216 
Biology of Pediculus humanus 
THE INFLUENCE OF COLOURED RAYS OF LIGHT ON PIGMENTATION 
IN LICE MAINTAINED ON WHITE FABRICS. 
Having established that lice do not become pigmented when maintained 
in absolute darkness, and that exposure to a very small amount of light 
suffices for them to become pigmented if maintained on black and red back¬ 
grounds, etc., it seemed of interest to observe if coloured rays of light exert an 
influence on pigmentation. 
Method. Seven cylindrical glass specimen jars, ca. 12 cm. high by 5 cm. 
in diameter were partly filled with solutions of the seven dyes enumerated on 
p. 203. The jars were closed by corks of suitable size bored centrally to hold 
short test tubes which were pushed through up to the rim. The tubes thus 
became surrounded by a uniform layer of dye solution except at the neck. 
The jars were stood in alcoves made of cardboard boxes covered with white 
paper and facing the glazed door of the thermostat opposite a window, the 
tubes being capped by a piece of white cardboard. Each tube contained a 
strip of white flannel (ca. 3*5 x 0-7 cm.) standing vertically inside, to which the 
lice (second and third stage larvae, 10 per tube) that were introduced could 
cling. The lice were fed in pill boxes whose interior was coloured with the 
corresponding dye solutions to those contained in the jars, the insects being 
transferred to the boxes on the pieces of flannel. 
Experiment I. Pale lice were maintained for 48 hours in the apparatus; 
they moulted therein and all of them were found to be pale or very slightly 
pigmented. Under the conditions of the experiment the coloured rays there¬ 
fore appeared to exert no effect. The experiment was therefore repeated and 
varied, the insects being maintained a longer time exposed to coloured rays. 
Experiment II. Newly emerged first-stage larvae were placed in the 
apparatus and raised to maturity whilst exposed continuously to light. During 
the day-time the light emanated from the window and during the evening 
and night from a 60-candle power electric lamp suspended close to the glazed 
door outside the thermostat. To ensure a maximum of illumination, a looking- 
glass was placed at the back of the thermostat to reflect the infalling rays into 
the tubes that contained the lice, whilst the inside of the thermostat was 
lined with white paper. A piece of starched white cotton netting was placed 
in each tube for the lice to cling to in lieu of white flannel because the latter 
afforded a slight amount of shade. The experiment lasted two weeks. 
The adults were carefully graded in respect to degrees of pigmentation 
and classified as follows, the sign x denoting separate individuals: 
Pigmentation 
Colour of 
Total adults 
( — 
-- 
A 
— 
- 
tube 
raised 
dark 
medium 
slight 
very slight 
absent 
Violet . 
9 
X 
XXX 
X X 
XXX 
# 
Indigo . 
10 
. 
XXX 
XXX 
X X X X 
• 
Blue 
8 
X 
X 
X 
X 
X 
XXX 
Green . 
9 
# 
X X 
X 
X X 
X 
X 
X X 
Yellow . 
11 
m 
X 
m 
X 
X 
xxxxxxxx 
Orange . 
12 
. 
. 
X 
X X 
X 
X 
X X X X X X X 
Red 
9 
X 
X 
X 
X 
X 
X X X X 
