82 



John Henry Comstock 



V, 



In the fore wings of the Psychidae it frequently happens 

 that the basal part of vein VIII disappears, and then vein IX 

 appears to be forked outwardly (Fig. 22). 



A good illustration of the coalescence of principal veins in 



another order of insects is presented by the dragon-flies {^Odo- 



natd). Here veins III, IV and V are united into one from 



the base of the wing to the arculus. This coalescence is from 



the base of the wing outward, as is the coalescence of the 



main stem of media with either radius or cubitus in the Lepi- 



doptera. But most instances of coalescence in the Lepidoptera 



begin on the disc of the 



m.^Hj m3+4 wing and extend in 



m 



either direction. In 



the Diptera a third 

 mode of coalescence is 

 common. In this order 

 it frequently happens 

 that two longitudinal 

 veins come together at 

 their tips and unite, the 

 coalescence proceeding 

 from the margin of the 

 wing towards the base. 

 The result is that a cell 

 which normally opens 

 on the margin of the wing is closed at a greater or less distance 

 before the margin ; and the extent of this distance will be an 

 indication of the degree of divergence from the primitive type. 

 The coalescence of two veins may be complete resulting in 

 the reduction of the number of veins in the wing. This fre- 

 quently happens especially with the branches of radius of the 

 fore wings in the Lepidoptera. This vein is naturally five- 

 branched ; when a less number of branches occurs it is because 

 the coalescence of some of the branches has proceeded to 

 the margin of the wing. 



The number of veins in the wing may be reduced, however, 

 in another way : a vein may simply fade out. The most com- 

 mon instances of this kind in the I^epidoptera occur in the 



IX vin 

 Fig. 26 — Acaloiihus. 



