D. Keilin and Cl. H. F. Nuttall 
(b) The percentage of Hermaphrodites observed in crosses between 
Pediculus humanus capitis and corporis. 
The origin of the hermaphrodites observed by us in Pediculus humanus 
seems clearly indicated by the study we have made of Bacot’s material. As this 
material is derived from the crossing of capitis with corporis , it is desirable 
to mention briefly the main differences which characterize these forms: 
Capitis when compared with corporis usually appears smaller, more 
strongly chitinized, its antennae being relatively broader and shorter, the 
legs stouter, and the inter-segmental grooves at the sides of the abdomen 
more incised, etc. Besides these morphological differences, the insects differ 
somewhat in their biology, thus capitis is more difficult to raise experimentally 
and prefers to oviposit on hair. The foregoing differences fluctuate, and 
frequently intermediate forms are encountered in a single lot of capitis. 
In this connection it is of fundamental importance to note that capitis , 
when raised under experimental conditions similarly to corporis, i.e. in the 
laboratory, after 4-5 generations changes its character and becomes similar 
to corporis both in respect to morphology and biology. This interesting 
phenomenon has been demonstrated by Sikora, whose observations we have 
confirmed (see Appendix I, p. 324). 
Capitis and corporis, as already affirmed by one of us (Nuttall, 1917), 
may therefore be regarded as races or varieties of a single species which have 
become adapted to a life of parasitism mainly on the head and body 
respectively. 
On discovering a large number of hermaphrodites in Bacot’s material, 
we proceeded to determine their percentage in relation to the number of 
normal individuals, but were immediately struck by the abnormal sex-ratio. 
The following table includes a full enumeration of the males, females and 
hermaphrodites which we have made from Bacot’s material, the successive 
generations being duly indicated: 
Tabular enumeration of Bacot’s material. 
The material consists of 6 lots, the progeny of as many pairs or sets of 
parents. The first pair consisted of corporis $ and capitis $, the remainder 
of capitis dd and corporis The letters AS (in brackets) relate to the list 
of tubes as classified and received from Mr Bacot. The signs F^, F 2 , F 3 denote 
the three successive generations descended from a given pair or pairs; the 
signs <£, denote hermaphrodites of <$ or 2 character. The percentages are 
given roundly. 
Cross I. Corporis S + capitis $. 
S 
$ 
it 
F i (A) 
35 
23 
0 
F 2 ( B) 
143 
101 
0 
f 3 ( C) 
64 
63 
0 
F 1 + F 2 + F 3 
242 (56 %) 
187 (44 %) 
0 (0 %) 
