migan, change their color, being brown in summer
and white in winter. So are the Arctic fox and the ermine, to whom it
is then an advantage to be white, not to avoid danger, but in order
that they may be the more easily able to steal unperceived upon their
prey.

Many of the cases in which certain insects escape danger by their
similarity to plants are well known; the leaf insect and the
walking-stick insect are familiar and most remarkable cases. The larvae
of insects afford, also, many interesting examples, and in other
respects teach us, indeed, many instructive lessons. It would be a
great mistake to regard them as merely preparatory stages in the
development of the perfect insect. They are much more than this, for
external circumstances act on the larvae, as well as on the perfect
insect: both, therefore, are liable to adaptation. In fact, the
modifications which insect larvae undergo may be divided into two
kinds--developmental, or those which tend to approximation to the
mature form; and adaptational or adaptive, those which tend to suit
them to their own mode of life.

It is a remarkable fact, that the forms of larvae do not depend on
those of the mature insect. In many cases, for instance, very similar
larvae produce extremely dissimilar insects. In other cases, similar,
or comparatively similar, perfect insects have very dissimilar larvae.
Indeed, a classification of insects founded on larva would be quite
different from that founded on the perfect insects. The group to which
the bees, wasps, and ants belong, for instance, and which, so far as
the perfect insects are concerned, form a very natural division, would
be divided into two; or rather one portion of them--namely, the
saw-flies--would be united to the butterflies and moths. Now, why do
the larvae of saw-flies differ from those of their allies, and resemble
those of butterflies and moths? It is because their habits differ from
those of ants and bees, and they feed on leaves like ordinary
caterpillars.

In some cases the form changes considerably during the larval state.
From this point of view, the transformations of a small beetle, called
_Sitaris_, which have been carefully observed by M. Fabre, are
peculiarly interesting.

The genus Sitaris, which is allied to the blister-fly and to the
oil-beetle, is parasitic on a kind of solitary bee which excavates
subterranean galleries, each leading to a cell. The eggs of the
beetle, which are deposited a