BIOLOGY OF THE THYSANOPTERA. II 



DR. A. FRANKLIN SHULL 

 University op Michigan 

 II. SEX AND THE LIFE CYCLE 

 Introduction 



From observations made on the abundance of males in 

 several species, Jordan (1888) was led to believe that 

 there might be among Thysanoptera, as in aphids, an 

 alternating life cycle ; that is, that there might be a series 

 of parthenogenetic generations during the summer, fol- 

 lowed by a generation of males and sexual females in the 

 latter part of the summer or in the fall. Coupled with 

 this he suspected that there were winged forms in the 

 parthenogenetic part of the cycle, and at least occasional 

 wingless individuals in the sexual phase. 



Uzel (1895), however, was unable to detect any indica- 

 tions of such a cycle. He held that there could be no 

 question of parthenogenesis in a species in which males 

 were abundant all the time or at intervals. Only in spe- 

 cies in which the males were too rare to impregnate all 

 the females would lie admit parthenogenesis. To prove, 

 in such a species, an alternating cycle like that of the 

 aphids, it must, in Uzel's opinion, be shown that the males 

 are abundant only at certain seasons. As Uzel was ac- 

 quainted with no European species in which males were 

 plentiful at but one season, he rejected Jordan's sugges- 

 tion regarding an alternating cycle, and his view seems 

 to have been accepted by thysanopterists since that time. 



To UzePs argument it may be objected that the pres- 

 ence of males, and even the occurrence of copulation, is 

 no proof that parthenogenesis is wanting. For among 

 the aphids and rotifers, the parthenogenetic and sexual 

 females exist side by side. Nor is parthenogenesis in 

 these two groups facultative (optional), as L T zel appears 

 to assume for Thysanoptera; a female is either only 

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