218 
Biology of Pediculus humanus 
black cloth (Expt. IV). Conversely, the palest lice were obtained in boxes 
that were white inside and contained white cloth, a few showed slightly in¬ 
creased pigmentation when the box was black inside and the cloth white 
(Expt. V). 
Lice raised in the light on violet, indigo, blue and red, were as dark as 
those raised on black (Expts. VI (red), XI, XII), whilst those raised on green, 
yellow, and orange (Expts. VI (yellow), XI, XIII) were pale or slightly pig¬ 
mented in the order yellow (acting like white), green and orange. 
Lice kept in absolute darkness on black and white, do not show pig¬ 
mentation (Expt. IX), but when maintained on black, a very small trace of 
light admitted to them causes pigmentation to take place (Expts. VII a, 
II B). 
Pale larvae sojourning on black for 36-48-72 hours prior to moulting, 
show progressive degrees of pigmentation (Expt. VIII). 
Dark adults transferred to a white background and pale adults transferred 
to a black background do not subsequently undergo modifications in pig¬ 
mentation. Their coloration is fixed for life. 
When many lice inhabit a white box, the colour of their bodies, gut con¬ 
tents and excreta darken the background with the result that the insects kept 
therein become more or less pigmented (Expt. XI and repeated experience). 
Contrary to the experiments with variously coloured backgrounds, those 
in which lice were exposed on white flannel for 48 hours to the action of 
coloured rays of light did not show differences in so far as all remained pale 
or very slightly pigmented.- 
A longer exposure to coloured rays exerts an influence on pigmentation, 
for lice exposed continuously to coloured light whilst being raised to maturity 
were, as a whole, decidedly darker in the violet and indigo tubes than in the 
others. There were intermediate in the blue and green, and mostly pale in 
the yellow, orange and red tubes. 
The experiments explain the great variation in the pigmentation of lice 
found on different races of man and variability in the pigmentation of lice occur¬ 
ring on white races, the colour of the skin, hair, and clothing, all exerting their 
effect on the insects. The statements are doubtless correct (a) that dark lice 
from negroes may turn pale when transferred to whites and vice versa, and 
(6) that head-lice on dark-haired whites are darker than on the fair-haired. 
The latter statement is being investigated. 
The change in pigmentation of lice, due to the colour of the background, 
doubtless affords a measure of protection to these parasites in nature, especially 
in the case of man, who, like the monkey, uses his eyes in the search for the 
insects. 
The observations recorded in Section (a), but especially in Section (6) show 
that Hindle ( Parasitology , ix. p. 265) was unfortunately misled in his experi¬ 
ments designed to breed pigmented and unpigmented strains of P. humanus. 
Hindle readily reared colourless strains but had difficulty with dark ones, and 
