The adult larvæ of five species are known, and to these I can now
add a sixth. In Gray the genus contains twenty-six species.[105] I
cannot find any figures or descriptions of the young stages of these
caterpillars, and I have myself only observed the complete ontogeny of
one species.
By placing a captured female _M. Stellatarum_ in a capacious
breeding-cage, in the open air, I was enabled to procure eggs. The
moth hovered about over the flowers, and laid its small, grass-green,
spherical eggs (partly when on the wing), singly, on the leaves, buds,
and stalks of _Galium Mollugo_. Altogether 130 were obtained in three
days.[106]
_First Stage._
After about eight days the caterpillars emerge. They are only two
millimeters in length, and are at first yellowish, but soon become
green, set with small single bristles, and they possess a short
greenish caudal horn, which afterwards becomes black. The head is
greenish-yellow. The young larvæ are entirely destitute of marking.
(Pl. III., Fig. 1).
_Second Stage._
The first moult takes place after four days, the caterpillar now
acquiring the marking which it essentially retains to pupation.
Fine white subdorsal and spiracular lines appear, and at the same time
a dark green dorsal line, which, however, does not arise from the
deposition of pigment, as is generally the case, but from a division in
the folds of the fatty tissue along this position. (Fig. 2, Pl. III.)
The colour is now dirty green in all specimens, the skin being finely
shagreened.
_Third Stage._
The second moult, occurring after another period of four days, does not
bring any change of marking, the colour only becoming somewhat darker.
Length, twelve millimeters.
_Fourth Stage._
The third moult (after another four days) likewise brings only a
change of colouring, which is of such a nature that the caterpillar
becomes dimorphic. At the same time that peculiar roughening of the
skin takes place which, in the case of _Chærocampa_, was designated as
“shagreening.” The colour is now light grass-green in some specimens,
and dark green in others; in these last the subdorsal line is edged
above with dark brown, and the spiracles are also of this colour.
Length, seventeen millimeters.
_Fifth Stage._
Four days later, after the fourth ecdysis, the dimorphism becomes a
polymorphism. Five chief types can be distinguished:--
_Variety I._--Light green (Fig. 7, Pl. III.); dorsal line,
blackish-green, strongly marked; subdorsal line broad, pure white,
edged above with dark green; spiracular line, chrome-yellow; horn,
black, with yellow tip and blue sides. Spiracles, blackish-brown, with
narrow yellow border; legs, and extremities of prolegs, vermilion-red.
_Variety II._--Blackish-brown (Fig. 6, Pl. III.); head and prothorax,
yellowish-brown; markings the same as above.
_Variety III._--Blackish-green or greenish-black (Figs. 10 and
11, Pl. III.); subdorsal line with blackish-green border above,
gradually passing into a light green ground-colour; spiracular line,
chrome-yellow; head and prothorax, greenish-yellow.
_Variety IV._--Light green (Figs. 4 and 12, Pl. III.); dorsal line
quite feeble; subdorsal broad, only faintly edged with dark green;
subspiracular line, faint yellowish; head and prothorax, green.
_Variety V._--Brownish-violet (Fig. 8, Pl. III.); the black dorsal line
on a reddish ground either narrow or broad.
From these five varieties we see that the different types do not
stand immediately next to one another; they are, in fact, connected
by numerous transitional forms, the ground-colour varying greatly,
being dark or light, yellowish or bluish. (Compare Figs. 4, 5, 7, and
12.) The markings remain the same in all, but may be of very different
intensities. The dorsal line is often only very feebly indicated, and
the subdorsal line is frequently but faintly edged; the latter is
also sometimes deep black above and bordered rather darkly beneath,
the sides then being of a dark green, often with blackish dots on
the yellow spiracular line (Fig. 5, Pl. III.), this likewise being
frequently edged with black. Only the horn and legs are alike in all
forms. The green ground-colour passes into blackish-green, greenish or
brownish-black, and again, from reddish-brown to lilac (Fig. 3), this
last being the rarest colour.
The designation “polymorphism” may here appear very inapplicable,
since we have no sharply distinct forms, but five very variable
ground-colours connected by numerous intermediate modes of coloration.
Should, however, the term “variability” be suggested, I am in
possession of an observation which tends to show that the different
colours have to a certain extent become fixed. I found a brown
caterpillar, the five front segments of which were light green on
the left side, and the fifth segment brown and green mixed (Fig. 9,
Pl. III.). Such parti-coloration can evidently only appear where we
have contending characters which cannot become combined; just as in
the case of hermaphrodite bees, where one half of a segment is male
and the other half female, the two characters never becoming fused so
as to produce a truly intermediate form.[107] From this observation,
I conclude that some of the chief varieties of _Stellatarum_ have
already become so far removed from one another that they must be
regarded as intermediate fixed forms, the colours of which no longer
become fused together when they occur in one individual, but are
developed in adjacent regions. Other facts agree with this conclusion.
Thus, among the 140 adult larvæ which I bred from the batch of eggs
above mentioned, the transition forms were much in the minority. There
were forty-nine green and sixty-three brown caterpillars, whilst only
twenty-eight were more or less transitional.
On these grounds I designate the phenomenon as “polymorphism,” although
it may not yet have reached, as such, its sharpest limits. This would
be brought about by the elimination of the intermediate forms.[108]
Immediately before pupation, all the caterpillars, both green and
brown, acquire a lilac coloration. The fifth stage lasts seven days,
and the whole larval development twenty-three days, the period from
the deposition of the eggs to the appearance of the moth being only
thirty-one days.
I have treated of the polymorphism of _Stellatarum_ in detail, not
only because it has hitherto remained unknown, and an analysis of such
cases has been completely ignored,[109] but more particularly because,
it appears to me, that important conclusions can be drawn therefrom.
Moreover, such an extreme multiplicity of forms is interesting, since,
so far as I know, polymorphism to this extent has not been observed in
any insect.
The theoretical bearing of this polymorphism will be treated of
subsequently. It is not in any way connected with a more advanced
development of the markings, since _M. Stellatarum_ shows in this
respect a very low state of development. This species displays only
two stages:--(1), complete absence of all markings; and (2), a simple
subdorsal, with dorsal and spiracular lines. We must therefore admit
that the phyletic development of the markings has for a long time
remained at a standstill, or, what expresses the same thing, that the
marking which the adult larva now possesses is extremely old.
In order to complete my observations on _M. Stellatarum_, I now add
some remarks on the pupa, the colour variations of which it appeared
of importance to investigate, owing to the extraordinary variability
of the caterpillar. The pupa varies but very slightly; the ochreous
yellow ground-colour sometimes passes into reddish, and sometimes
into greenish; the rather complicated blackish-brown marking of
streaky lines is very constant, especially on the wing portions, being
at most only more or less strongly pronounced. The minute colour
variations of the pupa therefore have no connection with the colour
of the caterpillar, both green and brown larvæ furnishing sometimes
reddish-yellow and sometimes greenish-yellow pupæ.
The comparison of _M. Stellatarum_ with the other known species of the
genus, brings scarcely any addition to our knowledge of the phyletic
development. Thus, the two European species of which the caterpillars
are known, viz. _M. Fuciformis_ and _Bombyliformis_,[110] show
essentially the same markings as _Stellatarum_, the chief element being
a well-developed subdorsal line. The Indian _M. Gilia_, Herrich-Schäf.,
possesses also this line,[111] and, together with the East Indian _M.
Corythus_, Walk.,[112] has oblique stripes in addition; the stripes
do not, however, cross this line, but commence underneath it, and
probably originated at a later period than the subdorsal line. Should
this be the case, we must regard _M. Corythus_ as representing a
later phyletic stage. According to Duponchel’s figures, in both _M.
Fuciformis_ and _Bombyliformis_ small oblique stripes (red) occur near
the spiracles, but these have nothing to do with the oblique stripes
of _M. Gilia_ just mentioned, as they run in a contrary direction. Of
the two European species, I have only seen the living caterpillar of
_Fuciformis_, and this possessed no oblique stripes.
To these five species I am now enabled to add a sixth, viz.
_Macroglossa Croatica_,[113] a species inhabiting Asia Minor and
Eastern Europe, of which a specimen and notice were kindly forwarded
to me by Dr. Staudinger. The adult caterpillar much resembles that of
_M. Stellatarum_ in form and marking, but the subdorsal line appears
much less distinctly defined, and the dorsal and spiracular lines seem
to be entirely absent. The colour is generally green, but varies to
red, and the subdorsal is more distinct and sharper in the young than
in the adult larva. The markings of this species do not therefore in
any way surpass those of _Stellatarum_, but are, on the contrary, much
simpler.[114]