In this stage also there remains almost always on the three to six front segments, a more or less distinct residue of the subdorsal, which extends backwards from the head as a whitish line intersecting the foremost oblique stripes. (Fig. 70, Pl. VII.)
RESULTS OF THE DEVELOPMENTAL HISTORY OF SMERINTHUS TILIÆ, POPULI AND OCELLATUS.
From the meagre materials furnished by these three obviously nearly related species, we may at least conclude that, with respect to marking, three stages of development can be distinguished:--(1) Simple (green) coloration without marking; (2) subdorsal lines crossed by seven pairs of oblique stripes; (3) more or less complete absence of the subdorsal lines, the oblique stripes remaining, and showing a tendency to become edged with a red border.
Which of the three species is the oldest I will not attempt to decide. If we might venture to form any conclusion from the frequency of the red spots, _Tiliæ_ would be the youngest, _i.e._, the species which has made the farthest advance. But this does not agree with the fact that the oblique stripes appear somewhat later in this species. Both these distinctions are, however, too unimportant to enable us to build certain conclusions on them. Neither does a comparison of the adult larvæ with other species of _Smerinthus_ furnish any further information of importance.
Of the genus _Smerinthus_, Latr., thirty species were catalogued by Gray,[101] of which I am only acquainted with the larvæ of eight (five European, and three North American). None of these in the last stage possess a complete subdorsal line together with oblique stripes. Neither, on the other hand, do any of them show a more advanced stage of development in having the red spots constantly formed into coloured border-stripes. We must therefore admit that they have all reached nearly the same stage of phyletic development. On turning to the doubtfully placed genus _Calymnia_, Boisduval, which is represented in Gray by only one species, figured by Westwood[102] as a _Smerinthus_, we first meet with an older stage of development of the genus.
The adult caterpillar of _C. Panopus_, from the East Indies, possesses, in addition to the oblique stripes, a completely developed subdorsal line,[103] and thus corresponds to the first stage of _S. Populi_. This species may possibly retain in its ontogeny a stage in which the oblique stripes are also absent, whilst the subdorsal line is present. From the early disappearance of the subdorsal line in the species of _Smerinthus_, we may venture to conclude that this character appeared at an early stage of the phylogeny, whilst the oblique stripes represent a secondary form of marking, as shall be further established subsequently.[104]