_Ontogeny of the Noctua larvæ._--References have already been given
in a previous note (67, p. 166) to observations on the number of legs
and geometer-like habits of certain _Noctua_-larvæ when newly hatched.
This interesting fact in the development of these insects furnishes
a most instructive application of the principle of ontogeny to the
determination of the true affinities, _i.e._ the blood-relationship of
certain groups of Lepidoptera. While the foregoing portions of this
work have been in course of preparation for the press, some additional
observations on this subject have been published, and I may take the
present opportunity of pointing out their systematic bearing--not,
indeed, with a view to settling definitively the positions of the
groups in question, as our knowledge is still somewhat scanty--but with
the object of stimulating further investigation.
Mr. H. T. Stainton has lately recorded the fact that the young larva
of _Triphæna Pronuba_ is a semi-looper (Ent. Mo. Mag. vol. xvii. p.
135); and in a recently published life-history of _Euclidia Glyphica_
(_Ibid._ p. 210) Mr. G. T. Porritt states that this caterpillar is
a true looper when young, but becomes a semi-looper when adult. To
these facts Mr. R. F. Logan adds (_Ibid._ p. 237) that “nearly all
the larvæ of the _Trifidæ_ are semi-loopers when first hatched.” The
_Cymatophoræ_ appear to be an exception, but Mr. Logan points out
that this genus is altogether aberrant, and seems to be allied to the
_Tortricidæ_. Summing up the results of these and the observations
previously referred to, it will be seen that this developmental
character has now been established in the case of species belonging
to the following families of the section _Genuinæ_:--_Leucaniidæ_,
_Apameidæ_, _Caradrinidæ_, _Noctuidæ_, _Orthosiidæ_, _Hadenidæ_,
and _Xylinidæ_, as well as the other _Trifidæ_ (excepting
_Cymatophora_).[215] The larvæ of the _Minores_ and _Quadrifidæ_ are
as a rule semi-loopers when adult and may be true loopers when young,
although further observations on this point are wanted. These facts
point to the conclusion that the _Noctuæ_ as a whole are phyletically
younger than the _Geometræ_, whilst the _Genuinæ_ and _Bombyciformes_
have further advanced in phyletic development than the _Minores_ and
_Quadrifidæ_. The last two sections are therefore the most closely
related to the _Geometræ_, as correctly shown by the arrangement given
in Stainton’s “Manual;” whilst that adopted in Doubleday’s “Synonymic
List,” where the _Geometræ_ precede the _Noctuæ_, is most probably
erroneous.
_Additional descriptions of Sphinx-larvæ._--In the foregoing essay on
“The Origin of the Markings of Caterpillars,” Dr. Weismann has paid
special attention to the larvæ of the _Sphingidæ_ and has utilized
for this purpose, in addition to his own studies of the ontogeny of
many European species, the figures in the chief works dealing with
this family published down to the time of appearance of his essay
(1876).[216] In order to amplify this part of the subject I have added
references to more recent descriptions and figures of Sphinx-larvæ
published by Burmeister and A. G. Butler, and I have endeavoured in
these cases to refer the caterpillars as far as possible to their
correct position in the respective groups founded on the ontogeny
and phylogeny of their allies. It is, however, obvious that for the
purposes of this work figures or descriptions of adult larvæ are of but
little value, except for the comparative morphology of the markings;
and even this branch of the subject only becomes of true biological
importance when viewed in the light of ontogeny. As our knowledge of
the latter still remains most incomplete in the case of exotic species,
it would be at present premature to attempt to draw up any genealogy
of the whole family, and I will here only extend the subject by adding
some few descriptions of species which are interesting as having been
made from the observations of field-naturalists, and which contain
remarks on the natural history of the insects.
Mr. C. V. Riley in his “Second Annual Report on the Noxious,
Beneficial, and other Insects of the State of Missouri, 1870,” gives
figures and describes the early stages and adult forms of certain
grape-vine feeding larvæ of the sub-family _Chærocampinæ_. The
full-grown larva of _Philampelus Achemon_, Drury, “measures about 3½
inches when crawling, which operation is effected by a series of sudden
jerks. The third segment is the largest, the second but half its size,
and the first still smaller, and when at rest the two last-mentioned
segments are partly withdrawn into the third.... The young larva is
green, with a long slender reddish horn rising from the eleventh
segment and curving over the back.” Mr. Riley then states that full
grown specimens are sometimes found as green as the younger ones, but
“they more generally assume a pale straw or reddish-brown colour, and
the long recurved horn is invariably replaced by a highly polished
lenticular tubercle.” The specimen figured was the pale straw variety,
this colour deepening at the sides, and finally merging into a rich
brown. The markings appear to consist of an interrupted brown dorsal
line, a continuous subdorsal line of the same colour, and six oblique
scalloped white bars along the side. Whether the colour and marking
is adapted to the vine, as is the case with the two varieties of the
dimorphic _Chærocampa Capensis_ (_q.v._), is not stated. The larva of
_Philampelus Satellitia_, Linn., when newly hatched, and for some time
afterwards is “green with a tinge of pink along the sides, and with an
immensely long straight pink horn at the tail. This horn soon begins
to shorten, and finally curls round like a dog’s tail.” The colour
of the insect changes to a reddish-brown as it grows older, and the
caudal horn is entirely lost at the third moult. The chief markings
appear to be five oblique cream-yellow patches with a black annulation
on segments 6-10, and a pale subdorsal line. The caterpillar crawls
by a series of sudden jerks, and often flings its “head savagely from
side to side when alarmed.” “When at rest, it draws back the fore
part of the body and retracts the head and first two joints into the
third.” Two points in connection with these species are of interest
with respect to the present investigations. The green colour and the
possession of a long caudal horn when young shows that these larvæ,
like those of _Chærocampa Elpenor_ (p. 178), _C. Porcellus_ (p. 184),
and _Philampelus Labruscæ_ (p. 195, note), are descended from ancestors
which possessed these characters in the adult state.[217] The next
point of interest is the attitude of alarm assumed by these larvæ,
and effected by withdrawing the head and two front segments into the
third.[218] The importance of this in connection with the similar
habit of ocellated species will be seen on reading the remarks on page
367 bearing upon the initial stages of eye-spots. The other species
figured by Mr. Riley are _Chærocampa Pampinatrix_, Smith and Abbot, and
_Thyreus Abboti_, Swains. The latter has already been referred to (p.
256).
In a paper “On a Collection of Lepidoptera from Candahar” (Proc. Zoo.
Soc., May 4th, 1880), Mr. A. G. Butler has described and figured, from
materials furnished to him by Major Howland Roberts, the larvæ of
three species of _Sphingidæ_. _Chærocampa Cretica_, Boisd., feeds on
vine; out of 100 specimens examined, there was not one black variety,
while in another closely allied species, found at Jutogh and Kashmir,
the larva is stated to be as often black as green. The general colour
of the caterpillar harmonizes with that of the underside of the vine
leaves; it possesses a thread-like dorsal, and a pale yellow subdorsal
line; also “a subdorsal row of eye-spots, each consisting of a green
patch in a yellow oval, the first spot on the fifth segment being the
largest and most distinct, those on each following segment becoming
smaller, more flattened, and less distinct, till lost on the twelfth
segment, sometimes becoming indistinct after the seventh or eighth
segment; these spots are only distinct as eye-spots on the fifth and
sixth segments, that on the sixth being flatter than that on the fifth,
those on the remaining segments appearing like dashes while the larvæ
is green, but more like eyes on its changing colour when full fed.”
The change here alluded to is the dark-brown coloration so generally
assumed by green Sphinx-larvæ previous to pupation, and which, as I
have stated elsewhere (Proc. Zoo. Soc., 1873, p. 155), is probably an
adaptation advantageous to such larvæ when crawling over the ground
in search of a suitable place of concealment. Making the necessary
correction for the different mode of counting the segments, it will be
seen that the primary ocelli of this species are in the same position
as those of the other species of this genus as described in a previous
part of this essay, and that it belongs to the second phyletic group
treated of at p. 193. The interesting fact that this species does not
display dimorphism, whilst the closely allied form from Kashmir is
dimorphic, shows that in the present species the process of double
adaptation has not taken place; and this will probably be found to be
connected with the habits of life, _i.e._ the insect being well adapted
to the colour of its food-plant may not conceal itself on the ground
by day. The caterpillar of _Deilephila Robertsi_, Butl., is found at
Candahar on a species of _Euphorbia_ growing on the rocky hills, and
is so abundant that at the end of May every plant with any leaves left
on it had several larvæ feeding upon it. “The larvæ are very beautiful
and conspicuous, and are very different in colouring according to their
different stages of growth.” The general colour is black with white
dots and spots; a subdorsal row of large roundish spots, one on each
segment, either white, yellow, orange or red; dorsal stripe variable
in colour, and sometimes only partially present or altogether absent.
“At the end of May most of the larvæ found presented a different
appearance; the black disappears more or less, and with it many of
the small white spots. In some cases the black only remains as a ring
round the larger white spots; the ground-colour therefore becomes
yellowish-green or yellow, varying very considerably.” The larva does
not change colour previous to pupation. This species, according to
the outline figure given (_loc. cit._, Pl. XXXIX., Fig. 9), appears
to belong to the first of Dr. Weismann’s groups, comprising _D.
Euphorbiæ_, _D. Dahlii_ and _D. Nicæa_ (see p. 199), and is therefore
in the seventh phyletic stage of development (p. 224). From the
recorded habits it seems most probable that the colours and markings
of this caterpillar are signals of distastefulness. It is much to
be regretted that Major Roberts has not increased the value of his
description of this species by adding some observations or experiments
bearing on this point. _Eusmerinthus Kindermanni_, Lederer, feeds on
willow. “General colour green, covered with minute white dots and seven
long pale yellow oblique lateral bands. (The ground-colour is the same
as the willow-leaves on which the larva feeds, the yellow stripes
the same as the leaf-stalks, and the head and true legs like the
younger branches).” As no subdorsal line is mentioned or figured, this
species must be regarded as belonging to the third stage of phyletic
development (see p. 242).
I have recently had an opportunity of inspecting a large number of
drawings of Sphinx-larvæ in the possession of Mr. F. Moore, and of
those species not mentioned in the previous portions of this work the
following may be noticed:--_Chærocampa Theylia_, Linn., like _Ch.
Lewisii_ (note 82, p. 194), appears to be another form connecting the
second and third phyletic groups of this genus. _Ch. Clotho_, Drury,
belongs to the third group (figured by Semper; see note 3 to this
Appendix). The larva of _Ch. Lucasii_, Walk., offers another instance
of the retention of the subdorsal line by an ocellated species. The
larva of _Ch. Lycetus_, Cram., of which Mr. Moore was so good as to
show me descriptions made at the various stages of growth, presents
many points of interest. It belongs to the third phyletic group, and
all the ocelli appear at a very early stage. The dimorphism appears
also in the young larvæ, some being green, and others black, a fact
which may be explained by the law of “backward transference” (see
p. 274). A most suggestive feature is presented by the caudal horn,
which in the young caterpillar is stated to be _freely movable_.
It is possible that this horn, which was formerly possessed by the
ancestors of the _Sphingidæ_, and which is now retained in many genera,
is a remnant of a flagellate organ having a similar function to the
head-tentacles of the _Papilio-larvæ_, or to the caudal appendages of
_Dicranura_ (see p. 289).
_Lophostethus Dumolinii_, Angas.--The larva of this species differs so
remarkably from those of all other _Sphingidæ_, that I have thought it
of sufficient interest to publish the following description, kindly
furnished by Mr. Roland Trimen, who in answer to my application sent
the following notes:--“My knowledge of the very remarkable larva
of this large and curious Smerinthine Hawk-moth is derived from a
photograph by the late Dr. J. E. Seaman, and from drawings and notes
recently furnished by Mr. W. D. Gooch. The colour is greenish-white,
inclining to grey, and in the male there is a yellow, but in the female
a bluish, tinge in this. All the segments but the second and the head
bear strong black spines, having a lustre of steel blue, and springing
from a pale yellow tubercular base. The longest of these spines are in
two dorsal rows from the fourth to the eleventh segment, the pairs on
the fourth and fifth segments being longer than the rest, very erect,
and armed with short simple prickles for three-fourths of their upper
extremity. The anal horn, which is shorter than the spines, is of the
same character as the latter, being covered with prickles, and much
inclined backwards. Two lateral rows of similar shorter spines extend
from the fourth to the 12th segment, and on each of the segments
6-11 the space between the upper and lower spines is marked with a
conspicuous pale yellow spot. Two rows of smaller similar spines extend
on each side (below the two rows of larger ones) from the second to the
thirteenth segment, one spine of the lowermost row being on the fleshy
base of each pro-leg. All the pro-legs are white close to the base, and
russet-brown beyond. Head smooth, unarmed in adult, greenish-white with
two longitudinal russet-brown stripes on face.
“The young larvæ have proportionally much longer and more erect spines
with distinct long prickles on them. There is a short pair besides,
either on the back of the head or on the second segment. Moreover,
the dorsal spines of the third and fourth segments, and the anal horn
(which is quite erect, and the longest of all), are longer than the
rest, and distinctly _forked_ at their extremity.
“Mr. Gooch notes that these young larvæ might readily be mistaken for
those of the _Acrææ_, and suggests that this may protect them. He also
states that the yellow lateral spots are only noticed after the last
moult before pupation, and that the general resemblance of the larva as
regards colour is to the faded leaves of its food-plant, a species of
_Dombeia_.”
The forked caudal horn in the young larva of this species is of
interest in connection with the similar character of this appendage in
the young caterpillar of _Hyloicus Pinastri_, p. 265.
_Retention of the Subdorsal Line by Ocellated Larvæ._--It has already
been shown with reference to the eye-spots of the _Chærocampa_-larvæ,
that these markings have been developed from the subdorsal line, and
that, in accordance with their function as a means of causing terror,
this line has in most species been eliminated in the course of the
phylogeny from those segments bearing the eye-spots in order to give
full effect to the latter (see p. 379). In accordance with the law
that a character when it has become useless gradually disappears, the
subdorsal is more or less absent in all those species in which the
ocelli are most perfectly developed; and it can be readily imagined
that in cases where adaptation to the foliage exists the suppression
of this line would under certain conditions be accelerated by natural
selection. On the other hand, it is conceivable that the subdorsal
line may under other conditions be of use to a protectively coloured
ocellated species by imitating some special part of the food-plant,
under which circumstances its retention would be secured by natural
selection.
Such an instance is offered by _Chærocampa Capensis_, Linn.; and as
this case is particularly instructive as likewise throwing light upon
the retention of the subdorsal by certain species having oblique
stripes (see p. 377, and note 166, p. 378), I will here give some details
concerning this species which have been communicated to me by Mr.
Roland Trimen, the well-known curator of the South African Museum, Cape
Town. The caterpillar of _C. Capensis_, like so many other species
of the genus, is dimorphic, one form being a bright (rather pale)
green, and the other, which is much the rarer of the two, being dull
pinkish-red. Both these forms are adapted in colour to the vine on
which they feed, the red variety according to some extent with the
faded leaves of the cultivated vines, but to a greater extent with the
young shoots and underside of the leaves of the South African native
vine (_Cissus Capensis_), on which it also feeds. There are two
eye-spots in this species in the usual positions; they are described as
being blue-grey in a white ring, and raised so as to project a little.
The subdorsal is white, and is bordered beneath by a wide shade of
bluish-green irrorated with white dots, and crossed by an indistinct
white oblique ray on each segment. These last markings are probably
remnants of an oblique striping formerly possessed by the progenitor of
this and other species of the genus (see, for instance, Fig. 25, Pl.
IV., one of the young stages of _C. Porcellus_). It is possible that
these rudimentary oblique stripes are now of service in assisting the
adaptation of the larva to its food-plant, but this cannot be decided
without seeing the insect _in situ_.
The subdorsal line extends from immediately behind the second eye-spot
to the base of the very short and much curved violet anal horn.
With reference to the protective colouring Mr. Trimen writes:--“The
difficulty of seeing these large and beautifully-coloured larvæ on
the vines is quite surprising; six or more may be well within sight,
and yet quite unnoticed. The subdorsal stripe greatly aids in their
concealment, as it well represents in its artificial light and shade
the leaf-stalks of the vine.” When this larva withdraws its front
segments the eye-spots stand out very menacingly; but in spite of this
it is greedily eaten by fowls and shrikes (_Fiscus Collaris_), and
Mr. Trimen also found that a tame suricate (_Rhyzæna Suricata_) and a
large monitor lizard (_Regenia Albogularis_) did not refuse them. The
failure of the eye-spots in causing terror in these particular cases
cannot be regarded as disproving their utility in all instances. It
must always be borne in mind that no protective character can possibly
be of service against _all_ foes; natural selection only requires that
such characters should be advantageous with respect to the majority
of the enemies of any species, and further experiments with this
caterpillar may show that in the case of smaller foes the eye-spots
are effective as a means of causing alarm. The dimorphism of the larva
of _C. Capensis_ is of special interest, although we are not yet
sufficiently acquainted with the habits of this species to offer a
complete explanation. According to Dr. Weismann’s conclusions (p. 297),
the dimorphism of the _Chærocampa_-larvæ is due to a double adaptation,
the insects first having acquired the habit of concealing themselves
by day, and the dark form having then been produced by the action of
natural selection, in order to adapt such varieties to the colour of
the soil, whilst others retained the green colour which adapts them
to the foliage of their food-plants. In accordance with this, _C.
Capensis_ may have a similar habit of concealment, or (should this be
found not to be the case) it is possible that this insect at a former
period possessed this habit and fed upon some other plant, when it
would have become dimorphic in the manner explained, and the _existing_
dimorphism may be a survival of the more ancient dimorphism, the red
form (corresponding to the older dark form) having been subsequently
modified so as to become also adapted to the new food-plant. Much light
would be thrown upon this by studying the ontogeny of the species.
_Phytophagic Variability._--A number of observations bearing on the
phytophagic variability of the Sphinx-larvæ and other caterpillars
have been recorded in a previous note (p. 305), and reference has also
been made to the food-plants of _Acherontia Atropos_ in South Africa
(note 121, p. 263). I am now enabled to add some further observations
on this species, from notes furnished to me by Mr. Roland Trimen, who
states that for many years he has noticed that at the Cape this larva
varies greatly in the depth and shade of the green ground-colour, the
variability being in strict accordance with the colour of the leaves
of the particular plant on which the individual feeds. The phenomenon
was particularly noticeable in larvæ feeding on _Buxia Grandiflora_, a
shrub in common cultivation in gardens, and of which the foliage is of
a very dull pale greyish-green. Another striking instance was noticed
in some very fine caterpillars feeding on a large shrubby _Solanum_,
which, excepting the bright yellow bands bordering the dorsal violet
bars, were generally dull ochreous-yellow, like the leaves and stalks
of the _Solanum_. On plants with bright green or deep green leaves,
the colour of the larvæ is almost in exact agreement. Mr. Trimen
adds:--“These remarks apply principally to the underside and pro-legs
and lower lateral regions, the dorsal colours of violet and yellow
varying but little. The protection afforded is very considerable, as
the larvæ almost always cling to the lower side of the twigs of their
food-plants, so that their uniformly-coloured under-surface is upwards,
and turned towards the light, and their variegated upper surface turned
downwards.”
These observations are of the highest importance, not only as adding
another instance to the recorded cases of phytophagic variation, but
likewise as showing that with this variability a protective habit
has been acquired. It is to be hoped that such a promising field
for experimental investigation as is offered by this and analogous
cases will not long remain unexplored. In attacking the problem two
chief questions have in the first place to be settled: (1) Is the
variability truly phytophagic, _i.e._ are the colour variations
actually brought about by the chemico-physiological action of the
food-plant? and (2) Are the larvæ at any period of growth susceptible
to the action of phytophagic influences? The first question could be
decided by feeding larvæ from the same batch of eggs on different
food-plants from the period of their hatching. The second question
could be settled by changing the food-plants of a series of selected
specimens at various stages of growth, and observing whether any
change of colour was produced. In accordance with the principles
advocated in a previous note (p. 305), it is conceivable _à priori_
that phytophagic variability may occur by direct chemico-physiological
action, quite irrespective of any of the changes of colour being of
protective use. In the case of brightly-coloured distasteful species
phytophagic variability might thus have full play, but in the case of
protectively-coloured edible species, phytophagic variability would be
under the control of natural selection. These considerations raise a
question of the greatest theoretical interest in connection with this
phenomenon. If phytophagic variability can have full play uncontrolled
by natural selection in brightly-coloured caterpillars, ought not this
phenomenon to be of more common occurrence in such species than in
those protectively coloured? Although our knowledge of this subject is
still very imperfect, as a matter of fact brightly coloured larvæ, so
far as I have been able to ascertain, do not appear to be susceptible
of phytophagic influences. But this apparent contradiction, instead
of opposing actually confirms the foregoing views, as will appear on
further consideration. The colours of protected species are as a whole
much inferior in brilliancy to those of inedible species, so that any
phytophagic effect would be more perceptible in the former than in the
latter, in which the highest possible standard of brilliancy appears
in most cases to have been attained. Now phytophagic variations of
colour appear to be of but small amount, or, in other words, such
variations fluctuate within comparatively restricted limits, and as
the cases at present known are mostly _adaptive_ it is legitimate
to conclude that they have been produced and brought to their
present standard by natural selection, _i.e._ that they have arisen
from phytophagic influences as a cause of variability. The initial
stages of phytophagic variations must therefore have been still less
perceptible than the now perfected final results; and this leads to the
conclusion that minute variations of this character were of sufficient
importance to protectively-coloured species to be taken advantage of
by natural selection. But minute variations in a dull-coloured larva
would, as previously pointed out, produce a comparatively much greater
effect than such variations in a brilliantly-coloured species; and as
protection is required by the former, the initial phytophagic effects
would be accumulated, and the power of adaptability conferred by the
continued action of natural selection, whilst in vividly-coloured
species where no power of adaptability is required this cause of
variation would not only produce a result which, as compared with its
effects upon dull species, may be regarded as a “vanishing quantity,”
but this result would be too insignificant to be taken advantage of
by natural selection, which is in these cases dealing only with large
“quantities,” and striving to make the caterpillars as brilliant as
possible. The fact that vividly-coloured distasteful larvæ do not show
phytophagic variation is to my mind explained proximately by these
considerations; the ultimate cause of phytophagic variability regarded
as a chemico-physiological action requires further investigation.
_Sexual Variation in Larvæ._--Since most of the markings of
caterpillars can be explained by the two factors of adaptation and
inheritance, or, in other words, by their present and past relations
to the environment, and since sexual selection can have played no
direct part in producing these colours and markings, I feel bound to
record here some few observations on the sexual differences in larvæ in
addition to the cases of _Anapæa_ and _Orgyia_ already recorded (note
i., p. 308) and of _Lophostethus Dumolinii_ (p. 527).
Mr. C. V. Riley states[219] with reference to the larva of _Thyreus
Abboti_ that the ground-colour appears to depend upon the sex, Dr.
Morris having described the insect as “reddish-brown with numerous
patches of light green,” and having expressly stated that “the _female_
is of a uniform reddish-brown with an interrupted dark-brown dorsal
line and transverse striæ.” Mr. W. D. Gooch, who has reared the South
African butterflies _Nymphalis Cithæron_ and _N. Brutus_ from their
larvæ, states[220] that these “differed sexually in both instances.”
Of _Brutus_ only a few were bred, but of _Cithæron_ many. “The sexual
difference of the latter was that the females had a large dorsal
sub-cordate cream mark, which was wanting, or only shown by a dot, in
the males, and the colour was more vivid in the edgings to the frontal
horns.”
Although such cases appear to be at present inexplicable, they are of
interest as examples of those “residual phenomena” which, as is well
known, have in many branches of science so often served as important
starting-points for new discoveries and generalizations.[221]