Females captured on the wing, laid in the breeding-cage single eggs of
a light green colour, spheroidal in form, and very similar to those of
_C. Elpenor_.
_First Stage._
The caterpillars on first hatching measure 3.5 millimeters in length,
and are of a uniform light green colour, with a fine white transverse
line on the posterior edge of each segment, precisely similar to that
which appears in the second stage of _C. Elpenor_. They resemble the
latter species still further in showing a fine white subdorsal line,
which can easily be recognized by the naked eye (Fig. 24). Although
the adult larva is distinguished from all the other known species of
_Chærocampa_ by the absence of a caudal horn, a distinct but very small
one is nevertheless present at this first stage, and is indeed retained
throughout the entire course of development, but does not increase
further in size, and thus gradually becomes so small in proportion to
the size of the caterpillar that it may be entirely overlooked.
The first moult takes place after 4-5 days.
_Second Stage._
The blue-green coloration remains unchanged; but a somewhat darker
green dorsal line becomes apparent down the middle of the back (the
dorsal vessel?), and the subdorsal line now becomes very broad and pure
white, being much more conspicuous than in any stage of _C. Elpenor_
(Fig. 25). The tapering of the three front segments occurs at this
stage, and oblique, dark green striations on a lighter ground stand out
distinctly on the spiracles. As with _C. Elpenor_, the first traces of
the future eye-spots appear during the second stage; not in the present
case as a curvature of the subdorsal line, but as a spot-like widening
of the latter, of a brighter white than the somewhat greenish colour of
the remainder of the line.
_Third Stage._
After the second moult, the formation of the dark “ground-area” of the
eye-spots commences by the appearance of a little brown on the under
edge of the foremost of the white spots, this coloration gradually
increasing in extent and in depth. At the same time both spots become
more sharply distinguishable from the subdorsal line, which becomes
constantly greener (Fig. 27). The brown colour soon grows round the
white of the front eye-spot, which becomes so far perfected; whilst
the completion of the hind spot is effected slowly afterwards. The
formation of the eye-spots does not therefore proceed any more rapidly
in this species than in _C. Elpenor_.
At the end of the present stage the length of the caterpillar is about
four centimeters; the ground colour is still sea-green; the subdorsal
line is much diminished, completely fading away at its lower edge, but
remaining sharply defined above, against the green ground-colour (Fig.
26).
_Fourth Stage._
After the third moult all the caterpillars (5) became brown, this
change occurring therefore one stage earlier than is generally the case
with _C. Elpenor_. In single instances the brown colour appeared in the
third stage. The subdorsal line had disappeared from all the segments
but the three first and the last. The eye-spots now rapidly attained
complete development; they contained a black pupil, and gave the insect
a truly repulsive appearance when, on being threatened by danger, it
drew in the front segments, and expanded the fourth (Fig. 28). The
eye-spots of the fifth segment are much less developed than in _C.
Elpenor_; they remain small, and are not readily detected. On the other
hand, there now appear on all the segments with the exception of the
last, just as in the sixth stage of _C. Elpenor_, distinct rudiments of
eye-spots, which present the appearance of irregular, roundish, black
spots on the front borders of the segments, at the height of the former
subdorsal line. In this latter region the black pigment is disposed as
a longitudinal streak, and to this a median line is added, the whole
forming a marking which perhaps makes the caterpillar appear still more
alarming to its foes. This marking is, however, only to be distinctly
recognized on the three first segments. The “dorsal spots” mentioned in
the case of _C. Elpenor_ then appear very distinctly on segments 5-11.
The caterpillars continued to feed for eleven days after the third
moult, at the end of which period the fourth moult took place, but
without the occurrence of any change of marking. The larvæ then buried
themselves, the complete development having taken 28-29 days.
The development of the _Porcellus_ caterpillar was twice followed;
in 1869 in twelve, and in 1874 in five specimens. In no case did I
obtain caterpillars which remained green throughout the entire course
of development, although this colour is stated in the books to occur
occasionally in these larvæ; neither have I been able to find any
figure of an adult green specimen, so that it must in the meantime be
admitted that such specimens, if they occur at all, are exceptional
instances.[71] The theoretical bearing of this admission will appear
later on.
RESULTS OF THE DEVELOPMENT OF CHÆROCAMPA ELPENOR AND C. PORCELLUS;
COMPARISON OF THESE WITH THE OTHER KNOWN SPECIES OF CHÆROCAMPA.
The first stage of _Elpenor_ shows that the most remote ancestor of
the genus possessed no kind of marking, but was uniformly green. At a
later period, the white longitudinal stripe which I have designated
the “subdorsal line” made its appearance, and at a still later period
this line vanished, with the exception of a few more or less distinct
remnants, whilst, at the same time, from certain portions of it, the
eye-spots of the fourth and fifth segments became developed. After the
perfecting of the eye-spots, weak repetitions of the latter appeared as
black spots on all the segments except the last.
In _Porcellus_ the caterpillar emerges from the egg with the subdorsal
line, the first stage of _Elpenor_ being omitted. From this fact we may
venture to conclude that _Porcellus_ is the younger species, or, what
comes to the same thing, that it has further advanced in development.
The whole subsequent history of _Porcellus_ agrees with this view,
its course of development being essentially but a repetition of the
phenomena displayed by _Elpenor_, and differing only in one point, viz.
that all new characters make their appearance one stage earlier than
in the latter species. This is the case with the transformation of the
green into a brown ground-colour; with the repetition of the eye-spots
on the remaining segments in the form of suffused black spots; and
with the appearance of the light “dorsal spots.” Only the eye-spots
themselves appear, and the snout-like tapering of the front segments
occurs in the same stage as in _Elpenor_, _i.e._ the second.
From these data alone, we may venture to infer the occurrence of four
chief stages in the phyletic development of the genus. The first stage
was simply green, without any marking; the second showed a subdorsal
line; the third, eye-spots on the third and fourth segments; and
the fourth stage showed a repetition of the eye-spots, although but
rudimentary, on all the remaining segments with the exception of the
twelfth.
Now if we compare the other known species of _Chærocampa_ larvæ with
the above, we shall arrive at the interesting conclusion that all these
species can be arranged in three groups, which correspond exactly with
the three last phyletic stages as just deduced from the ontogeny of _C.
Elpenor_ and _Porcellus_.
Of the genus _Chærocampa_,[72] over fifty species have been
described,[73] of which the larvæ of only fifteen are known in the
form which they possess at the last ontogenetic stage.
GROUP 1.--I can furnish but little information with respect to this
group. The first species with which I became acquainted was _Chærocampa
Syriaca_,[74] of which I saw two blown caterpillars in Staudinger’s
collection, and which I have figured in Pl. IV., Fig. 29. The larva
is green, and has the short oblique stripes over the legs common to
so many species of _Chærocampa_, the only marking besides these being
a simple white subdorsal line, without any trace of eye-spots. This
species exactly corresponds therefore with the second ontogenetic
stage of _C. Elpenor_ and _Porcellus_. The account of the species,
both in the larval and perfect state, is unfortunately so imperfect,
that we cannot with certainty infer the age of the two caterpillars
from their size. If the moth were of the same size as _Elpenor_, then
the caterpillar figured, having a length of 5.3 centimeters, would not
be in the last but in the penultimate stage, and it remains doubtful
whether it may not acquire eye-spots in the last stage.
That species exist, however, which in their last stage correspond to
the second stage of _Elpenor_, is shown by two of the forms belonging
to Walker’s genus _Darapsa_, which was founded on the characters
of the imagines only. Ten species of this genus are given in Gray’s
catalogue, the adult larva of two of these being known through the
excellent figures of Abbot and Smith.[75] These two caterpillars
possess the characteristic tapering form in a very marked degree;
one is figured in the attitude so often assumed by our species of
_Chærocampa_ on the approach of danger, the three front segments
being withdrawn into the fourth. (Fig. 34, Pl. IV., is copied from
this Plate). There are no eye-spots either in _D. Myron_ or _D.
Chœrilus_,[76] but only a broad white subdorsal line; underneath which,
and to a certain extent proceeding from it, there are oblique white
stripes, precisely similar to those which meet the subdorsal line in
the third stage of _C. Porcellus_.[77]
GROUP 2.--This group contains numerous species which, like our native
_C. Elpenor_ and _Porcellus_, show eye-spots on the fourth and fifth
segments, whilst these markings are absent, or at most only present in
traces, on the remainder. To this section there belong, besides the
two species mentioned, five others, viz. in Europe, _C. Celerio_ and
_Alecto_ (not certainly known?);[78] in India, _C. Nessus_, Drury, and
_Lucasii_, Boisduval;[79] and an unnamed species from Port Natal.
In the species belonging to this group the subdorsal line may be more
or less retained. Thus, _C. Celerio_, according to Hübner’s figure,
has a broad yellow line extending from the horn to the sixth segment,
whilst it is completely absent on the three front segments. In the
unnamed species from Port Natal[80] the subdorsal line extends to the
front edge of the fifth segment, and on the fourth segment only is
there a perfect eye-spot, whilst on the succeeding segments traces of
such markings can be recognized as dark spots similar to those in
_Elpenor_ and _Porcellus_. The transition to the third group is through
another unnamed species from Mozambique,[81] in which rather large
eye-spots have become developed on the fourth and fifth segments and
these are followed by a subdorsal line, which only appears distinctly
at certain places. On this broken subdorsal line, and not completely
separated from it, there are small, roundish eye-spots, situated near
the front edge of each segment; these being, therefore, a somewhat more
perfect repetition of the front eye-spots.[82]
GROUP 3.--In the species of this group the eye-spots are repeated on
all the segments. I am acquainted with seven such _Chærocampa_ larvæ,
of which _C. Bisecta_, Horsfield,[83] shows some affinity to the
foregoing group, since the eye-spots on segments 6-11 have not yet
attained full perfection. In _C. Odenlandiæ_, Fabr.,[84] and in _C.
Alecto_ from India,[85] the eye-spots appear to be perfectly alike on
all the segments; whilst in _C. Acteus_, Cram.,[86] and in the North
American _C. Tersa_[87] (Pl. IV., Fig. 35) they are smaller on the
other segments than on the fourth; and in _C. Celerio_, Linn., from
India,[88] the size of the spots diminishes from the head to the tail.
In this group also the subdorsal line is retained in a very variable
degree. In some species it appears to have completely vanished (_C.
Acteus_, _Celerio_); in others it is present as a light stripe
extending along all the segments (_C. Alecto_); whilst in others it
is retained as a broad white stripe, which extends only to the fourth
segment (_C. Tersa_, Fig. 35). In species possessing eye-spots, the
subdorsal line is thus a very variable character. It is, however, an
interesting fact that even in the present group, which has made the
greatest step forward, the subdorsal line is of general occurrence,
because the eye-spots in all these species may have almost a similar
development to those of _Elpenor_ and _Porcellus_. The ontogeny of the
tropical species would alone give a definite reply on this point, but
unfortunately we are not acquainted with any of the young forms, so
that we can but presume that some of them at least would show only in
the first stage the simple subdorsal line without eye-spots; that in
the second stage the primary pairs of eye-spots would be formed on the
fourth and fifth segments, whilst the transference of these spots to
the remaining segments would take place in the last stage.
The foregoing assumption is based immediately on the ontogeny of
_Elpenor_ and _Porcellus_; it is supported by the considerable size
attained by the eye-spots in many species of the third group, and
would receive additional confirmation by observations on the Indian
_C. Celerio_, supposing that Horsfield’s statements do not arise from
a confusion of species. This skilful observer, who was the first to
breed systematically a large number of tropical larvæ, has given a
figure of the Indian caterpillar of _C. Celerio_, according to which
this species possesses eye-spots on all the segments from the fourth to
the tenth. The European form of this same species has eye-spots only on
segments four and five, a fact which does not appear to have been known
to Horsfield, as no mention of it is made in his notice of the Indian
species. If the caterpillar figured is really that of _Celerio_, which
I consider to be by no means improbable, not only is it thus shown that
in the species of the third group the ocelli on the hind segments have
a secondary origin through a repetition of the primary ones of the
front segments, but we can also establish that the same species in two
different regions may arrive at two different phyletic stages.
If, finally, we sum up the facts taught by the ontogeny of the two
German species, and the adult forms of the other species, we can form
therefrom a tolerably complete picture of the course of development of
the genus _Chærocampa_. Of the four phyletic stages indicated by the
ontogeny of _Elpenor_ and _Porcellus_, three still form the terminus
of the development of existing species. The great differences among
the caterpillars of this genus can be very simply explained on the
view that they stand at different levels of phyletic development; some
species having remained far behind (Group 1), others having advanced
further (Group 2), and others having reached the highest point of
development (Group 3). The fact that the species of the third group
are only tropical accords well with this view, since many facts prove
that phyletic development proceeds more rapidly in the tropics than in
temperate climates.
The striking markings of the _Chærocampa_ larvæ may, in brief, be
stated to originate from a local transformation of two portions of the
subdorsal line into eye-spots, and the subsequent transference of
these two primary ocelli to the other segments. The eye-spots always
originate on segments four and five, and from these the transference
mostly occurs backwards, although in certain cases it takes place at
the same time forwards. Herein, _i.e._ in the origin of the eye-spots,
there lies a great distinction between the genus _Chærocampa_ and the
genus _Deilephila_, with which it was formerly associated, and in which
the origin of a very similar kind of marking can be traced to quite
another source.