Evolution and Taxonomy 



In the hind wings of all Lepidoptera, except Hepialis (Fig. 

 27) and Micropteryx (Fig. 28), all of the branches of radius 

 are united into one. But the condition of radius in the two 

 genera named shows that it is normally five-branched in the 

 hind wings as well as in the fore wings. 



In the discussion of media, given on a previous page, atten- 

 tion was called, so far as concerns its coalescence with other 

 veins, merely to its branches ; but the principal stem of this 

 vein may become joined either to radius, as in the fore wing 

 of Castnia (Fig. 15), or to cubitus, as in the fore wing of 

 Prionoxystus (Fig. 12). 



It will be observed that here is a character which is of value 

 as indicating a dichotomous division of the line of descent. 



I do not recall any instance where cubitus is coalesced with 

 an anal vein to a marked degree, except in the Papilionidae 

 (PI. II, Fig. 2) ; but the 

 growing together of dif- 

 ferent anal veins is a 

 very common occur- 

 rence. This condition 

 is preceded phylogenet- 

 ically by the formation 

 of a cross vein. Such 

 a vein exists between 

 veins IX and XI of the 

 fore wings of Castnia 

 (Fig. 15), and between 

 veins VIII and IX of 

 Thyridopteryx (Fig. 22) . 

 Following this stage the 

 two veins are drawn to- 

 gether. See veins IX 

 and XI of the fore wings 

 of Thyridopteryx (Fig. 

 22), and the same veins 

 \u Megalopyge (Fig. 25). Usually, however, when these veins 

 are joined in this way, that part of vein XI beyond the point of 

 union disappears, and vein IX presents the appearance of 

 being forked towards the base. See Adoneta (Fig. 17). 



Fig. 25. — Megalopyge. 



