6. 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 was 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. 
Number of} Number of} Total /| Percent of — 
Generation. Year. males. females. | reared. females. 
1917 168 276 62. 2 
Sabra rset hc ei ee ey eee Re ee 1918 146 235 381 61.7 
1919 623 1,031 1, 654 62.3 
Fall 1917 749 1,510 50.4 
Sat PU BY can Sa sa cE er) TS aS Se gee ea 1918 1, 493 1,313 2, 806 46.8 
1917 cz 471 47.8 
Wolunteer wheatseic. 2 eh. 2s su eee ees ees 1918 51 45 96 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 1 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 nave to be left to the imagi- 
nation at the present time. 3 
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. 
