No spots ever occur on the other segments at this stage; they only appear in the last stage, but as they may be entirely wanting, they must have arisen as the result of internal laws of correlation, that is, they must be recapitulations of the hindmost spots which arose in the phylogeny through natural selection. We may conclude this, at least, if we believe in the truth of the fundamental proposition of the biogenetic law, and admit that there is in the ontogeny some more or less distinct recapitulation of the phylogeny.
Fig. 118. Two stages in the life-history of the Spurge Hawk-moth (Deilephila euphorbiæ). A, first stage, the caterpillar dark blackish-green, without marking. B, second stage, the row of spots is distinctly connected by a light streak, the vestige of the sub-dorsal stripe.
This proposition may be recognized as true in the case of Deilephila also, if we compare the different species with one another as regards their ontogeny. We find here too that not only the sub-dorsal, that is, the phyletically oldest marking of the Sphingid caterpillars, occurs everywhere in the young stages, but also that it is being shunted back to younger and younger stages, in proportion to the degree of the development of the spot-marking reached in the full-grown caterpillar. Thus, for instance, in the caterpillar of Deilephila euphorbiæ the highest form of spot-marking is reached, and in this species the sub-dorsal line is no longer the sole marking element at any stage. Leaving out of the question the absolutely unmarked little caterpillar which emerges from the egg (Fig. 118, A), there appears at once in the second stage a series of ring-spots connected by a fine white sub-dorsal line (Fig. 118, B). In the following stage, the third, this sub-dorsal line disappears without leaving a trace, and there remains only the spot-marking, which is subsequently duplicated.
Let us compare with this the ontogeny of the bed-straw hawk-moth, Deilephila galii (Fig. 117). The full-grown caterpillar possesses only a single row of ring-spots (B), and accordingly the young stages of the caterpillar up to the fourth show a distinct sub-dorsal line (A), although spots are seen upon it. A still earlier phyletic stage of development is illustrated by Deilephila livornica, in which the ring-spots are all connected by the sub-dorsal line.
It can thus hardly be doubted that the biogenetic law is guiding us aright when we conclude from a comparison of the ontogeny of the different species of Deilephila, that the oldest ancestors of the genus possessed only the longitudinal stripes, and that from these small pieces were cut off as ring-spots, and that these were gradually perfected and ultimately duplicated, while at the same time the original marking, the longitudinal stripe, was shunted back further and further in the young stages, until it finally disappeared altogether.
Let us now refer for a moment to the third form of marking in the caterpillars of the Sphingidæ—transverse striping. This has not arisen out of the sub-dorsal line, but quite independently and at a later date. This is proved with great certainty by the ontogeny of species of the genus Smerinthus. The full-grown, and usually also the young caterpillars, of these species have quite regularly the seven broad oblique stripes which run in the direction of the tail-horn at equal intervals on the lateral surfaces of the body ([Fig. 3]). They are absent only from the three anterior segments, and upon these a part of the older marking, the sub-dorsal stripe, has persisted. But we find this fully developed in the youngest stages of other species. In Smerinthus populi, the little caterpillar, which has no markings at all when it leaves the egg, very soon shows the white sub-dorsal line, and simultaneously with it the seven transverse stripes, which cut obliquely through it; in the older caterpillars the sub-dorsal then disappears (Fig. 119).
When I was investigating these matters at the beginning of the seventies I did not succeed in procuring eggs of the species of the genus Sphinx, which likewise almost all exhibit the oblique striping in their full-grown stages. But from what I knew of the ontogeny of Smerinthus species I was able to predict that, among the young stages of Sphinx, there must be some with sub-dorsal lines. This was confirmed later, for Poulton found in Sphinx convolvuli that in the first stage there are no oblique stripes, but only the sub-dorsal stripe, while in Sphinx ligustri both kinds of marking were present at the same time.
Fig. 119. Caterpillar of Smerinthus populi, the Poplar Hawk-moth, at the end of the first stage, showing both the complete sub-dorsal stripe and the oblique stripes.