178 
Biology of Pediculus humanus 
eggs that hatched in the light was somewhat smaller than in the dark, 
the difference is too slight to possess significance and may be due to 
experimental error, for of 1158 other eggs raised at 30° C. in the dark 
(see p. 153) but 69-6% hatched. 
REACTIONS TO WARMTH. 
Thermotactic reactions. Various authors have tested the thermotactic 
reactions of corporis, some with positive, others with negative results. 
Wulker (1915, p. 630) found no evidence of a thermotactic sense, and 
Nocht and Halberkann (1915. p. 626) observed no effect when the insects 
were brought in proximity to man. Other authors find that lice only 
react at a short distance from man, at 2 cm. (Hase, 1915, cited by Muller, 
1915, p. 66), at most 3 cm. (Silcora, vm. 1915, p. 533); or that they 
wander aimlessly unless quite close to man (Swellengrebel, 1916, p. 13). 
Frickhinger (1916, p. 1254) enclosed lice in a photographic plate-holder, 
a part of which was warmed to 20-25° C., and states that the insects 
sought the warmed part of the holder. Sikora (toe. cit.) found that lice 
were not attracted to warm glass, whilst Major F. M. Howlett informs 
me that in experiments he conducted in India, lice were attracted to 
a tube containing warm water 1 . Some of these apparent contradictions, 
as we shall see, are readily explained. 
Apparently some of the discordant results above mentioned are due 
to the experimental conditions, which, however, are not specified,. 
Judging from personal experience, it is essential that the insects should 
not be torpid, and it is best for them to be hungiy if well marked 
reactions to warmth are to be observed. With these conditions fulfilled, 
it is easy to observe that lice promptly react to a source of warmth, 
either the hand, or a tube containing warm water. Thus, having 
placed some 200 corporis on a piece of cloth, in a room at ca. 16° C., 
in daylight, the insects, after a time, had all retreated away from the 
light, beneath the cloth, or along its shady side. On holding the hand 
ca. 2 cm. above the cloth, they promptly swarmed to the upper surface 
and followed the finger moved about over the cloth very much like iron 
filings follow a magnet. Breathing on the cloth likewise produced an 
immediate swarming to its upper surface. I may add that the warm 
tube, held at a moderate distance from the cloth, attracted the lice, 
in a similar manner to the finger. 
When the hot water tube approached them too closely, they fled, 
1 See this author's paper, p. 186. 
