114 



several types of branching of the radial sector. Each of 

 these figures represents a condition which exists in some 

 insect. 



Figure iia represents the typical or dichotomously 

 branched radial sector. Figure i ilf represents a typical ra- 

 dial sector with the addition of some accessory veins on the 

 caudal side of vein I^^. In this case the radial sector is 

 nearly pectinate, but not quite so, owing to the forked con- 

 dition of vein ^44.5. In the form represented by Figure jic 

 veins J^^ and J^^ coalesce to the margin of the wing ; and in 

 this way the pectinate type is attained. In another insect 

 (Fig. 11^) the pectinate type has been attained by fission in- 

 stead of coalescence. Here veins i?^ and J?^ have split apart 

 till vein i?^ arises from the main stem of radius. 



When many cross-veins are present, the dichotomy of the 

 branching of the sector may be suppressed in still another 

 way, by the transference of the base of vein J^^ to vein 

 J?^+^. All stages of this switching of the base of vein R^ 

 occur in the ant-lions ; and two of them are represented by 

 Figures ii<? and 11/. 



In the genus represented by Figure 11^ the base of vein 

 /?^ appears to be forked ; one arm of the fork arising from 

 vein -^g, the other from vein ^3+3. The former is the true 

 base of vein i?^ ; the latter is a cross-vein which is assuming 

 the function of a base of this vein. In the genus repre- 

 sented by Figure 11/ the switching has been completed, 

 vein ^^ arising from vein J^,+y 



In the foregoing illustrations comparisons of allied insects 

 have been made in order to determine the ways in which 

 the wings are being modified ; frequently a comparative 

 study of the fore and hind wings of a single insect is equally 

 suggestive, for it often happens that the two pairs of wings 

 exhibit different degrees of the same kind of modification, 

 and thus the course of the change is indicated. 



