108
THE ESSEX NATURALIST.
weather can only be traced immediately after a thunder shower,
and even then only beneath some dense bush, where little air
can circulate.
They were collected separately in small tins kept moist with
a little leaf-mould or moss, but were isolated as soon as possible,
for when confined together for long the mortality rate is rather
high among those species with cannibalistic tendencies—notably
the omnivorous O. asellus, and to a lesser extent P. dilatatus.
Apart from this, however, the tins served as suitable receptacles
for quite long periods, provided sufficient moisture is present—
yet not so much as to flood the creatures.
It was soon discovered that the choice of habitat of the
different species depends to a very large extent on the degree of
humidity present in the surroundings, and their actual distri-
bution appears to be restricted more by this factor than by any
other. The humidity requirements of the different species,
however, differ considerably; so much so, in fact, that, given a
situation of a known state of dampness, and with a knowledge
of the moisture requirements of each species (for which, see
below), one can foretell with a considerable degree of accuracy
which particular species are likely to be found there. Indeed,
in collecting the different species I have found this criterion of
moisture requirements to be a far more reliable guide to their
respective haunts than such factors as food material, light, etc.
This observation no doubt accounts for the statement by
W. M. Webb in 1906 that "the crustaceans may be entirely
"absent from many tree trunks, while when another is examined
"which seems to differ very slightly, if at all, in condition or
"situation, they are found in swarms." The common species
O. asellus, for example, appears from my observations to be
actually very particular about which tree among many, or
beneath which stone among many, it lives. Such situations
must be of just the correct dampness, and it is in this respect that
those situations favoured by the Woodlouse differ from the rest.
I have on several occasions watched the fate of a fallen tree-
trunk ; whilst the tree remains moist it will be swarming with
woodlice, but as the wood gradually dries they desert it, until
when the log has lain for some time and become quite dry and
dead (unless it be kept damp by the constant drip of water