NORTH AMBKICAN EUCOSMINAE. 5 



The male costal fold, on the other hand, is thoroughly unreliable. I 

 have used it only in one place (Thiodia), and there simply for con- 

 venience, to separate an unvp^ieldly genus and there again only be- 

 cause the species which would fall under Thiodia are closely related 

 otherwise and in venation average a considerable advance over those 

 with the fold (3-4 of hind wing are very frequently long stalked or 

 united in Thiodia, seldom so in Eucosma proper). Since the generic 

 name already exists, nothing is added to the synonomy by the separa- 

 tion. I believe that the Eucosminae as a group originated from a 

 form possessing the fold, and that its loss is part of the general 

 progress, but it is too easily and frequently lost in any of the groups 

 for the loss to be significant here. Neither have I been able to use 

 differences in pectination of the male antennae or vestiture of the 

 palpi. The differences are too gradual and slight and as marked 

 between species within a given group as between the groups them- 

 selves. 



In the Olethreutidae, generic divisions have not the same signifi- 

 cance as in groups where natural limitations are sharply defined by 

 consistent structural differences. This is particularly true of the 

 Eucosminae, which is in reality one large composite genus with few 

 or no distinct gaps between the species, and with exasperating spe- 

 cific fluidity. In fact, because of the tendency to modify under local 

 and food-plant conditions, to split off into local races and varieties 

 differing in structure, color, and size, the fixing of specific limits is 

 often a difficult matter. With the genera the limits are even more ob- 

 scure. Fundamental structural differences between the subfamilies 

 are none too rigidly fixed. The transition, for example, from a hind 

 wing with vein 5 bent and closely approximate to 4 at the base to 

 one with 5 straight and parallel with 4 (the Laspeyresiin charac- 

 ter) is not sudden. Indeed, several of the Eucosminae have vein 5 

 well separated from 4, and, in some cases, were it not for habitus and 

 genitalic characters there would be considerable doubt of their posi- 

 tion. With all other characters it is the same. Nothing holds rigidly. 

 There are, however, within the family several definite tendencies at 

 work indicating diverging lines of development. Not all the species 

 are tending the same way. Groups here and there show markedly 

 opposite tendencies, and in the farthest advanced species the result 

 is striking. But the difficulty is that not one or two but many tenden- 

 cies are manifest, and in no two groups is the same proportion or rate 

 of change among the various structures maintained. Some, for ex- 

 ample, will retain a developed uncus while exhibiting an advanced 

 type of neuration. Others again in losing the uncus exhibit a tendency 

 to narrow and split the organ, while still others reduce it in an en- 

 tirely different way. Such tendencies are significant, as they show 

 the influence of heredity, different in the descendants of different 



