THE WINGS OF LEPIDOPTERA 325 



reduction; and, as already stated, this coalescence almost invariably pro- 

 ceeds outward. This method of the coalescence of the branches of a vein 

 is often shown, byastudy of aseries of allied forms in which different degrees 

 of it can be observed, the point of separation of two branches being nearer 

 to the margin of the wing in successive forms until the margin is reached and 

 a single vein remains where there were two before. 



There are also many instances where the reduction is due to the fading 

 out of veins. Frequently where a vein has atrophied a vestige of it remains, 

 either as a faint line in the position formerly occupied by the vein, or as a short 

 fragment of the vein; but in other cases no trace of the lost vein exists. 



{d) THE PRIMARY DIVISIONS OF THE LEPIDOPTERA INDICATED BY THE 

 STRUCTURE OF THE WINGS 



In papers published in 1892 and 1893 I called attention to the fact that 

 in certain moths the two wings of each side are united by an organ borne by 

 the fore wing which I termed the jugum and in all other Lepidoptcra the 

 wings are united either by an organ borne by the hind wing, which had 

 long been known as the frenulum, or by a substitute for the frenuliun, a 

 greatly expanded humeral angle of the hind wing. 



The discovery of the fact that there are two distinct modes of uniting 

 the wings during flight suggested the inference that in the primitive Lepi- 

 doptcra the wings were united in neither way ; for it is not easy to see how 

 one mode could have been developed from the other. 



Correlated with each of the two methods of uniting the two wings of 

 each side there is a distinctive feature of the wing-venation. In those 

 moths in which the wdngs are united by a jugum the venation of the two 

 pairs of wings is similar ; in other Lepidoptera there is a striking difference 

 in the venation of the two pairs of wings, due to the fact that in the hind 

 wings the radius is greatly reduced. 



If in the primitive Lepidoptera there were neither a jugum nor a frenu- 

 limi, and in some of the descendants of these primitive Lepidoptera there 

 was developed a jugum and in others a frenukun, there arose in this manner 

 two distinct lines of descent. These supposed two lines of descent I con- 

 sidered of subordinal value and proposed the temis JugatcB and FrenatcB 

 for them respectively. 



Suborder Jugatse. — The suborder Jugatae includes the descendants of 

 those ancient Lepidoptera in which a jugum was developed and in which 

 the venation of the two pairs of wings is similar (Fig. 334)- 



To these, the most obvious characteristics of the Jugatae, can be added 

 the following: In the Jugatas the cubitus and the first anal vein of the 

 fore wings coalesce at the base of the wing; the basal portion of the free 

 part of cubitus has the appearance of being an oblique cross vein; and a 



