The Oleander Hawk-moth is also an interesting case.  Many of the Hawk-moth caterpillars have eye-like spots, to which I shall have to allude again presently.  These are generally reddish or yellowish, but in this species, which feeds on the periwinkle, they are bright blue, and in form as well as color closely resemble the blue petals of that flower.  One other species, the Sharp-winged Hawk-moth, also has two smaller blue spots, with reference to which I can make no suggestion.  It is a very rare species, and I have never seen it.  Possibly, in this case, the blue spots may be an inherited character, and have no reference to the present habits.  They are, at any rate, quite small.

No one who looks at any representations of Hawk-moth caterpillars can fail to be struck by the peculiar coloring of those belonging to the Pine Moth, which differ in style of coloring from all other sphinx larvae, having longitudinal bands of brown and green.  Why is this?  Their habitat is different.  They feed on the leaves of the pinaster, and their peculiar coloring offers a general similarity to the brown twigs and narrow green leaves of a conifer.  There are not many species of butterflies or moths which feed on the pine, but there are a few:  and most, if not all of them, have a very analogous style of coloring to that of the Pine Moth, while the latter has also tufts of bluish-green hair which singularly mimic the leaves of the pine.  It is still more remarkable that in a different order of insects we again find species—­for instance one of the saw-flies—­which live on the pine, and in which the same style of coloring is repeated.

Let us now take a single group, and see how far we can explain its various colors and markings, and what are the lessons which they teach us.  For this purpose, I think I cannot do better than select the larvae of the Hawk-moths, which have just been the subject of a masterly work by Dr. Weissmann, from which most of the following facts are taken.

The caterpillars of this group are very different in color—­green, white, yellow, brown, sometimes even gaudy, varied with spots, patches, streaks, and lines.  Now, are these differences merely casual and accidental, or have they a meaning and a purpose?  In many, perhaps in most cases, the markings serve for the purpose of concealment.  When, indeed, we see caterpillars represented on a white sheet of paper, or if we put them on a plain table, and focus the eye on them, the colors and markings would seem, if possible, to render them even more conspicuous; but amongst the intricate lines and varied colors of foliage and flowers, and if the insect be a little out of focus, the effect is very different.

[Illustration:  FIG. 3.—­THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH (Chaerocampa elpenor).  Full grown.  Natural size.]

[Illustration:  FIG. 4.—­THE CATERPILLAR OF THE ELEPHANT HAWK-MOTH (Chaerocampa elpenor).  First stage.]