354 J- T - PATTERSON. 



dark, more or less mottled color. One can therefore readily 

 detect a mixed brood under the microscope. 



The 162 broods studied were taken at random from the field, 

 and therefore in all probability the data on numbers and sex 

 yielded by them represent the approximate sex ratio for the 

 species. These 162 broods contained a total of 31 ,001 individuals, 

 or an average of over 191 to the brood. Ninety of these, or 55.56 

 per cent., contained only female parasites, 62, or 38.27 per cent., 

 contained only male parasites, and 10, or 6.17 per cent., con- 

 tained mixed broods of males and females. 



There are therefore not only a larger number of female broods 

 than male, but the average number of individuals in the former 

 exceed that of the latter. Female broods average a little over 

 198 individuals to the brood (Table III.), while male broods 

 average only about 175 (Table IV.). The range in the number of 

 individuals in these broods (from 25 to 395 in the female, and 

 from 41 to 345 in the male) makes it evident that these averages 

 are of little significance, except, perhaps, to show that the 

 fertilized egg is slightly more prolific than the unfertilized egg. 



Of the total number of individuals (31,001), 63.41 per cent, are 

 females and 36.59 per cent, males; but obviously the true sex 

 ratio can not be based on these figures. It must be determined 

 from the number of male and female broods. It would not be a 

 difficult matter to determine this ratio were it not for the un- 

 certainty of the origin of some broods. There is always the pos- 

 sibility in these insects that more than one parasitic egg has 

 been laid in the egg of the host, and hence the parasites which 

 later emerge may not constitute a true polyembryonic brood, but 

 in fact represent two or even more such broods. Under the 

 circumstances, the best that one can do is to determine approxi- 

 mately the sex ratio for the species. This can be done in the 

 following manner. If we assume, as all previous workers have 

 done, that each of the mixed broods is the product of at least two 

 eggs, then, in accordance with the law of probability, we can 

 determine the number of unmixed male and female broods, each 

 of which must also have been produced from two eggs. Worked 

 out on this basis, it is found that the ratio of females to males 

 is 106/76 or a sex ratio of approximately 3 : 2 



