The common shore-crab shows many different colours and mottlings,
especially when it is young. It may be green or grey, red or brown, and
so forth, and it is often in admirable adjustment to the colour of the
rock-pool where it is living. Experiments, which require extension, have
shown that when the crab has moulted, which it has to do very often when
it is young, the colour of the new shell tends to harmonise with the
general colour of the rocks and seaweed. How this is brought about, we
do not know. The colour does not seem to change till the next moult, and
not then unless there is some reason for it. A full-grown shore-crab is
well able to look after itself, and it is of interest to notice,
therefore, that the variety of coloration is mainly among the small
individuals, who have, of course, a much less secure position. It is
possible, moreover, that the resemblance to the surroundings admits of
more successful hunting, enabling the small crab to take its victim
unawares.
Professor Poulton's experiments with the caterpillars of the small
tortoise-shell butterfly showed that in black surroundings the pupæ tend
to be darker, in white surroundings lighter, in gilded boxes golden; and
the same is true in other cases. It appears that the surrounding colour
affects the caterpillars through the skin during a sensitive period--the
twenty hours immediately preceding the last twelve hours of the larval
state. The result will tend to make the quiescent pupæ less conspicuous
during the critical time of metamorphosis. The physiology of this
sympathetic colouring remains obscure.