This is the only representative of the fifth and oldest group known to
me. The moth resembles _D. Euphorbiæ_ to the extent of being sometimes
confounded with it, a circumstance which is made the more remarkable by
the fact that the caterpillars are so completely different.
The adult larva of this local moth has been made known by the figures,
more or less exact, in the works of Hübner, Boisduval, and Duponchel.
Wilde also gives a description of it, although from a foreign source.
I will not here delay myself by criticizing the different descriptions
and figures; they are partly correct, partly inexact, and sometimes
altogether erroneous; they were of no avail for the question which here
primarily concerns us, and new observation had to be undertaken.
I have been able to compare altogether about forty caterpillars,
thirty-five of which were living. All these specimens possessed nearly
the same greyish-green ground-colour, and most of them had exactly
the simple marking as represented, for instance, in Hübner’s figure,
_i.e._, a rather broad greenish-white subdorsal line, somewhat faded
at the edges, and without a trace of spots on any of the segments
with the exception of the eleventh, on which there was a yellowish,
black-bordered mirror-spot, with a broad, diffused, vivid orange-red
nucleus. Specimens also occur, and by no means uncommonly, in which no
other markings are to be seen than those mentioned; there were nine
among twenty-eight examples compared from this point of view.
In many other individuals of this species small red spots appear on
the subdorsal line, exactly in the positions where the ring-spots are
situated in the other species of the genus (Fig. 60), so that these
spots are thus repetitions of the single ring-spot--a fact which must
appear of the greatest interest in connection with the development
of the markings throughout the whole genus. But this is not all, for
again in other specimens, these red spots stand on a large yellow
“mirror,” and in one individual (Fig. 59), they had become developed
into well-formed ring-spots through the addition of a black border. We
have thus presented to us in one and the same stage of a species, the
complete development of ring-spots from a subdorsal line.
These facts acquire a still greater interest, as showing how new
elements of marking are produced. The spots on the subdorsal line
decrease from the posterior to the anterior segments, so that they
must undoubtedly be regarded as a repetition or transference of the
ring-spot previously developed on the eleventh segment. I will now
proceed to furnish proofs in support of this statement.
I have never met with any specimens having ring-spots on all the
segments--in the most prominent instances these spots were present
on segments 10-5. This was the case in three out of the twenty-eight
caterpillars minutely examined. On all these segments, however, the
ring-spots were not equally developed, but increased in perfection
from the posterior towards the anterior segments. In the larva
represented in Fig. 59 for example, there is a completely developed
ring-spot on segment 10, which, although possessing but a feeble black
“ground-area,” is still distinctly bordered; on segment 9 this border
is less sharp, and not so dark, and it is still less sharp and much
lighter on segments 8 and 7, whilst it has completely disappeared from
segment 6, the yellow “mirror” having at the same time lost in size. On
segment 5, only two small contiguous reddish spots, the first rudiments
of the nucleus,[94] can be recognized on close inspection.
Specimens in which the spots extend from the eleventh to the seventh
segment are of more frequent occurrence, five having been found
among the twenty-eight. In these the spots diminish anteriorly in
size, perfection, and intensity of colour. Still more frequently (in
eleven specimens) are the ring-spots or their rudiments restricted to
the tenth and ninth segments, the spot on the latter being without
exception less developed than that on the former segment.
An anteriorly progressing formation of ring-spots thus undoubtedly
occurs, the spots generally diminishing in perfection very suddenly
towards the front segments; and specimens, such as that represented in
Fig. 60, Pl. VII., in which traces of ring-spots are to be seen on all
the segments from the tenth to the fifth, are of rare occurrence.
From what elements of marking are these _secondary_ ring-spots
resulting from transference developed? They do not, as in the case
of the _primary_ eye-spots of the _Chærocampinæ_, originate in the
separation of one portion of the subdorsal line, and the subsequent
formation of this detached spot into a “mirror;” but they arise from
the formation of a nucleus, first one and then two of the shagreen-dots
on the subdorsal line acquiring a yellowish or reddish colour (Fig.
61, Pl. VII., segments 6 and 7). The ground on which these two spots
are situated then becomes yellow (Fig. 61, Pl. VII., segment 8), and
a more or less distinct black border, having the form of two small
crescents, is afterwards formed. At a later period these two crescents
and also the two primary nuclei coalesce, producing a ring-spot which,
as in Fig. 61, Pl. VII., segment 9, can be distinctly resolved into two
portions.
It certainly cannot be denied that these facts may also be
theoretically interpreted in a reverse sense. We might interpret
the phenomena in this case, as also in that of _D. Zygophylli_, as
a gradual disappearance from the front towards the hind segments of
ring-spots formerly present, a view which could only be refuted by the
ontogeny of the species. I have not been fortunate enough to procure
eggs of _D. Hippophaës_, so that the younger stages are unknown to me.
Among my caterpillars, however, there were two in the fourth stage of
development, but these did not show ring-spots on all the segments, as
we should expect on the above view; on the contrary, no trace of such
spots could be seen on any of the segments with the exception of the
eleventh, on which there was a ring-spot less perfectly developed than
in the last stage.
In this fourth stage the larva of _D. Hippophaës_ is of a lighter green
(Fig. 58), the subdorsal yellowish with sharp boundaries, and the
infra-spiracular line pure white, as in the next stage. The shagreening
is present, but none of the shagreen-dots are red or reddish, and no
trace of a ring-spot can be detected on the subdorsal line with the
exception of that on the eleventh segment. In this last position this
line is somewhat widened, and a long, diffused, rose-red spot can there
be recognized upon it (Fig. 58 A). The black “ground-area” present
in the fifth stage is as yet absent, and the spot is not so sharply
separated anteriorly from the subdorsal line as it becomes later.
From these observations we might venture to expect that in the third
stage of _Hippophaës_, the subdorsal line would also be free from this
spot on the eleventh segment, and it is possible that in the second
stage this line is itself absent.