66 



AN INTRODUCTION TO ENTOMOLOGY 



short fragments of the veins. A much more common way in which 

 the number of veins has been reduced is by the coalescence of adja- 

 cent veins. In many wings the basal parts of two or more principal 

 veins arc united so as to appear as a single vein; and the nimibcr of 

 the branches of a vein has been reduced in very many cases by two or 

 more branches becoming united throughout their entire length. 



When a vein consists of two or more of the primitive veins united, 

 the name applied to the compound vein should indicate this fact. In 

 the wing of Rhyphus (Fig. 78), for example, radius is only three- 

 branched; but it would be misleading to designate these branches as 

 Ri, R2, and R3, for this would indicate that veins R4 and R5 are lacking. 

 The first branch is evidently»Ri ; the second branch is composed of the 



Fig- 79- — -^ wing of Tabanv.s. 



coalesced R2 and R3, it is, therefore, designated as R2+3; and the 

 third branch, which consists of the coalesced R4 and R5, is designated 

 as R4+5- 



A second method of coalescence of veins is illustrated by a wing of 

 Tahanus (Fig. 79). In this wing the tips of cubitus-two and the 

 second anal vein are united ; here the coalescence began at the margin 

 of the wing and is progressing towards the base. The united portions 

 of the two veins are designated as 2d A+Cu2. 



When it is desired to indicate the composition of a compound 

 vein it can be readily done by combining the terms indicating its 

 elements. But in descriptions of hymenopterous wings where a 

 compound vein may be formed by the coalescence of several veins the 

 logical carrying out of this plan would result in a very cumbersome 

 terminology, one that it is impracticable to use in ordinary descrip- 

 tions. In such cases th.e compound vein is designated by the term 

 indicating its most obvious element. Thus, for example, in the fore 



