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. [Illustration: 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.]