Of this genus (in the narrow sense employed by Gray) I have only been
able, in spite of all trouble, to obtain fertile eggs of one species.
The females cannot be induced to lay in confinement, and eggs can only
be obtained by chance.
I long searched in vain the literature of this subject for some account
of the young stages of these caterpillars, and at length found, in a
note to Rösel’s work, an observation of Kleemann’s on the young forms
of _Sphinx Ligustri_, which, although far from complete, throws light
on certain points.
From a female of _S. Ligustri_ Kleemann obtained 400 fertile eggs. The
caterpillars on emerging are “at first entirely light yellowish-green,
but become greener after feeding on the fresh leaves;” the horn is also
at first light green, and then becomes “darker.” The young larvæ spin
webs, by which they fasten themselves to the leaves of their food-plant
(this, so far as I know, has not been observed in any species of
_Sphingidæ_). They moult four times, the border round the head and the
purple stripes appearing after the third moult, these stripes “having
previously been entirely white.” The ecdyses follow at intervals
of about six days, increasing to about ten days after the fourth
moult.[118]
From this short account we gather that in the third stage the marking
consists of seven oblique white stripes, which acquire coloured edges
in the fourth stage, a fact which I have myself frequently observed.
On the most important point Kleemann’s observations unfortunately give
no information--the presence or absence of a subdorsal line in the
youngest stages. That he does not mention this character, can in no way
be considered as a proof of its actual absence. I am rather inclined to
believe that it is present in the first, and perhaps also in the second
stage. There occur, however, species of the genus _Sphinx_ (_sensû
strictiori_) which possess a subdorsal line when young, as I think may
be certainly inferred from the fact that the remains of such a line
are present in the adult larva of _S. Convolvuli_.
This conclusion becomes still more certain on comparing the markings
with those of a nearly allied genus; without such comparison the
separation of the genus _Macrosila_, Boisd., from _Sphinx_ is scarcely
justifiable. If to these two genera we add _Dolba_, Walk., and
_Acherontia_, Ochs., we must be principally struck with the great
similarity in the markings, which often reaches to such an extent that
the differences between two species consist entirely in small shades of
colour, while the divergence of the moths is far greater.
Of the genera mentioned, I am acquainted altogether with fourteen
species of caterpillars:--_Macrosila Hasdrubal_, _Rustica_,[119] and
_Cingulata_;[119] _Sphinx Convolvuli_, _Ligustri_, _Carolina_,[119]
_Quinquemaculata_,[119] _Drupiferarum_,[119] _Kalmiæ_,[119] and
_Gordius_;[119] _Dolba Hylæus_;[119] _Acherontia Atropos_, _Styx_,[120]
and _Satanas_.[120] With one exception all these caterpillars possess
oblique stripes of the nature of those of the _Smerinthus_ larvæ,
and most of them are without any trace of a subdorsal line; one
species--the North American _M. Cingulata_--has a completely developed
subdorsal; and the typical European species, _S. Convolvuli_, has a
rudimentary subdorsal line. The ground-colour in most of these species
is of the same green as that of the leaves of their food-plants; some
are brown, _i.e._ earth-coloured, and in these the markings do not
appear so prominently; others again possess very striking colours (_A.
Atropos_), the oblique stripes in these cases being very vivid. Only
_M. Hasdrubal_[121] separates itself completely from this system of
classification, since this species is deep black with narrow yellow
rings, the horn and last segment being red.
The large and most striking caterpillar of _M. Hasdrubal_ is the same
which Wallace has made use of for his theory of the brilliant colours
of caterpillars. The explanation of the origin of this widely divergent
mode of marking could only be furnished by the ontogeny, in which
one or another of the older phyletic stages will certainly have been
preserved.
Strictly speaking the same should be said of the other
species--nevertheless their comparison with the so similarly marked
_Smerinthinæ_, together with the circumstance that in certain species
a subdorsal line can be traced, makes it appear correct to suppose
that here also the subdorsal was the primary marking, this line being
subsequently entirely replaced by the oblique stripes. The _Sphinginæ_
would therefore be a younger group than the _Smerinthinæ_, a conclusion
which is borne out by the fact that in the former the oblique stripes
have reached a higher development, being always of two, and sometimes
even of three colours (_S. Drupiferarum_, white, red, black), whilst in
the species of _Smerinthus_ they only occasionally possess uniformly
coloured borders.