150 EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 



species of the first division are more closely related among themselves than to 

 any of the species of the second division or of Cremastobomhycia. 



An ancestral form, more primitive in respect to generic structure as well as 

 color pattern and transversely marked with seven bands of a uniform pale 

 yellowish color, has evidently given rise first to a series of species in which dark 

 margins have developed earliest on the outer edges of bands — in this case, the 

 white streaks and fasciae are said to be internally dark margined. Later, or 

 perhaps contemporaneously, in another species of the same ancestral strain, 

 the opposite tendency has developed, and there has been evolved a series of 

 species in which dark margins appear earliest on the inner edges of the bands — 

 in this case, the white streaks and fasciae are said to be externally dark margined. 

 A few of these latter species, constituting the subgenus Cremastobombycia, have 

 retained a more primitive generic structure; the remainder have followed the 

 same course of evolution structurally as the species of the first division of the 

 genus. It is, of course, possible that this second tendency originated twice 

 and independently. 



Several factors combine to render it probable that Cremastobombycia (in 

 its modern form) and the ''flat-larval group'' originated at a somewhat later 

 period than the typical LithocoUetis (Fig. 26). The restricted geographical 

 range of the ''flat-larval group'' and of Cremastobombycia, which with one 

 exception are confined to America, indicates their comparatively recent origin. 

 Considerations based upon larval characters confirm this view. The later larval 

 stages of the typical LithocoUetis are identical with those of Gracilaria, which is 

 without question accepted as the ancestor of all these groups. The corresponding 

 stages in the larva of Cremastobombycia have a more flattened and triangular- 

 shaped head, without, however, the structural modifications which have de- 

 veloped in the "flat-larval group." It may be concluded, then, with reasonable 

 certainty, that after the origin of the typical LithocoUetis, the species of the 

 ancestral stock retained for a short period their imaginal structure. During 

 this time, changes in the larval form were initiated. In the ancestral form of 

 Cremastobombycia, development along this line did not progress far, and the 

 larvse have preserved the normal mode of feeding; hence there has been no 

 attendant structural modification. 



Cremastobombycia may have originated after a certain amount of evolution 

 in the color pattern had already taken place; this view is supported by the 

 observation that none of its species conserves as primitive a type of marking as 

 is to be found in several species of the "flat-larval group." 



We may conclude, therefore, that the immediate ancestor of all the groups 

 under discussion was of a somewhat more primitive structure than Cremasto- 

 bombycia as we now know it. The structural relations of the four groups with 

 reference to the ancestral stem are illustrated diagrammatically by Fig. 25; the 

 relative time of origin of each by Fig. 26. The similarity of the color pattern of 

 the species of Cremastobombycia to that of some of the species of the second 



