On Certain Grass-Eating Insects. 91 



crossing difficult and in time lead to the separation of a species. 

 The great variations in these important organs found in this genus 

 lead us to this conclusion. 



The record of species now living shows that variation in the time 

 of breeding has most probably occurred in the genus, and is even 

 now taking place. C. laqtceatelhis has a compact breeding season 

 of about five weeks ; C. interminellus has a breeding season ex- 

 tending through June and July and with a few individuals ap- 

 pearing the latter part of August, while C. ^nutabilis flies through 

 June, July, "August and into September. See Time of Flight of 

 Species of Crambus. (Table p. 58.) This shows there is great 

 variation in the length of the period of flight. If variation in time of 

 flight is the first step in the separation of a species, we would expect 

 closely allied species to have nearly the same period of flight. 

 Secondly, we should expect that only the most specialized forms 

 would fly earliest and latest. This is also borne out by the record. 

 C. laqiieatelliis, mutabilis, and C. vulgivagelliis are highly spec- 

 ialized forms and stand at the heads of different lines of devel- 

 opment. The periods of flight of these species diverge most 

 widely from the time when the more generalized forms fly. The 

 closely allied species of these highly specialized forms have periods 

 of flight which overlap the periods of their more specialized 

 relatives. 



That variation occurs in the genitalia is shown by members of 

 the Unistriatellus Group. With the exception of C unistriatelhis , 

 I have been unable to separate the species of this group by the 

 character of the genitalia. 



The genus Cra?nbiis has been adapted to its environment by a 

 multiplication of species, which resemble each other in many 

 respects, yet are well separated, as a rule, by variations in the 

 male genitalia. A glance at the time of flight of the species 

 shows that members of the various species fly throughout the 

 growing season and that they are most destructive in June and 

 July, the time when their food is most abundant. 



