Vol. 1.] Woodwo)-th. — Wi)ig Veins of Insects. 47 



origin in the precursor of the wing entirely distinct from the 

 trachea- -suggests the idea that no essential relation exists, or 

 ever has existed, between these structures. 



This view^ is further supported by the well-known fact that 

 there is considerable variation in the tracheation within a group, 

 certain veins being in some cases with or in others without 

 trachea^, though otherwise not different, and particular vein-s 

 being served from entirely different trachea\ These variations 

 occur in the lower groups, as well as in the higher; in the 

 former at least, a definite and constant relationship should be 

 maintained. 



A large amount of very strong evidence would be needed to 

 explain away the essential identity of structure in tracheated 

 and non-tracheated veins; the evidence obtainable seems all to 

 indicate identity rather than difference. We must conclude, 

 then, that the presence or absence of trachese is an incident 

 of structure of no special significance in comparing veins. 



Other evident differences in the structure of veins and in the 

 degree of constancy in structure or position within the same 

 species or small group, where homology is easily traced, have 

 given occasion for the idea that there are two different kinds 

 of veins. But the line separating these two sorts is placed dif- 

 ferently by different authorities, in the ways already pointed 

 out. The structural differences in addition to those already 

 discussed in this section are mostly matters of size and pig- 

 mentation. 



In reference to the size of veins, it is only necessar}' to point 

 out that while, in particular cases, veins can be readily grouped 

 into two classes on the basis of relative size, still one need "go 

 but a short way in the study of related forms to find that this 

 character is of no significance. Longitudinal and cross veins 

 are the ones in which differences of this kind most commonly 

 occur. In most cases no decided difference can be made out, 

 and often Avhere cross veins have become parallel with the 

 costa they are so similar as to be easily confounded with true 

 longitudinal veins. 



The pigmentation of veins is a very characteristic and rather 

 complicated matter. We must recognize at least three topo- 

 graphically distinct regions of pigmentation as occurring on 

 veins. The pigments of these three regions are very uniform, 

 all being brownish or black oxidization products in the cuticle. 



