244 THE AMERICAN NATURALIST [Vol. XLVIII 



the presence of numerous males is dependent on climatic 

 conditions, or whether it is a racial difference, there is at 

 present no way of deciding. The weather was unusually 

 cold during the summer in which these records were 

 made, and it is desirable that the effect of temperature be 

 experimentally determined. The presence of males in 

 goodly numbers throughout the summer, the occurrence 

 of copulation in nature, and the failure of an attempt to 

 breed the species parthenogenetically, leave, as the only 

 reason for suspecting that it may have been partheno- 

 genetic at Douglas Lake, the fact that it is parthenoge- 

 netic elsewhere. But if the species is parthenogenetic in 

 one region and sexual in another, it is not difficult to be- 

 lieve that it may be both parthenogenetic and sexual in 

 the same region. It is difficult to decide whether the well- 

 marked increase in the proportion of males in August 

 and early September should be regarded as evidence of 

 such an alternation, or as due to a period of cold weather 

 or other climatic factor, or as a hereditary remnant of 

 the sexual phase of an alternating cycle once possessed 

 by the species. Only experiment, and perhaps cytological 

 study, can decide this question. 



A similar but less marked increase in the number of 

 males is seen in Anthothrips verbasci, also in August. In 

 that month the proportion of males rose from about 20 

 per cent, to 40, or even nearly 50 per cent. In this species 

 the increase may be due to the late date at which the first 

 brood of larvae becomes mature. The life history of this 

 species is longer than that of most of the suborder Tere- 

 brantia, and may appear to be still longer because ene- 

 mies destroy many of the larger larvae. For these rea- 

 sons, in the region of Douglas Lake, the first generation 

 of larvae may not become mature until nearly August. If 

 this assumption is correct, the proportion of males found 

 prior to August is the proportion that survive the winter. 

 This explanation receives support from the cytology of 

 the germ cells. As stated above, there is a lagging chro- 

 mosome in the spermatocyte divisions, which suggests 



