D. Keilin and G. H. F. Nuttall 
317 
Note on normal sex-ratio. 
In this connection it should be recalled that normally the preponderate 
over dd (see Nuttall, Parasitology , x. pp. 114-117, xi. pp. 219-220). To deter¬ 
mine the sex-ratio the mortality taking place during development should be 
excluded to obtain correct figures. This has only been done once (Nuttall, 
loc. cit.) where 100 out of 102 first stage larvae were raised from the egg to 
maturity and yielded 43 per cent, dd and 57 per cent. 
The preponderance of $$ is also shown in counts made of normal “wild” 
lice or such as have been raised without regard to the mortality during de¬ 
velopment. Thus in a publication by Hase (1915, p. 28), which has recently 
become available, this author states that among 4382 “wild" lice the different 
stages were present in the proportions: 100 dd, 175 $$ and 233 larvae; whence 
we reckon that the $$ represented ca. 63 per cent, of the adults. Moreover, 
our recent examination of 2769 such normal adults shows 59 per cent, of $$ 1 . 
Cross II. This gave a numerous progeny, about 700 individuals in which 
the males predominated (53 per cent.), but one hermaphrodite being found. 
Therefore, if a larger number of individuals had been present in the case 
of Cross I, doubtless hermaphrodites would have been found therein. 
Cross III possesses no interest because of the small number of individuals 
derived therefrom. 
Crosses IV and V are interesting since they yield 6 per cent, of herma¬ 
phrodites in the first generation. 
Cross VI is the most interesting because the progeny was raised through 
three generations and comprised a large number of individuals (970) including 
110 hermaphrodites. The first generation, F ± , of this cross, although devoid 
of hermaphrodites, exhibits a strong numerical superiority of males over 
females (128:16). The F 2 generation yielded 71 hermaphrodites, their 
appearance being accompanied by a great increase in the proportion of males 
to females (368:54). The same result is visible in the F 3 generation, but the 
figures are somewhat less striking (197:97). 
The foregoing evidence demonstrates that an abnormal sex-ratio occurs in 
the progeny of crosses betiveen capitis and corporis, a great diminution in the 
proportion of females to males is observable , this being accompanied by the ap¬ 
pearance of a large number of hermaphrodites 2 . The phenomenon is analogous 
to that observed by Goldschmidt to which we refer on p. 313. 
Since a large number of hermaphrodites can be obtained by crossing 
capitis with corporis , it is obvious that the occasional occurrence of herma¬ 
phrodites among “wild” lice is readily understandable. Frequent oppor- 
1 These figures, based on the descendants of numerous males and females, taken collectively, 
are not affected by the observation of Hindle (1917, Parasitology , ix. 259) that certain pairs yield 
either <$. $, or offspring. 
2 The circumstance that Bacot (as he informs us) removed a very few specimens here and 
there from the tubes containing the material, does not affect our conclusions, for the insects 
removed were of the nature of “random samples,” not being examined critically. 
