( vi ) 



In a numerous colony of a gregarious insect casual sex- 

 production on the part of any given pair will result in a nearly 

 equal proportion of (J cJ and $ ?, and might serve to minimise 

 the dangers of too close inbreeding. In this series, however, 

 sex-production does not appear to be entirely casual, as there 

 is an evident bias on the $ $ side. 



The sex-percentages of the F. 1 generation of four cross 

 pairings between ^ P. capitis and $ P. humanus are as 

 follows : — 



No. 4 76 = 68% 35 = 32% 



328 = 71% 131 = 29% 



It will be seen that in this series the bias is heavily on 

 the (J side, which is possibly due to the effects of crossing. 

 I must admit, however, that in view of the erratic per- 

 centages from normal pairings of one of the parent races 

 my argument on the basis of abnormal sex-production on the 

 part of the hybrid insects can carry but little weight. 



An experiment dealing with the egg-laying instincts of the 

 two species affords clearer evidence. The following method 

 was adopted. Glass-bottomed boxes of 1 J inch diameter and 

 I an inch deep, were lined on the vertical sides with a strip 

 of flannel, such as is used for Army shirts, human hairs being 

 placed in central area so as to form a web, but not a felted 

 mass. The hairs came in contact with the glass bottom, the 

 flannel-lined sides and the gauze cover through which the 

 lice were fed. An equal number of pairs of P. capitis and 

 P. humanus were placed in each box. At the end of a week 

 the $ $ of P. capitis had laid 80% of their eggs on the hair 

 and 20% on the flannel, while the P. humanus $ $ had laid 

 20% on the hair and 80% on the flannel. As the insects had 

 been taken from congested stock boxes that had been in use 

 for many months, the P. capitis box containing a felted 

 mass of hairs and the P. humanus box a rolled flannel strip 



