Evolution and Taxonomy 



79 



in, 



of costa of the hind wings is preserved. This is well shown in 

 Papilo (PI. II, 2). This vein has been observed and figured by 

 niany writers ; but it has always been considered a precostal 

 (/. <?. , humeral) vein. But I believe it is essentially different 

 from the humeral vein or veins of the Lasiocampidae. The hu- 

 meral veins of this family of moths are secondary developments ; 

 while the spur in the humeral angle of butterflies is a rem- 

 nant of one of the primitive veins, the costa. 



In the hind wings of many moths a coalescence of subcosta 

 and radius also 

 takes place to a 

 greater or less ex- 

 tent. These two 

 veins may be joined 

 for a short di.stance, 

 as in Packardia, 

 (Fig. i6), or they 

 ma}^ be merged in- 

 to one for a consid- 

 erable proportion of 

 their length as in 

 Megalopyge, (Fig. 

 25), and in theSesi- 

 idae. Bvery degree 

 of coalescence be- 

 t w e e n these ex- 

 tremes exist. 



Fig. 22. — Thyredopteryx . 



It has been customary in cases like the last to consider the 

 subcostal vein wanting ; but it is only necessary to count the 

 branches of the principal veins to see that 

 all are present ; moreover, in most cases the 

 two coalesced veins are separate for a short 

 distance near the base. 



There are, however, forms in which the 

 basal part of radius is wanting. This con- 

 dition is brought about in this way. First, 

 something interferes with the growth of the 

 basal part of radius, and this vein becomes weaker than the oth- 



F1G.23. — Diagram 

 of part of hind 

 wing of Thyre- 

 dopteryx. 



