342 
Pediculus and Plithirus 
Contradiction in the literature. The critical presentation of the foregoing 
evidence appears called for because of the continually recurring misstatements 
in text-books and the contradictions in the writings of various authors. Of 
recent writers, we find Cholodkowsky (1904), Enderlein (1905), Fahrenholz 
(1912), Hase (1915), Heymann (1915), Cummings (1915), and Alessandrini 
(1916) contending that capitis and corporis are two distinct species, whilst 
Meinert (1891) more correctly regards them as at most varieties, and Neumann 
(1910) ranks them as sub-species. 
Evidence of raising experiments. The foregoing statements, which I base on 
the examination of a very large material collected from man, may perhaps ap¬ 
pear insufficient to those in whom the belief has become rooted that capitis and 
corporis represent two species. There is however a final piece of evidence, 
i.e. typical capitis lose all their distinctive morphological characters when 
raised experimentally on man under conditions which are favourable for the 
propagation of corporis , and they acquire all the morphological characters of 
corporis after four or more generations {vide my paper with Dr Keilin, this 
vol.). 
The conclusion reached on the basis of morphological data above enumera¬ 
ted, is that typical capitis and corporis represent the extremes in the variation of 
the one species Pediculus humanus, and that they are identical in all essential 
points of structure 1 . 
(b) Biological differences. 
Having found that morphological differences afford no support for the 
contention that capitis and corporis constitute two species, we may now con¬ 
sider the biological differences which characterize typical specimens of both 
forms. 
Habitat on the host. It has been repeatedly asserted that capitis occurs 
only on the head and corporis only upon the clothing, this being regarded as 
a sign of the forms being two species. Even assuming that such a biological 
difference existed, it could not be regarded as specific. A closer examination 
of this supposed biological difference shows, however, that it is inconstant. 
Recent evidence (see Nuttall, xi. 1917, p. 86; Keilin & Nuttall, 1919, this vol.) 
proves that capitis does occur on man’s body and that it lays its eggs upon his 
body-hair, even occasionally upon his clothing in the vicinity of the head. The 
assertion that corporis only oviposits on man’s clothing has been disproved, 
it having been found to lay on his body-hair ( loc. cit.), therefore it is practically 
certain that corporis is capable of occasionally ovipositing on the hair of the 
head. Whilst there is no denying that the one form shows a preference for 
the head and the other for the body, the foregoing evidence proves at any rate 
that their habitats are inconstant and that their areas of distribution overlap 
to no inconsiderable degree. 
1 These are the grounds for my statement made in an earlier paper (n. 1917, p. 294, footnote) 
and which was contested by some entomologists who were not aware of the evidence on which 
I based it. 
