THE WINGS OF LEPIDOPTERA 341 



the third anal vein when this vein is two-branched. The distal parts of the 

 two veins coalesce, while the basal parts remain separate; and thus is 

 formed a single vein which is forked at the base. 



The wings of Caccecia (Fig. 353), illustrate several of the conditions 

 described above. In the fore wing, the first anal vein is represented by the 

 anal furrow, except at the tip where a short vestige of it remains; in the 

 hind wing this vein is well-preserved. In both wings, the second anal vein 

 is forked at the base. In the fore wing the third anal vein is represented 

 only by the hind branch of this fork ; while in the hind wing both branches 

 of the two-branched third anal vein persist; the first branch being joined 

 to the second anal vein and the second branch extending free to the margin 

 of the wing. In a case of this kind, the identity of the first branch of the 

 third anal vein is not obvious ; but studies of the tracheation of the wings of 

 pwpse show clearly that the apparent forking at the base of the second anal 

 vein is due to the coalescence of this vein with the first branch of the third 

 anal vein. This is shown in the figure of the fore wing of a pupa of Pieris 



(Fig- 333)- 



While the extent to which the reduction of the anal area has proceeded 

 may merely indicate the degree of divergence from the primitive type, a 

 wing ha\ang two anal veins being more specialized than one having three 

 and less specialized than a wing having a single anal vein, a comparison of 

 the extent of the reduction in the two pairs of wings in different insects may 

 show a difference in kind of specialization indicating a dichotomous division 

 of a line of descent. 



As an illustration of an application of this principle the separation of the 

 families Papilionidee and Pieridas, which were formerly classed together as a 

 single family, may be cited. In the fore wings of the Papilionidae (Fig. 

 350) all three of the anal veins are at least partly preserved, while in the 

 hind wings there is only a single anal vein. On the other hand in the 

 Pieridffi (Fig. 342) the anal area of the fore wings is more reduced than the 

 anal area of the hind wings, the former having a single anal vein, the latter 

 two. From this it follows that in tiie Papilionidas the reduction of the anal 

 area of the hind wings preceded the reduction of the anal area of the fore 

 wings; while in the Pieridas the reverse was the case. It is evident there- 

 fore that at the time the separation of these two families occun^ed there had 

 been no reduction of the anal areas of either pair of wings. 



The difference in the direction of the migi-ation of the base of vein M2 

 in these two families, already indicated, confirms the conclusion that their 

 separation occurred very early in the history of that division of the Frenatas 

 represented by the butterflies. At the time when it occurred there had 

 been no reduction of the anal areas, and vein Mo had not begun its migra- 

 tion towards either radius or cubitus. This is as generalized a condition of 

 wing structure as exists in any of the living Frenatae. 



