110 EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 
exceedingly elongate, linear scales, so that toward their bases they appear 
almost as fine as the cilia themselves. Such a scale, one of the blue iridescent 
scales in the cilia of L. crategella, is illustrated by Fig. 5. The scales forming 
the apical spot, a characteristic of a considerable group 
of species, are smaller than the average and proportion- 
ately broader and are of almost uniform size. The scales 
in the black patch before the apex of such species as L. 
crategella are very similar to these. 
All of the scales just described are marked with a 
Ped. Wide. series of nearly parallel strize, there being six or eight 
such ridges extending into each tooth. 
In a few species, which for additional reasons to be adduced later are to be 
regarded as among the farthest advanced phylogenetically, a peculiar highly 
specialized type of scale has developed. The 
extremely brilliant luster of the white mark- 
ings of L. morrisella and ostensackenella and 
P. desmodiella and of the bronzy purple me- 
tallic reflections on the thorax and base of the wings of P. desmodiella is pro- 
duced by these scales. These white scales, one of which is illustrated by Fig. 
6, are broader than the usual type, and the sides curve outward from the base. 
The outer edge of the scale, instead of being coarsely and acutely dentate as in 
__ the ordinary scales, is obtusely dentate or, sometimes, the teeth 
are entirely absent. The edge then appears scalloped, in a man- 
EE 
LIES 
Fie. 5. Apical wing scale. 
== пег similar to the edge of a pecten. The depressions between 
= the striæ, which appear rather broader and more widely sepa- 
. . rated than usual, are concave. The concavities between these 
Fic. 6. White wing ; š 2 ; | 
scale of L.morrisella. Striæ appear as if formed by a sloping out of the sides of the 
strie. The assumption of a concave surface will explain the 
brilliant and slightly opalescent luster of these scales; the various colors being 
produced by essentially the same processes as they are in a soap film. These 
scales contain no pigment whatever; the effects observed are due entirely to 
the effect of light passing through a medium of varying thickness. It may be 
noted here that in L. robiniella, a species very closely related to L. morrisella, 
this differentiation has not reached the same degree of perfection. The white 
scales are, however, broader and with fewer stri: than those of the more typical 
species. The metallic purple scales of the base of the wings and thorax of P. 
desmodiella are very similar to the white scales structurally but differ in that 
they contain a brownish pigment. Their metallic iridescence may undoubtedly 
be ascribed to the same agencies. 
(c) COMPARATIVE STUDY OF THE ADULT MARKINGS. 
The following study of the adult markings was instituted for the purpose 
of discovering whether, as was suggested earlier in the paper, the primitive 
