Fig. 115. Caterpillar of the Humming bird
Hawk-moth, Macroglossa stellatarum. sbd, the sub-dorsal
line.
Transverse striping occurs as the sole mode of marking in species which live on bushes and trees whose leaves have strong lateral veins, such as willows, poplars, oaks, privet, syringa, and so on, and these markings associated with the leaf-green of their colouring protect them most effectively from discovery.
The third scheme of marking, namely by spots, occurs in various forms in species of the genera Deilephila and Chærocampa, and it varies in its biological significance; in many species the spots serve as a warning colour, by making the caterpillar conspicuous and easily seen from a distance (Deilephila galii, [Fig. 117]); in others they imitate the eyes of a larger animal, and have a 'terrifying' effect, as we have already said (Fig. 4); in still other and rarer cases they heighten the resemblance of the caterpillar to its food-plant by mimicking parts of it, as, for instance, the red berries of the buckthorn (Deilephila hippophaës, Fig. 8, r).
Fig. 3 (repeated). Full-grown caterpillar of the
Eyed Hawk-moth, Smerinthus ocellatus. sb, the sub-dorsal
stripe.
Thus all three modes of marking possess a biological value, and protect the soft and easily wounded animal in some way, and, in the case of at least two of them, it is clear that they must have arisen at the very end of the caterpillar's development, since they can only be effective as the animal is approaching full size, and would be valueless in the very young caterpillar. The transverse striping only makes the caterpillar like a leaf when the stripes bear about the same relation to each other as those on the leaf, and eye-spots can only scare away lizards and birds when they are of a certain size. Only longitudinal striping is effective as a protection in the case of young caterpillars, supposing, that is, that they live in or on the grass (Fig. 116).
Fig. 4 (repeated). Full-grown caterpillar of the Elephant Hawk-moth, Chærocampa elpenor, in its 'terrifying attitude.'
