HABITS AND LIFE-HISTORIES OF WOODLICE. 141
Under conditions of suitable humidity certain species of
woodlice, particularly those most adapted to the terrestrial life,
such as A. vulgare, will, in fact, remain in brilliant light and
move about normally without making any attempt to hide.
Presumably, then, though most woodlice do show experimentally
a definite response to light, phototaxis as an ecological factor is
normally subordinate to hygrokinesis in affecting their behaviour.
Finally, the temperature factor is worthy of some considera-
tion. Small temperature changes, within the range of approxi-
mately 10°, do not appear to influence woodlice to any
appreciable extent, since they may be found in equal abundance
on warm and cool days alike ; the dominating factor, as has been
already pointed out, being the humidity conditions.
A similar result was obtained by Gunn (1937) using a tempera-
ture gradient apparatus with specimens of P. scaber. He found
that the animals moved to the most moist place irrespective of
where that may be in the temperature gradient, showing that
if there is a preferred temperature, the humidity preference
is so strong as to hide it. Also Allee (1926), working on the
factors controlling bunching in land isopods, found that tempera-
ture does not affect the bunching together so markedly or
completely as does the moisture content of the substratum.
Woodlice are, however, influenced by more profound seasonal
changes, and during the winter all the species except T. pusillus
and P. muscorum are scarce, and can only be found in very
remote crevices and holes, in a semi-lethargic state. They
generally disappear from their usual haunts after the first cold
spell at the beginning of winter, usually in November ; which
goes to prove the influence of profound temperature changes,
since at this season the humidity conditions are usually on the
increase: and they do not emerge from their remote hiding places
until the following February, March or April, according to the
species. Occasionally, during a spell of exceptionally mild
weather in mid-Winter, they may be attracted out, and those
species which are found in the neighbourhood of damp houses,
such as P. scaber, O. asellus and A. vulgare, may at such times
suddenly appear in large numbers, and then as suddenly dis-
appear again with the return of cold weather.