274 Sex Inheritance in Pediculus humanus var. corporis 



and females alternated fairly regularly in the order of their emergence 

 from the third stage larvae. Records of the order of hatching were kept 

 in each case and the following example is typical of the manner in which 

 the sexes alternated : 



Family N 6. %^^ ?</</d'</ Wc/cT./ ¥ ? ? ?^</ ¥• 



Sterility. 



Although it was not very uncommon to meet with cases in which 

 pairs had no offspring, it is probable that most of them could be explained 

 by the assumption that one or other of the parents was unhealthy and 

 thereby prevented from performing any sexual functions. However, 

 one pair {N 10) seems to constitute an example of mutual sterility, for 

 unlike most of these cases, the female laid as many eggs as an ordinary 

 fertile individual, and none of them showed any signs of development. 

 When an unfertilized female is isolated it usually lays a few shrivelled 

 eggs, but the numbers are very much less than in the case of a fertilized 

 individual. It is possible, of course, that in the case mentioned, the 

 mere mechanical effect of the act of copulation may have stimulated 

 the process of Qgg laying, but there was such a well-marked difference 

 in the numbers and appearance of the eggs laid, that it seems more 

 reasonable to interpret it as an example of sterility. 



The same male crossed with two different females. 



Three experiments were made in order to determine whether the 

 same male produced the same type of family when crossed with different 

 females. Families A 1 and A\a constitute the first example, from which 

 it appears that when crossed with one female only male offspring were 

 produced, whilst when crossed with another, a mixed family was obtained. 

 Families 2 and 18 form another example, but unfortunately only 

 one adult, a male, was reared from the first cross, whilst the second cross 

 resulted in one male and three females. The results, therefore, of this 

 experiment are inconclusive. In a third experiment, families 7 and 

 019, the first cross resulted in three females, whilst the second one 

 produced four males and one female, but in the two latter cases the 

 results are complicated by the fact that the female had also been crossed 

 with another male, as mentioned below. Nevertheless, the result of the 

 first example is supported by the others, and strongly suggests the 

 existence of two types of females in the body louse. 



