176 
Biology of Pediculus humanus 
28-30° C. the blood in the gut had become dark or black in 3-4 hours; the 
red blood corpuscles being destroyed in 5 hours at latest and replaced 
by detritus consisting of amorphous particles. The empty gut is faintly 
acid (this spoils Giemsa staining unless neutralized by ammonia vapour), 
but on feeding, a weak alkaline protease is given off into the gut. 
Widmann found the gut contents to be sterile in 25 lice examined for 
aerobic and anaerobic bacteria; from 5 out of 25 lice that had been 
dead 10-12 hours he isolated a small anaerobic bacillus. 
Widmann found the faeces to be faintly acid. When fresh faeces 
are viewed in salt solution they are seen to consist of fine granular 
detritus and yellow-brown prismatic crystals, occurring in bundles or 
rosettes; the crystals are indistinct, insoluble in water, alcohol, ether, 
chloroform, and the ordinary mineral acids, but they dissolve in dilute 
alkalies. These crystals, probably haemoglobin or its derivatives, 
apparently form outside on contact with air as they are not found in 
the gut. I have occasionally observed excreta having a whitish colour, 
and doubtless derived from the Malpighian tubes; in an old adult a 
long mass of this character was found protruding from the anus of the 
dying insect. 
Some further details relating to digestion will be considered in the 
section on anatomy that will follow. 
Some Reactions of Pediculi. 
REACTIONS TO LIGHT. 
Heliotropic reactions. According to Hase (1915, pp. 48-50, cited by 
Muller), corporis shun the light unless they have hungered 12-24 hours, 
in which case they wander toward a source of light; if, however, they 
are disturbed, they seek darkness. Galli-Valerio (1916, p. 37) states 
that capitis wander to the light when hungry and to the dark’ when 
gorged. Bacot (n. 1917, p. 233) found corporis and capitis moved 
toward the shadow of dark objects; when placed upon squares of black 
or white paper, they mostly wandered to the shady side; he makes no 
mention of whether the insects he tested were hungry or not. 
I have made a few experiments with corporis at ca. 15° C. in diffuse 
daylight about 5 feet away from a window. About 200 hungry insects 
in all stages of development were placed on a square of dark cloth 
measuring about 10 x 8 cm., in the centre of a large glass dish. When 
agitated, on first being placed on the cloth, they repeatedly ran to the 
upper illuminated surface, when the cloth was reversed at intervals of 
