140 THE ESSEX NATURALIST.
may be for the same purpose, and Theobald (1904) similarly
noted that large quantities of bottle cork were sometimes
devoured by O. asellus. The other possible explanation is that
these substances contain minute quantities of some other essential
constituent not yet known and the creatures are not necessarily
feeding upon what they eat, but only upon the associated
substances, as Uvarov (1928) has shown to be the case in some
insects. In any case it is a noteworthy feature in woodlice, first
remarked upon by Collinge (1913), that they will apparently
consume an enormous amount of "food" when such is plentiful,
and yet an extremely small amount has been shown to be
sufficient to keep them alive for long periods.
As regards the light factor, most woodlice appear to show a
negative phototaxis in daylight, but since they usually live in the
dark it is probable that during the greater part of their time light
is not an influential factor in their lives. The reaction of wood-
lice to light has been demonstrated experimentally and analysed
by Abbott (1918) working on Oniscus and Porcellio. From his
results he concludes that the influence of phototaxis is an impor-
tant ecological factor in Isopods, and although ordinary daylight
is so diffuse that it cannot direct their course as unmistakeably
as under experimental conditions, yet its influence, he claims,
is sufficient to turn them back whenever they wander into it in
the course of their ordinary activities.
However this may be, in a state of nature the response of
Isopods is in my own experience largely overruled by their
hygrokinetic reactions, and when disturbed in their natural
haunts they scurry away to shelter presumably not so much to
avoid the daylight as to reach a damp situation again.
It has on several occasions (Pierce, 1907, Collinge, 1914)
been noted that woodlice come out in large numbers at the
approach of dusk, becoming more active as darkness increases ;
this is more probably due to the increasing humidity conditions
rather than to the fading light, to which it has previously been
attributed.
Allee (1926), too, though finding woodlice to be strongly
photonegative, maintained that the light factor is distinctly
subsidiary to humidity conditions, as a factor affecting the
bunching, or aggregation, of these creatures.