The green eggs much resemble those of _Populi_, as also do the newly
hatched caterpillars, which, as in the case of this last species, are
entirely without markings. As with _Populi_, the markings are formed in
the course of the first stage, and are distinctly visible before the
first moult. The long caudal horn is of a red colour.
After two to three days the caterpillars moult, their length then being
one centimeter; the seven beautiful oblique white stripes, and the fine
white subdorsal line, are more strongly pronounced, the latter becoming
broader in front. They differ from _Populi_ in having the oblique
stripes united in the dorsal line.
The second moult occurs after another three days, and brings no
important change; only the fine subdorsal line becoming somewhat
fainter. Neither is the third moult, which takes place four days later,
associated with the appearance of any essentially new character. The
oblique stripes remain as before, but their upper portions now stand
on a somewhat darker green ground-colour, whilst the subdorsal line
vanishes, leaving distinct traces only on the three or four front
segments.
The fourth moult follows after a period of seven days, and my bred
larvæ underwent scarcely any alteration in marking. Only small
differences in coloration became perceptible in the head and horn,
these changing to bluish. Specimens occur, although but rarely, which
show in this last stage red spots in the vicinity of the oblique
stripes, just in the same manner as with _Populi_, in which species,
however, they occur more commonly. I only once found an adult larva of
_Ocellatus_ possessing reddish-brown spots above and below the oblique
stripes,[99] exactly as in one of the specimens figured by Rösel.[100]
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]