+-----------------+---------+---------+---------+---------+---------+
| Deilephila. | Ontogeny| Ontogeny| Ontogeny| Ontogeny| Ontogeny|
| | Stage 1.| Stage 2.| Stage 3.| Stage 4.| Stage 5.|
+-----------------+---------+---------+---------+---------+---------+
| 1. Hippophaës | ? | ? | ? | III. | III.-IV.|
| 2. Zygophylli | ? | ? | ? | ? | III.-IV.|
| 3. Lineata | ? | ? | ? | ? | IV. |
| 4. Livornica | ? | ? | ? | IV. | V. |
| 5. Galii | ? | ? | IV. | V. | VI. |
| 6. Vespertilio | ? | II. (?) | IV. | V. | VI. |
| 7. Mauritanica | ? | ? | ? | ? | VI. |
| 8. Dahlii | I. | ? | VI. | VII. | VII. |
| 9. Euphorbiæ | I. | V. | VI. | VII. | VII. |
| 10. Nicæa | ? | ? | VII. | VII. | VII. |
+-----------------+---------+---------+---------+---------+---------+
From this very incomplete table we perceive that, in certain instances,
the stages can be represented as a continuous series of phyletic
steps, as in the case of _D. Galii_; that in others certain steps
may be omitted, as with _D. Euphorbiæ_, in which grade I. of stage 1
is immediately followed by grade V. in stage 2. In reality the gap
caused by this omission is still greater than would appear, as grade
V. is only indicated, and not actually reached, the subdorsal not
being present as a sharply-defined line, but only as a faint stripe.
The suppression of phyletic steps increases with the advancement in
phyletic development. The higher the step to which a species finally
attains, the greater is the tendency of the initial stages to be
compressed, or omitted altogether.
From what has thus far been seen with respect to the development of
_D. Hippophaës_, there may be drawn what to me appears to be a very
important conclusion, viz. that the ring-spots of _Deilephila_ first
originated on the segment bearing the caudal horn, and were then
gradually transferred as secondary spots to the preceding segments.
Complete certainty would be given to this conclusion by a knowledge
of the young forms of other phyletically retarded species, especially
those of the American _D. Lineata_, and perhaps also those of
_Zygophylli_ and _Livornica_. The few observations on the development
of _D. Galii_ already recorded give support to this view, since
the absence of ring-spots on the three front segments in the young
caterpillar (one instance), or their less perfect formation on these
segments (second instance), indicates a forward transference of the
spots.
If the foregoing view be accepted, there follows from it a fundamental
difference between the development of the genera _Chærocampa_ and
_Deilephila_. In the former the formation of the eye-spots proceeds
from a subdorsal line, but they first appear on two of the front
segments, and are then transferred to the _posterior_ segments. In
_Deilephila_, on the other hand, a single ring-spot is formed on the
penultimate segment bearing the caudal horn, and this is repeated on
the _anterior_ segments by secondary transference. With respect to
the origination of the ring-spot also, there is a distinction between
this genus and _Chærocampa_, inasmuch as the first step towards the
eye-formation in the latter consists in the separation of a curved
portion of the subdorsal line, whilst in _Deilephila_ the nuclear
spot first seems to originate and the separation of the mirror-spot
from the subdorsal line appears to occur secondarily. It is difficult
here to draw further conclusions, since the first appearance of the
primary ring-spot has not yet been observed, and no more certain
inference respecting the history of the formation of the _primary_
ring-spots can be drawn from the manner in which the _secondary_
ring-spots are formed. Because in _Hippophaës_ the formation of the
secondary ring-spots begins with the red coloration of one or two
shagreen-dots, it does not follow that the primary spot on the eleventh
segment also originated in this manner; and this is not without
importance when we are concerned with the causes which underlie the
formation of ring-spots. In _Chærocampa_ also, the formation of the
primary eye-spots appears to differ from that of the secondary--in the
latter the black “ground-area” first appearing, and in the former the
“mirror-spot.” The secondary eye-spots certainly remain rudimentary in
this last genus, so that the evidence in support of this conclusion is
thus much weakened; but it must be admitted that we are here on ground
still too uncertain to admit of wider conclusions being based thereon.
As a final result of the investigation, we may advance the opinion
that the existing species of the genus _Deilephila_ have reached five
different phyletic stages, and that their very different external
appearance is explained by their different phyletic ages; the
appearance from these caterpillars of moths so extremely similar, can
otherwise be scarcely understood.
It may appear almost unnecessary to bring forward additional proofs in
support of this interpretation of the facts, but in a field where the
data are so scanty, no argument which can be drawn from them should be
considered as superfluous. The variations which occasionally occur
in the larvæ, however, to a certain extent furnish a proof of the
correctness of the theoretical interpretation offered.
When, in the ontogeny of these species, we actually see before us
a series of stages of phyletic development, we must admit that
ordinary reversion may occur, causing an adult caterpillar to show
the characters of the young. Forms reverting to an earlier phyletic
stage must, on the whole, occur but seldom, as this stage is removed
further back in the ontogeny. Thus, indications of the subdorsal line
must occur but rarely in the _adult_ larvæ of _Euphorbiæ_, and still
less frequently in _Nicæa_, whilst they must be expected to be of more
common occurrence in _Vespertilio_, and also, as has already been
seen, in _Dahlii_. In this last species, as also in _Vespertilio_,
the completely-developed subdorsal line is still present in the third
stage, whilst it is possessed by _Euphorbiæ_ only in the second stage,
and then in a rudimentary condition.
The state of affairs may in fact be thus described: Among several
hundred adult larvæ of _Dahlii_ found in Sardinia by Dr. Staudinger,
there were some which did not actually possess a distinct subdorsal
line, but in place thereof, and as its last indication, a feeble light
stripe. One of Dr. Staudinger’s caterpillars showed also a distinct
line between the closed eye-spots. In the last stage of _Vespertilio_
this line appears still more frequently, whilst in _Euphorbiæ_ it is
extremely rare, and when present it only appears as a faint indication.
This is the case with one of the specimens figured in Hübner’s work as
an “aberration,” and also with one in Dr. Staudinger’s collection. Of
_Nicæa_ I have at most seen only eight specimens, none of which showed
any trace of the long-vanished subdorsal line.
It must be expected that any ontogenetic stage would most readily
revert to the preceding phyletic stage, so that characters present
in the preceding stage are consequently those which would most
commonly arise by reversion. This postulate of the theory also finds
confirmation in the facts. Caterpillars which, when full grown,
belong to the _seventh_ phyletic stage, _e.g._ _D. Euphorbiæ_, not
unfrequently show variations corresponding to the _sixth_ stage,
_i.e._ only one instead of two rows of ring-spots--the upper and
first-appearing series. On the other hand, forms reverting to the
_fifth_ phyletic stage (ring-spots with connecting subdorsal line)
occur but very rarely. I have never met with such cases in adult
living caterpillars of _D. Euphorbiæ_, although in one instance such
a larva was found in the fourth ontogenetic stage; but the strikingly
dark, brownish subdorsal line which connected the otherwise perfectly
developed ring-spots, completely disappeared in the fifth stage of the
ontogeny. Those larvæ which, in the adult state, belong to the _sixth_
phyletic stage, not unfrequently show the characters of the _fifth_
stage more or less developed, as, for example, _D. Vespertilio_.[96]