HABITS AND LIFE-HISTORIES OF WOODLICE. 147
seventh pair of legs do not appear until the time of the third
moult, except in T. pusillus, where they appear at the fourth
and fifth moults respectively.
These second and third moults of the young occur at about
fortnightly intervals, whilst subsequent moults continue to take
place at fairly regular intervals of a little over three weeks
throughout the growth period, so that nine or ten moults occur
within the first six months of the life of the animal. It is
interesting to note that there is quite a marked regularity in the.
times of occurrence of these successive moults, characteristic
for each of the species ; they occur in the most rapid succession
in the comparatively large young of T. pusillus, and at the
longest intervals in the small young of A. vulgare. In the later
stages, however, the intervals between the successive moults
in the different species approximate more and more closely to
each other, so that in the sixth, seventh and eighth moults the
young of all the species moult at almost identical intervals.
Unlike the adults, the young woodlice normally continue to
moult at regular intervals throughout their first winter, under-
going no cessation of activity, but on the contrary increasing in
size at a fairly constant rate. After these first six months
moults continue to occur in regular succession at about monthly
intervals up to the commencement of the breeding season of the
following year, when they fall into line with the moults of the
adults.
Growth is fairly rapid for such small creatures, O. asellus (the
largest adult species) growing at the rate of one millimetre in
the first seven weeks, whilst A. vulgare grows half a millimetre
in the same period. Meanwhile the antennae lengthen and
become more slender, the integument becomes more calcified
and more deeply pigmented, so that by the end of three months
the genera are distinguishable by their surface markings alone ;
small abdominal limbs or pleopods become differentiated and
bear slight sexual distinctions ; the eyes enlarge and develop
facets, forming the characteristic compound Arthropod eyes,
until, by the time of the fifth moult—roughly the age of three
months—their development may be regarded as complete,
though they are as yet less than half the size of their parents,
and far less deeply pigmented. The seventh thoracic segment