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THE ESSEX NATURALIST.
IV. ECOLOGICAL FACTORS.
As has been already gathered, the ecological factor which
appears to influence the lives and distribution of woodlice more
than any other is the humidity factor. This in itself imparts a
particular interest to the study of the group, and, with a view to
obtaining more precise information concerning the operation
of this factor, I endeavoured to determine experimentally the
particular humidity requirements of each species. Even on
casual observation, these were seen to differ quite considerably,
ranging from the minute T. pusillus at one extreme, which lives
in an atmosphere of permanently high moisture content, to the
large A. vulgare at the other extreme, which can even survive
exposure for a time to brilliant sunshine on a dry, porous, warm
chalky surface.
I therefore collected large numbers of each species and placed
them under close observation in groups of three or four, housed
in covered glass petri dishes for several weeks at a time at
varying degrees of humidity, noting their respective reactions
under the different conditions. The humidity conditions, it may
be of interest to record, were maintained and controlled within
the dishes by circles of damp blotting paper, of the same diameter
as the dishes, and placed on the base so that they adhered all
round and prevented the woodlice from crawling beneath. The
circles were periodically re-moistened from a small pipette, to
replace that lost by gradual evaporation. The responses of the
woodlice were easy to observe, for it was soon discovered that
a shortage of moisture was indicated by a tendency on the part
of the animals to curl up, presumably to conserve moisture
around their gills, which are situated on the pleopods. Con-
versefy, excessive moisture caused them to stretch out, with
their segments wide apart. It is important to note that exces-
sive moisture proved to be just as harmful as insufficient moisture,
and would eventually cause the death of the. animals by clogging
up the pleopods.
Indeed, until the correct humidity requirements for each
species were thus determined by experiment, the mortality rate
of these creatures was strikingly high ; with this experience
once gained, however, the greatest difficulty in keeping woodlice
in captivity had obviously been overcome, and henceforward it