therefore, that 

 the study of them was so long delayed. 



Method of study of the tracheation of wings. — In the course of our 

 investigations Dr. Needham and I devised a method of study of the wings 

 of immature insects, which renders the observation of the tracheee in them a 

 simple matter. A description of this method, compiled from our joint 

 account of it, with a few changes made necessary by subsequently obtained 

 information is given in the outline of laboratory work included in Chapter 

 XXVI of this vohmie. 



By this method most beautiful objects can be prepared, which will 

 show the minutest ramifications of the tracheae. Plate II is a half-tone 

 reproduction of a photograph of an object prepared in this way. This 

 figure represents a small portion of a wing of a pupa of Corydalus cornutus. 



The development of wing-veins. — "Not only can the tracheae that 

 precede the wdng-veins be studied in this manner, but if the wing be taken 

 at the right stage, pale bands can be seen which indicate the position of 

 channels about which the veins are later developed; this is shown in the 

 figure of a wing of a pupa of Sialis (Fig. 2). An examination of a cross- 

 section of a developing wing (Fig. 3) will show the nature of these channels, 

 or vein-cavities, as they may be termed. 



"In all insect wings the two plates of the hypodermis constituting the 

 wing fold are at first separate, i. e. not fused internally. At the time when 



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