THE TRACHEATION OF WINGS 15 



"While this increases the difficulty of determining the homologies of the 

 wing-veins, it is oftet\ of great aid in taxonomic work, for it may afford an 

 indication of the degree of divergence from a primitive type in the structure 

 of a wing ; and when a series of forms is studied the course of this divergence 

 is often clearly indicated. 



"The figure also show that in some cases what appears as a single vein 

 is formed about two closely parallel trachea}. This is shown in the case of 

 the bases of the second and third tracheae, counting from the costal margin 

 of the wing, the radial and medial tracheee. This illustrates a fact of 

 frequent occurrence, that what appears to be a single vein may be formed 

 by the coalescence of two primitive veins. 



"In this figure the medial trachea appears not to extend to the base of 

 the wing. This is due to the fact that in the preparation photographed the 

 mounting medium had penetrated it for a distance, rendering the basal 

 portions of it invisible. 



"We will not go farther into the discussion of the technique of this 

 method of study. Enough has been said to show that we have at hand a 

 comparatively simple method of determining those questions of homologies 

 of wing-veins that have sorely puzzled all investigators that have attempted 

 to deal with them, and to indicate the nature of the material upon which we 

 have based the conclusions that we purpose to offer in succeeding chapters 

 of this paper" (Comstock and Needham 'qS-'qq). 



Following the definite delimiting of the channels through which the 

 tracheae pass, which results in the formation of the vein-cavities described 

 above, there takes place, during the final stages in the development of the 

 wings : first, a segregation of the hypodermis about the vein-cavities ; and 

 later, the production by this hypodermis of the thickened lines in the cuti- 

 cula of the wing that constitute the walls of the wing-veins. In fact the 

 hypodermal layers of the wing-sac are almost completely used up in the 

 formation of the cuticula of the wing, the wing membrane and the wing- 

 veins. Almost no trace of the hypodermis is to be found in the wings of 

 adults. 



The hypothetical primitive type of insect wings. — The most important 

 result obtained by a study of the tracheation of the wings of nymphs and of 

 pupae was the demonstration of the truth of the conclusion, already reached 

 by the study of the wings of adults, that the wings of all orders of insects are 

 modifications of a single primitive type ; and that consequently it is possible 

 to homologize the wing-veins of any of the orders with those of any other 

 order. 



Comstock and Needham made an extended comparative study of the 

 tracheation of the wings of n>Tnphs and of pupae of the several orders for 

 the purpose of determining what features were common to all; for, as it 

 is evident that features common to all must have been inherited from a 



