This explanation may, perhaps, at first sight appear far-fetched, but if the experiment be made of observing a caterpillar of _Sphinx Ligustri_ on its food-plant, not immediately before one’s eyes in a room, but at a distance as under natural conditions, it will be found that the violet edges do not stand out brightly, but show a colour very similar to that of the shadows playing about the leaves. The coloured edges, in fact, produce a more effective breaking up of the large green surface of the caterpillar’s body, than whitish stripes alone. Of course if the insect was placed on a bare twig in the sun, it would be easily visible at a distance; the larva never rests in such a position, however, but always in the deep shadow of the leaves, in which situation the coloured edges produce their peculiar effect. It may be objected that the oblique white stripes, standing simply on a dark green ground-colour, would produce the same effect, and that my explanation therefore leaves the bright colouring of these edges still unaccounted for. I certainly cannot say why in _Sphinx Ligustri_ these edges are lilac, and in _S. Drupiferarum_, _S. Prini_, and _Dolba Hylæus_ red, nor why they are black and green in _Macrosila Rustica_, and blue in _Acherontia Atropos_. If we knew exactly on what plants these caterpillars fed originally, we might perhaps indulge in comparing with an artistic eye the shadows playing about their leaves, seeing in one case more red, and in another more blue or violet. The coloured stripes of the _Sphingidæ_ must be regarded as the single strokes of a great master on the countenance of a human portrait. Looked into closely, we see red, blue, or even green spots and strokes; but all these colours, conspicuous when close, disappear on retreating, a general effect of colour being then produced, which cannot be precisely described by words.
Quite in accordance with this explanation, we see caterpillars with the brightest coloured stripes concealing themselves in the earth by day, and betaking themselves to their food-plants only in the dusk of the evening or dawn of morning and even during the night; _i.e._ in a light so faint that feeble colours would produce scarcely any effect. The bright blue of _Acherontia Atropos_, for example, would give the impression of oblique shadows without any distinctive colour.