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.
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