37^ N- M. Stevejis 



somes in other orders of insects has recently been so fully dis- 

 cussed in a paper by A. M. Boring ('07) that it seems hardly nec- 

 essary to go into the subject exhaustively here. The dimorphism 

 of the spermatozoa resulting from the maturation of the male 

 germ cells of the nine species of Diptera considered in this paper 

 is of the same character as that described by the author for 36 

 species of Coleoptera (see note, p. 49, Stevens '06), and by Wilson 

 ('05 and '06) for several species of Hemiptera heteroptera. The 

 dimorphism is brought about by the presence in the spermatogonia 

 and spermatocytes of an unequal pair of heterochromosomes, 

 w^hile in large numbers of other insects such dimorphism is due to 

 the presence of an odd chromosome in the male germ cells. These 

 flies have proved to be exceptionally favorable material for demon- 

 strating the occurrence in the female germ cells and somatic cells 

 of a pair of chromosomes, each equivalent to the larger hetero- 

 chromosome of the male. 



Here, as in similar cases previously described, it is perfectly clear 

 that an egg fertilized by a spermatozoon containing the smaller 

 heterochromosome produces a male, while one fertilized by a 

 spermatozoon containing the larger heterochromosome develops 

 into a female. The material does not, however, throw any fur- 

 ther light on the question whether the dimorphic spermatozoa 

 are themselves in some way instrumental in determining sex in 

 these insects; or whether sex is a character borne by the hetero- 

 chromosomes and segregated in the maturation of the germ cells 

 of each sex. If the latter supposition is true, sex is probably 

 determined by the dominant heterochromosome of the egg, and 

 fertilization is selective as has been shown in previous papers 

 (Wilson '05, '06; Stevens '06, p. 54; Nowhn '06; Boring '07). 



The only hope of determining whether sex is a Mendelian char- 

 acter seems at present to lie in breeding experiments with forms 

 that may be shown by cytological study to be favorable. It is 

 probable that in some cases at least, other characters may be so 

 correlated with sex that their behavior in heredity may throw 

 light on the sex question. 



As to the proportion of sexes in these flies, a few figures may be 

 given for Drosophila ampelophila. In the autumn and winter 



