th the resemblance is well
known in mimicry. A common aid to concealment is the adoption by
different individuals of two or more different appearances, each of
which resembles some special object to which an enemy is indifferent.
Thus the leaf-like butterflies (_Kallima_) present various types of
colour and pattern on the under side of the wings, each of which closely
resembles some well-known appearance presented by a dead leaf; and the
common British yellow under-wing moth (_Tryphaena pronuba_) is similarly
polymorphic on the upper side of its upper wings, which are exposed as
it suddenly drops among dead leaves. Caterpillars and pupae are also
commonly _dimorphic_, green and brown. Such differences as these extend
the area which an enemy is compelled to search in order to make a
living. In many cases the cryptic colouring changes appropriately
during the course of an individual life, either seasonally, as in the
ptarmigan or Alpine hare, or according as the individual enters a new
environment in the course of its growth (such as larva, pupa, imago,
&c.). In insects with more than one brood in the year, _seasonal
dimorphism_ is often seen, and the differences are sometimes appropriate
to the altered condition of the environment as the seasons change. The
causes of change in these and Arctic animals are insufficiently worked
out: in both sets there are observations or experiments which indicate
changes from within the organism, merely following the seasons and not
caused by them, and other observations or experiments which prove that
certain species are susceptible to the changing external influences. In
certain species concealment is effected by the use of adventitious
objects, which are employed as a covering. Examples of this
_allocryptic_ defence are found in the tubes of the caddis worms
(_Phryganea_), or the objects made use of by crabs of the genera _Hyas_,
_Stenorhynchus_, &c. Such animals are concealed in any environment. If
sedentary, like the former example, they are covered up with local
materials; if wandering, like the latter, they have the instinct to
reclothe. Allocryptic methods may also be used for aggressive purposes,
as the ant-lion larva, almost buried in sand, or the large frog
_Ceratophrys_, which covers its back with earth when waiting for its
prey. Another form of allocryptic defence is found in the use of the
colour of the food in the digestive organs showing through the
transparent body, and i