No. 568] BIOLOGY OF THE THY SAN OPT ERA 237 



sexual or only parthenogenetic. Moreover, in the roti- 

 fers, females incapable of fertilization copulate as fre- 

 quently as do those requiring fertilization, as was first 

 shown by the work of Maupas (1890) on the rotifer 

 Hydatina. 



Presence of males and occurrence of copulation are, 

 therefore, no proof of sexual reproduction. But even if 

 we accept, as Uzel does, this criterion of sexuality, Jor- 

 dan's view that there may be an alternating cycle would 

 receive some support if it could be shown that males are 

 more abundant at one season of the year than at other 

 times. Casual observations made by me several years ago 

 seemed to indicate this seasonal variation in the abund- 

 ance of males. As the data then available were meager, 

 no conclusion was drawn, but I subsequently undertook 

 to obtain such data on a larger scale, by making extensive 

 collections at all seasons of the year to determine the sex 

 ratio. The following pages give these data, along with 

 other observations bearing on sex or the life cycle. 



I desire to acknowledge the a>si>tance of my wife, by 

 whom much of the labor of determining species and 

 counting the sexes was done. 



The Sex Ratio in Various Species of Thysanoptera 

 In making collections for the purpose of determining 

 the sex ratio, the food plants were examined very care- 

 fully, torn apart if necessary, and every individual cap- 

 tured. This precluded the possibility of obtaining an 

 erroneous sex ratio because one sex was more easily dis- 

 turbed than the other. A few individuals escaped, but 

 they could not have affected the sex ratio very greatly, 

 and it was known from their size that they were some- 

 times of the one sex, sometimes of the other. 



The sex in the suborder Terebrantia is readily deter- 

 mined by the presence of an ovipositor in the female and 

 the rounded end of the abdomen in the male. In the sub- 

 order Tubulifera, the sex in Anthothrips verbasci was 

 determined by the presence of two short, heavy spines, 



