398 THE WINGS OF INSECTS 



wards, the point of separation of two veins moving nearer and nearer to the 

 margin of the wing until it is reached and the two veins are completely 

 united. The existence of this method of coalescence has been demon- 

 started by studies of series of allied forms in which the successive stages are 

 shown ; such series are easily found in the Lepidoptera, and it was probably 

 by this method that the number of the branches of radius was reduced in 

 Rhyphus and the other Diptera studied in this course. 



By the other method the coalescence begins at the margin of the wing 

 and procedes towards the base of the wing; two series illustrating this 

 method of coalescence are indicated below. 



Coalescence of veins Cuo and 2d A. — Study your drawings of the follow- 

 ing named wings: — 



In the wing of Rhyphus veins Cui; and 2d A are widely separated at the 

 tip. 



In the wing of Paniarhes veins Cuo and 2d A are approximate at the 

 margin of the wing but are still separate. 



In the wing of Erax these two veins coalesce for a short distance at the 

 margin of the wing. 



In the wing of Scenopinus the coalescence of these two veins has 

 progressed to a considerable distance. 



In the wing of Rhamphomyia the coalescence of these two veins has 

 progressed so far that vein Cuo extends towards the base of the wing. 



Coalescence of veins M3 and Cui. — Arrange your drawings of the wings 

 of Tahanits, Erax, and Pantarhes in the order named and note the successive 

 stages in the coalescence of veins M3 and Cui and in the obliteration of 

 cell Ms. 



The uniting of cells. — An-ange your drawings of the wings of Musca 

 domestica, Psilopodiiis sipho, and DoHchopus coquilletti in the order named. 

 Note that in Musca cells M and ist M2 are separated by the free part of 

 vein M3; in Psilopodins the free part of vein M3 is partly atrophied, only a 

 short spur remaining ; and in DoHchopus the free part of vein M3 is entirely 

 lost and consequently cells M and ist M2 are completely united. 



IDENTIFICATION OF THE WING-VEINS AND OF THE CELLS OF THE WINGS IN 



LEPIDOPTERA 



As the wings of Lepidoptera are covered with scales, it is difBcult to 

 determine the nature of their venation without specially preparing them 

 for this puri3osc. After a student has become familiar with the type of 

 venation characteristic of the order, he can usually determine the course of 

 any particular vein by jmtting a drop of chlorofonn on the part of the wing 

 to be examined; this will render the veins more distinct for a few seconds. 

 Or the scales can be removed from a small part of the wing with a small, 



