No. 56S] BIOLOGY OF THE THY SANOPTERA 245 



the probability that there are two classes of sperm asso- 

 ciated with sex, as in the bugs and many other animals, 

 and that therefore the sexes should be approximately 

 equal in numbers. The 40 to 50 per cent, of males in 

 August accord fairly well with this explanation. 



This explanation would not, however, account for the 

 increase in the number of males in late summer in a spe- 

 cies whose life history is much shorter than that of 

 Authothrips rcrbasci. Thus, in Anaphothrips striatus, 

 Hinds states that the entire life history is passed through 

 in 12 to 30 days. Even in a cold season, such as that of 

 1912 at Douglas Lake, therefore, the life history can not 

 have been so long that the first adults would emerge in 

 the middle of August. The increase in the number of 

 males of Anaphothrips in August and September is not 

 to be explained, therefore, as due to the first appearance 

 of a new brood at that time. 



Thrips tabaci likewise affords interesting, even if mea- 

 ger, evidence regarding the seasonal occurrence of males. 

 In this species males are exceedingly rare. Hinds (1902) 

 redescribed the male in quotation marks, from which it 

 is to be inferred that he did not have specimens. Tn my 

 own collecting, though the females were quite common, I 

 never saw a male until the summer of 1912. Then two 

 specimens were taken September 2, as shown in Table I. 

 These irregularly occurring males can hardly be func- 

 tional, so that Thrips tabaci is still probably to be re- 

 garded as wholly parthenogenetic. But their appearance 

 in late summer may be the vestige of a former sexual 

 phase, and may be caused now, as the sexual phase prob- 

 ably was in part formerly caused, by climatic conditions. 



Chirothrips manicatus presented, at Douglas Lake, an 

 anomalous condition. As shown in Table I, and stated 

 more explicitly above, females were abundant in a given 

 small area early in July, but practically no males were 

 present. Then, so far as I could determine by painstak- 

 ing collections, the females disappeared ; almost no adults 

 of either sex, and not many larvae, were to be found. Two 



