142 EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 



the others toward the adult condition; as in tiliacella, development must take 

 place more rapidly, since its margins reach their adult condition before those 

 of the other bands. Here, also, is the first manifestation of that tendency, so 

 characteristic of all the more highly differentiated species of the genus, toward 

 the darkening of scales in the apex. In this species, these scales are scattered 

 over the entire apex, instead of being limited to a definitely defined area where 

 the scales are modified structurally. This species does not, at any stage, show 

 the primitive type of marking; a considerable amount of development is, however, 

 necessary before the color areas attain their definitive final extent. 



In L. cratcegella (Fig. 36, PI. Ill), ostrycefoliella (Fig. 23, PL III), lucidicostella 

 (Fig. 44, PI. Ill), hageni (Fig. 17, PL III) and morrisella (Fig. 48, PL III), in the 

 basal half of the wing, where in the adult there is the greatest divergence from a 

 transversely banded type of marking, the color areas, upon the first appearance 

 of a uniform pale yellowish, have almost exactly the same shape and extent as 

 in the adult. They undergo very little change during development, except that 

 in these species, as weh as in tiliacella and tritcenianella, there is a progressive 

 and gradual deepening of the ground color. In the youngest specimens of 

 ostrycefoliella observed (Fig. I4), the first pair of streaks are still connected; 

 very soon, however, they become separated by the union of Band III with IV. 



In the apical half of the wing. Bands V, VI and VII appear as straight 

 transverse bands (except in morrisella) ; the transformation of the unpigmented 

 fasciae between them and between IV and V into pairs of opposite streaks is 

 brought about by the outward extension of Bands IV, V and VI along the cell 

 in a manner similar to that previously observed for Band IV in tiliacella and 

 tritcenianella. The obliteration of a white streak is brought about by the inward 

 extension of the extremity of a band to meet the outer edge of the preceding 

 band. In morrisella, the pattern is predetermined from the outset, the white 

 fascia and streaks being produced by structurally modified scales, which are 

 destined never to acquire pigment. 



There are no reasons favoring the view that such a type of marking as that 

 found in cratcegella (Fig. 36, PL III) is primitive: there are many reasons for re- 

 garding it as having been produced through a long process of evolution. It is a 

 matter of general observation that the pale colors appear first ontogenetically, 

 black of the pigmental colors being last to appear; it has been inferred that the 

 same is true also for the phylogeny. In the adults of these species, there have 

 developed, in addition to the usual dark markings adjacent to the unpigmented 

 areas, definitely limited black spots and streaks in the apex. The scales thus 

 pigmented are also differentiated structuraUy from the surrounding scales by 

 their smaller size. There is no such structural modification of the dark tipped 

 scales in the apex of tritcenianella. Obviously a considerable period of time 

 must have been required for the final differentiation of such characters as these. 

 That these species have undergone an extensive process of differentiation in the 

 production of certain characters of a kind which can only be acquired late in 



