246 THE AMERICAN NATURALIST [Vol. XLVIII 



weeks later, however, males were found in the same area 

 in large numbers. As these males were wingless, they 

 had probably not immigrated. The only other explana- 

 tion that occurs to me is that the larvae were present in 

 considerable numbers at the time of the earlier collec- 

 tions, but in the flowers, not among the spikelets of the 

 timothy, so that I did not discover them; and that the 

 female larvae reached maturity much earlier than the 

 males. In any case, it is difficult to see how the males can 

 have been functional, when the two sexes occurred at dif- 

 ferent times. If such conditions recur frequently, Chiro- 

 thrips manicatus, even though it produces many males, 

 must be parthenogenetic. 



Summary 



The principal conclusions reached in the second part of 

 this work may be stated as follows : 



Some species of Thysanoptera pass through the winter 

 in both sexes, in others the males perish. In none of 

 those studied does the egg or larva live over winter. 



Pupation of most of the species of Thysanoptera stud- 

 ied occurs on the food plants where the larvae live, not- 

 withstanding that the pupae seldom appear in collections. 



From the determination of the sex ratio, Euthrips 

 tritici shows no indication of an alternating life cycle. It 

 is probably sexual throughout the active season, though 

 this is not proven. 



Chirothrips manicatus occurred abundantly in both 

 sexes, but the two sexes appeared at different seasons. 

 The explanation of this phenomenon is doubtful. 



An increase in the number of males in Anthotli rips 

 verbasci in late summer may be explained as due to the 

 great length of the life history and to selective mortality 

 during the winter, without assuming an alternating life 

 cycle. 



Anaphothrips striatus, a species which has hitherto 

 been known almost wholly in the female sex, produced 

 about 25 per cent, of males at Douglas Lake. This may 



