EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 147 



to that of the species of the '^flat-larval group.'' The margins appear almost 

 simultaneously^ the scales changing directly from colorless to gray and thence to 

 black. These scales, at the time the yellow ground color spreads over the wing, 

 are not fully formed, thus contrasting with the similarly unpigmented, but 

 fully developed scales belonging to the white fascia and streaks. In all other 

 species examined, the scales were all equally and fully formed at the time of the 

 first appearance of pigment in any part of the wing. 



In many species, these dark streaks adjacent to white are the only defined 

 dark markings present; there may be a few scattered, darker tipped scales in 

 the apex. An increase in the amount of dark pigment producing additional 

 dark markings manifests itself in either of two ways. (1) The number of scales 

 thus affected may be increased, so that the actual extent of the wing occupied 

 by dark tipped scales is greater. This is the direction that evolution has taken 

 in the species of the '' flat-larval group'' (illustrated on Plate IV) and in a few of 

 the species of the '^ cylindrical-larval group" {e. g., celtisella, Fig. 56, PL III, and 

 celtifoliella, Fig. 4, PL III). Often, among the more highly differentiated of the 

 species of the '^ flat-larval group," the entire apex of the wing is covered with 

 black tipped scales {e. g., agrifoliella^ Fig. 79, PL IV). Similar areas may develop 

 in other parts of the wing; thus a patch of dark tipped scales may extend outwards 

 from the angle of a fascia (e. g., bethunella, Fig. 84, PL IV). (2) There maybe a 

 concentration of pigment in comparatively few scales, which with increasing 

 differentiation tend to be segregated in definitely limited areas marked out by 

 structural modifications of the scales (e. g., lucidicostella, Fig. 44, PL III, and 

 cratcegella, Fig. 36, PL III). In these cases, the scales are pigmented almost to 

 their bases, not merely dark tipped. With the single exception of obstrictella 

 (Fig. 64, PL IV) among the species of the second division of the genus, this 

 tendency is confined to the species of the '^ cylindrical-larval group," where it 

 manifests itself in the apex either as an apical dot or as a somewhat elongate or 

 irregular patch of scales, broadest just before the tip of the wing. A similar 

 longitudinal streak of black scales is present in the fold in robiniella (Fig. 50, 

 PL III), uhlerella (Fig. 49, PL III), and morrisella (Fig. 48, PL III). 



Finally, there may be a decided deepening of the ground color itself between 

 the white streaks. This process has reached its highest development in robini- 

 ella, uhlerella, and morrisella. 



In the ontogeny, the development of all of these specializations has been 

 much abridged, and concomitant with this, their time of appearance has been 

 pushed back farther and farther into the earlier stages of pupal development. 

 The dark tipped scales remain colorless during the time the yeflow scales are 

 attaining their adult condition; later, they change directly to gray and black. 

 The pigment in the areas of dark tipped scales in the apex and beyond the fascise 

 often appears simultaneously with that of the margins. Some variation is to 

 be noticed here; in L. bethunella, the gray pigment appears in all the scales 

 beyond the fascia, simultaneously with the beginning of the formation of the dark 



