e of the sloths--defenceless animals which
feed upon leaves, and hang from the branches of trees with their back
downwards. Most of the species have a curious buff-coloured spot on the
back, rounded or oval in shape and often with a darker border, which
seems placed there on purpose to make them conspicuous; and this was a
great puzzle to naturalists, because the long coarse gray or greenish
hair was evidently like tree-moss and therefore protective. But an old
writer, Baron von Slack, in his _Voyage_ _to Surinam_ (1810), had
already explained the matter. He says: "The colour and even the shape of
the hair are much like withered moss, and serve to hide the animal in
the trees, but particularly when it has that orange-coloured spot
between the shoulders and lies close to the tree; it looks then exactly
like a piece of branch where the rest has been broken off, by which the
hunters are often deceived." Even such a huge animal as the giraffe is
said to be perfectly concealed by its colour and form when standing
among the dead and broken trees that so often occur on the outskirts of
the thickets where it feeds. The large blotch-like spots on the skin and
the strange shape of the head and horns, like broken branches, so tend
to its concealment that even the keen-eyed natives have been known to
mistake trees for giraffes or giraffes for trees.

Innumerable examples of this kind of protective colouring occur among
insects; beetles mottled like the bark of trees or resembling the sand
or rock or moss on which they live, with green caterpillars of the exact
general tints of the foliage they feed on; but there are also many cases
of detailed imitation of particular objects by insects that must be
briefly described.[69]

_Protective Imitation of Particular Objects._

The insects which present this kind of imitation most perfectly are the
Phasmidae, or stick and leaf insects. The well-known leaf-insects of
Ceylon and of Java, species of Phyllium, are so wonderfully coloured and
veined, with leafy expansions on the legs and thorax, that not one
person in ten can see them when resting on the food-plant close beneath
their eyes. Others resemble pieces of stick with all the minutiae of
knots and branches, formed by the insects' legs, which are stuck out
rigidly and unsymmetrically. I have often been unable to distinguish
between one of these insects and a real piece of stick, till I satisfied
myself by touching it and found it to b