HABITS AND LIFE-HISTORIES OF WOODLICE. 113
became a comparatively simple matter to keep them for indefinite
periods under observation.
The instruments which proved most handy and reliable for
recording in absolute terms the humidity conditions within the
dishes were paper hygrometers, and the average readings of the
requirements ultimately determined for each of the species at a
mean temperature of 67°F.9 were as follows :—
T. pusillus 84% Relative humidity.
P. dilatatus 71% Relative humidity.
P. scaber 71% Relative humidity.
O. asellus 77% Relative humidity.
P. muscorum 80% Relative humidity.
A. vulgare 65% Relative humidity.
(Atmosphere 59%.)
Thus it will be seen that the relative humidity of the
atmosphere (which varied from a monthly average of 57% at
an average temperature of 70°F. in July, to 88% at an average
temperature of 44°F. in December at Kew in 1937) does not
normally appear to be sufficient for any of the species except
A. vulgare ; and this is confirmed by the familiar observation
that the other species are unable to survive prolonged exposure.
E. E. Unwin (1931) and G. Modlinger,10 working indepen-
dently, have both, moreover, demonstrated that the differing
moisture requirements correspond to different modifications of
their respiratory organs in adaptation, in a greater or lesser
degree according to the species, to the air-breathing habit—as
shown in the following extracts from the table in Unwin's
work :—
9 This represents the approximate mean temperature of the atmosphere in the months of
June and August. The relative humidity of the atmosphere for these months is approximately
59%.
10 Reported by Miller (1938).