VARIATION IN HEAD LENGTH OF SPERMATOZOA IN 

 SEVEN ADDITIONAL SPECIES OF INSECTS' 



CHARLES ZELENY and C. T. SENAY 



EIGHT FIGURES 



A study of variation in head length among spermatozoa from 

 single testes in fifteen species of animals from widely separated 

 groups was made by the senior author and E. C. Faust ('15 a 

 and b) . The frequency distribution of the size groups in a great 

 majority of the cases showed bimodality of such a character as 

 to make it highly probable that in these species the spermatozoa 

 are dimorphic as regards size. Furthermore it was shown to 

 be highly probable that this dimorphism is the result of chromo- 

 somal differences. There is a close agreement between the ratio 

 in the two groups as calculated on this hypothesis and the actual 

 ratio determined by measurement. 



These chromosomal differences as has now been abundantly 

 demonstrated are related to sex determination and the size groups 

 must be similarly related, fertilization of the eggs by spermato- 

 zoa of the upper group yielding females and by those of the 

 lower group, males. A probability is thus opened for controlling 

 sex as soon as living spermatozoa of the two sizes can be experi- 

 mentally separated. 



In view of the importance of the question of existence of two 

 size groups and in further preparation for the experimental 

 tests, measurements were made for seven additional species and 

 these are described in the present paper. 



Together with those formerly described twenty-two species 

 are now available for drawing a general conclusion. This num- 

 ber includes all that have been measured, with two exceptions. 

 These two are Helodrilus, a hermaphroditic form, and the 



1 Contribution from the Zoological Laboratory of the University of Illinois 

 No. 49. 



505 



