It is not here my intention to go through all the groups of Lepidoptera in this manner. The instances adduced are quite sufficient to prove that longitudinal stripes occur wherever we should expect to find them, and that they really possess the biological significance which I have ascribed to them. That these markings are occasionally converted into an adaptive imitation of certain special parts of a plant, is shown by the larvæ of many moths, such for example as Chesias Spartiata, which lives on broom (Spartium Scoparium), its longitudinal stripes deceptively resembling the sharp edges of the stems of this plant.[143]
3. Oblique striping. Can the lilac and white oblique stripes on the sides of a large green caterpillar, such as those of Sphinx Ligustri; or the red and white, or white, black, and red stripes of Smerinthus Tiliæ and Sphinx Drupiferarum respectively, be of any possible use? Have we not here just one of those cases which clearly prove that such a character is purely morphological, and worthless for the preservation of the individual? Does not Nature occasionally sport with purposeless forms and colours; or, as it has often been poetically expressed, does she not here give play to the wealth of her phantasy?
At first sight this indeed appears to be the case. We might almost doubt the adaptive importance of the green ground-colour on finding coloured stripes added thereto, and thus—as one might suppose—abolishing the beneficial action of this ground-colour, by making the insect strikingly conspicuous. But this view would be decidedly incorrect, since oblique stripes are of just the same importance as longitudinal stripes. The former serve to render the caterpillar difficult of detection, by making it resemble, as far as possible, a leaf; they are imitations of the leaf-veins.
Nobody who is in the habit of searching for caterpillars will doubt that, in cases where the oblique stripes are simply white or greenish-white, it is extremely difficult to see the insect on its food-plant, e.g. S. Ocellatus on Salix; not only because it possesses the colour of the leaves, but no less because its large body does not present an unbroken green surface, which would bring it into strong contrast with the leaves, and thus arrest the attention. In the case of the species named, the coloured area of the body is divided by oblique parallel stripes, just in the same manner as a willow leaf. In such instances of course we have not presented to us any special imitation of a leaf with all its details—there is not a perfect resemblance of the insect to a leaf, but only an arrangement of lines and interspaces which does not greatly differ from the division of a leaf by its ribs.
That this view is correct is shown by the occurrence of this form of marking. It is on the whole rare, being found, besides in many Sphingidæ, in isolated cases in various families, but is always confined to those larvæ which live on ribbed leaves, and never occurring in species which feed on grasses or on trees with needle-shaped leaves. This has already been shown with respect to the Sphingidæ, in which the oblique stripes are only completely developed in the subfamilies Smerinthinæ and Sphinginæ. The species of Smerinthus all live on trees such as willows, poplars, lime, oak, &c., and all possess oblique stripes. The genus Anceryx also belongs to the Sphinginæ, and these caterpillars, as far as known, live on trees with needle-shaped leaves. The moths of this last genus are very closely allied to the species of Sphinx, not only in form and colour, but also in many details of marking. The larvæ are however different, this distinction arising entirely from their adaptation to needle-shaped leaves, the Sphinx caterpillars being adapted to ordinary foliage. The species of Anceryx, as has been already shown, are brown mixed with green, and never possess even a trace of the oblique stripes, but have a latticed marking, consisting of many interrupted lines, which very effectively serves to conceal them among the needles and brown bark of the Coniferæ.
Of the Sphinginæ living on plants with ordinary foliage, not a single species is without oblique stripes. I am acquainted with ten species of caterpillars and their respective food-plants, viz. Sphinx Carolina, Convolvuli, Quinquemaculata, Prini, Drupiferarum, Ligustri; Macrosila Rustica and Cingulata; Dolba Hylæus and Acherontia Atropos.
Besides among the Sphinginæ, oblique stripes occur in the larvæ of certain butterflies, viz. Apatura Iris, Ilia, and Clytie, all of which live on forest trees (aspen and willows), and are excellently adapted to the leaves by their green colour. In addition to these, I am acquainted with the larvæ of some few moths, viz. of Aglia Tau and Endromis Versicolora, both of which also live on forest trees.
Oblique stripes also occasionally occur in the smaller caterpillars of Noctuæ, Geometræ, and even in those of certain Pyrales, in all of which they are shorter and differently arranged. In these cases also, my theory of adaptation holds good, but it would take us too far if I attempted to go more closely into them. I will here only mention the extraordinary adaptation shown by the caterpillar of Eriopus Pteridis. This little Noctuid lives on Pteris Aquilina; it possesses the same green colour as this fern, and has double oblique white stripes crossing at a sharp angle on each segment, these resembling the lines of sori of the fern-frond so closely, that the insect is very difficult to perceive.
After all these illustrations it can no longer remain doubtful that the oblique stripes of the Sphingidæ are adaptive. But how are the coloured edges bordering these stripes in so many species to be explained?
I must confess that I long doubted the possibility of being able to ascribe any biological value to this character, which appeared to me only conspicuous, and not protective. Cases may actually occur in which the brightly coloured edges of the oblique stripes make the caterpillar conspicuous—just in the same manner as any marking may bring about a conspicuous appearance by presenting a striking contrast of colour. I am acquainted with no such instance, however. As a rule, in all well-adapted caterpillars, considering their colour in its totality, this is certainly not the case. The coloured edges, on the contrary, enhance the deceptive appearance by representing the oblique shadows cast by the ribs on the under-side of the leaf; all these caterpillars rest underneath the leaves, and never on the upper surface.