BULLETIN 1008, U. S. DEPARTMENT OF AGRICULTURE. 



SEX RATIO. 



Before proceeding further with the discussion of the rate of multi- 

 plication, it is necessary to take up the question of the proportion 

 existing between the two sexes. In order to acquire data on this 

 subject, a beginning was made, more than two years ago, to record 

 the sex of all Hessian flies that emerged in the cages. These cages 

 contained only puparia separated from their surrounding plant tissue 

 and reared in small vials. The puparia had been obtained in various 

 localities in the northeastern portion of the United States. In this 

 way the sex has been recorded of practically every fly that has 

 emerged from a few thousand puparia each year. At the end of 

 the first year it appeared that the percentage of females in the 

 spring generation Avas remarkably high. It was not anticipated that 

 this excess of females in the spring generation would prove to be con- 

 stant, but Table 4 shows that for the second year the percentage of 

 females dropped only slightly. Although the rearing of flies from 

 the 1919 material has not been completed, a much larger number 

 than usual have already been obtained, and the percentage of 

 females for 1919 is almost the same as for the two preceding years. 



Table 4. 



-The sex ratio in the various Hessian fly generations, 1917, 1918, and 



1919. 



Generation. 



Year. 



Number of 

 males. 



Number of 

 females. 



Total 

 roared. 



Per cent of 

 females. 





f 1917 

 I 1918 

 [ 1919 

 / 1917 

 \ 1918 

 / 1917 

 \ 1918 



168 

 146 

 623 

 749 

 1,493 

 246 

 51 



276 



235 

 1.031 



761 

 1,313 



225 

 45 



444 



381 



1,654 



1,510 



2,806 



471 



96 



62.2 

 61.7 



Fall 



62.3 

 50.4 





47.8 





46.9 



For the three years the average percentage of females is 62.1, and 

 it seems quite safe to assume now that at least 60 per cent of the 

 spring brood develop into females. In the case of the fall genera- 

 tion the sexes appear to be approximately equal in numbers, although 

 figures for 1918 are rather low in females. In volunteer wheat there 

 are a very small number of cases from 1918, but it appears in gen- 

 eral that the sexes are about equal in the partial broods 'developed in 

 the summer. 



It is now evident enough that the spring brood is the one which 

 shows the greatest variation from normal, both in the high percent- 

 age of females produced and in the reduced number of eggs these 

 females can lay. Both may be related phenomena with a common 

 explanation, but this explanation will have to be left to the imagi- 

 nation at the present time. 



THE MEANING OF THESE FIGURES. 



In order to demonstrate fully the meaning of the figures which 

 have been submitted, two examples will be of considerable help. 



