d for the purpose of protection. Another exceptional case is
that of the very conspicuous caterpillar of the spurge hawk-moth
(Deilephila euphorbiae), which was at once eaten by a lizard, although,
as it exposes itself on its food-plant in the daytime and is very
abundant in some localities, it must almost certainly be disliked by
birds or by some animals who would otherwise devour it. If disturbed
while feeding it is said to turn round with fury and eject a quantity of
green liquid, of an acid and disagreeable smell similar to that of the
spurge milk, only worse.[97]

These facts, and Mr. Poulton's evidence that some larvae rejected by
lizards at first will be eaten if the lizards are very hungry, show that
there are differences in the amount of the distastefulness, and render
it probable that if other food were wanting many of these conspicuous
insects would be eaten. It is the abundance of the eatable kinds that
gives value to the inedibility of the smaller number; and this is
probably the reason why so many insects rely on protective colouring
rather than on the acquisition of any kind of defensive weapons. In the
long run the powers of attack and defence must balance each other. Hence
we see that even the powerful stings of bees and wasps only protect them
against some enemies, since a tribe of birds, the bee-eaters, have been
developed which feed upon them, and some frogs and lizards do so
occasionally.

The preceding outline will sufficiently explain the characteristics of
"warning coloration" and the end it serves in nature. There are many
other curious modifications of it, but these will be best appreciated
after we have discussed the remarkable phenomenon of "mimicry," which is
bound up with and altogether depends upon "warning colour," and is in
some cases the chief indication we have of the possession of some
offensive weapon to secure the safety of the species imitated.

_Mimicry._

This term has been given to a form of protective resemblance, in which
one species so closely resembles another in external form and colouring
as to be mistaken for it, although the two may not be really allied and
often belong to distinct families or orders. One creature seems
disguised in order to be made like another; hence the terms "mimic" and
mimicry, which imply no voluntary action on the part of the imitator. It
has long been known that such resemblances do occur, as, for example,
the clear-winged moths of the fami