130 University of California PuUications. [Entomology 



There is but little similarity between the venation of the 

 wings of these three groups. The venation in each case is repre- 

 sented by only a few veins; all the cross veins present are 

 specialized, and almost as constant and important as the lon- 

 gitudinal veins. This cross-vein specialization is the keynote 

 of the venations of the Neoptera. 



DIPTERA. 



The higher flies possess a venation which is very character- 

 istic and uniform. In no group is there as large a number of 

 species with so little variation in their vein arrangement as is 

 true in the higher families of flies. The lower flies present 

 abundance of variation in venation, so that at times it is 

 rather difficult to carry out homologies with them. 



The character which at once sharply separates the flies from 

 all other insects is the special modification of the hind wing 

 into an organ only remotely associated with flight. This 

 requires a somewhat different development of the front wing 

 to suit it for independent action. In other orders, where the 

 hind wungs are greatly reduced, there is still some connection 

 between the two wings. In the mayflies, for example, either 

 the reduced hind wing overlaps the base of the front wing, or, 

 in case it is so narrow that it would too readily slip off, there 

 has been developed a hook-like structure that holds the wings 

 together in much the same way as occurs in all Hymenoptera. 

 In the males of scale insects the hind wings are minute and 

 rod-like, but here, again, they are hooked to the front wings. 



In conformity with the peculiarities of Diptera just pointed 

 out, the venation exhibits a character in the hind areas which 

 contrasts strongly with that found in any front wing, except 

 in those Neuroptera where the wings remain somewhat inde- 

 pendent; but in these cases the resemblance is evidently not a 

 true homology. In the Sialidaj and other related neuropterous 

 insects, in the posterior region of the wing are three veins that 

 arise from a common center and are bound together by cross 

 veins, producing basal cells in the manner so common in 

 Diptera, but every other indication is to the effect that the 

 veins inclosing the basal cells in Diptera lie in front of those 

 surrounding similar cells in the Neuroptera. We doubtless 

 have here analogous structures produced by similar mechanical 

 needs. 



