No. 568] BIOLOGY OF THE THYSANOPTERA 



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is that males could not be found in the spring until the 

 females had been active long enough to have produced 

 one generation of offspring. Males occur late in autumn, 

 but must perish before the end of winter. Likewise, 

 neither eggs nor larvae live over winter, or larvae would 

 appear earlier in spring. In Thrips physopus, on the 

 other hand, males were found as early as the females; 

 hence, in the absence of any collection earlier than May 

 22, and in ignorance of the time required for develop- 

 ment, I should assume that both sexes survive the winter. 

 Both sexes of Anthothrips verbasci have been seen on 

 dead mulleins in winter. 



In species, like Euthrips tritici, whose males do not 

 survive the winter, if fertilization of the early spring 

 females takes place at all, it must occur in the fall. I do 

 not regard my breeding experiments as proof of par- 

 thenogenesis in this species, but it is by no means improb- 

 able that parthenogenesis occurs. More rigorous experi- 

 ments are needed. 



As regards the mode of reproduction during the rest of 

 the year, there is nothing in the sex ratio, as given in 

 Table I, to suggest an alternating cycle in Euthrips tri- 

 tici. In other species, it would be possible to interpret 

 certain facts to mean that an alternation of partheno- 

 genesis and sexual reproduction occurs, or did once 

 occur. There is a well-marked increase in the proportion 

 of males in Anaphothrips striatus, for example, in Au- 

 gust. This is a particularly interesting species. Hinds 

 (1902) saw only the female of this species, though he 

 mounted and examined over a thousand specimens, and 

 he bred it parthenogenetically in the laboratory for 

 months. What purported to be the male was described 

 by Cary (1902), from Maine, but the specimens described 

 were evidently those of another species. The first males 

 ever recorded were described by Shull (1909), two speci- 

 mens among probably two hundred females. It is re- 

 markable, therefore, that in the vicinity of Douglas Lake 

 there should be nearly 25 per cent, of males. Whether 



