The adult larva, as made known by many, and for the most part good
figures, has very complicated markings, which do not seem derivable
from any of the elements of marking in the _Sphingidæ_ hitherto
considered. I was therefore much surprised at finding a young
caterpillar of this species, only twelve millimeters in length, of
a light green colour, without any trace of the subsequent latticed
marking, and with a broad white subdorsal line extending along all
the twelve segments. (Pl. VII., Fig. 63). Judging from the size and
subsequent development, this caterpillar was probably in the third
stage.
The same colouring and marking remained during the following (fourth)
stage; but in the position occupied by the caudal horn in other
_Sphingidæ_, there could now be observed the rudiment of a future
ocellus in the form of a round yellowish spot (Pl. VII., Fig. 64). The
subdorsal line disappears suddenly in the fifth stage, when the larva
becomes dark green (rarely) or blackish-brown; the latticed marking
and the small oblique stripes are also acquired, together with the
beautifully developed eye-spots, consisting of a yellow mirror with
black nucleus and ground-area (Pl. VII., Fig. 65).
The North American _Pterogon Gauræ_ and _P. Abboti_[116] also show
markings precisely similar to those of this European species in the
adult state; but in the two former the markings are of special interest
as indicating the manner in which the primary Sphinx-marking has become
transformed into that of the apparently totally different adult _P.
Œnotheræ_. _P. Gauræ_ is green, with a complicated latticed marking,
which closer observation shows to arise from the dorsal line being
resolved into small black dots, whilst the subdorsal line is broken up
into black, white-bordered triangles. This caterpillar therefore gives
fresh support to the remarkable phenomenon that the animals as well as
the plants of North America are phyletically older than the European
fauna and flora, a view which also appeared similarly confirmed by
_Deilephila Lineata_, the representative form of _D. Livornica_. In
entire accordance with this is the fact that the larva of _P. Gauræ_ is
without the eye-spot on the eleventh segment, and instead thereof still
shows the original although small caudal horn. The perfect insect also
resembles our _P. Œnotheræ_ in colour and marking, but not in the form
of the wings.
That the caterpillars of the genus _Pterogon_ originally possessed the
caudal horn we learn from _P. Gorgoniades_, Hübn.,[117] a species
now inhabiting south-east Russia, and for a knowledge of which I am
indebted to Dr. Staudinger’s collection. There are in this about
eight blown specimens, from 3.7 to 3.9 centimeters in length, which
show a marking, sometimes on a red and sometimes on a green ground,
which unites this species with the young form of _P. Œnotheræ_, viz.,
a broad white subdorsal line, extending from the small caudal horn
to the head. In addition to this, however, the caterpillar possesses
an extraordinarily broad white red-bordered infra-spiracular line, a
fine white dorsal stripe, and a similar line between the subdorsal and
spiracular, _i.e._ a supra-spiracular line.
The caterpillars in Staudinger’s collection, notwithstanding their
small size, all belong to the last stage, as the moth itself does not
measure more than 2.6 centimeters in expanse, and is therefore among
the smallest of the known _Sphingidæ_. This species has therefore in
the adult condition a marking very similar to that of _Œnotheræ_ when
young--it bears to _Œnotheræ_ the same relationship that _Deilephila
Hippophaës_ does to _D. Euphorbiæ_, only in the present case the
interval between the two species is greater. _Gorgoniades_ is obviously
a phyletically older species, as we perceive from the marking and
from the possession of a horn. We certainly do not yet know whether
_Œnotheræ_ possesses a horn in its earliest stages, although in all
probability it does so; in any case the ancestor of _Œnotheræ_ had a
horn, since the closely allied _P. Gauræ_ now possesses one.
We thus see that also in the genus _Pterogon_ the marking of the
caterpillars commences with a longitudinal line formed from the
subdorsal; an infra-spiracular or also a supra-spiracular line
(_Gorgoniades_) being added. A latticed marking is developed from the
linear marking by the breaking up of the latter into spots or small
patches, which finally (in _Œnotheræ_) become completely independent,
their connection with the linear marking being no longer directly
perceptible.