(_Geometra_, _Tortrix_, _Pyralis_, &c.), have merely a few scattered
short hairs, scarcely perceptible except through a lens: others
(_Odenesis potatoria_, _Lasiocampa Rubi_) are covered with down
more or less thick: in others (_Eriogaster lanestris_, _Lasiocampa
Neustria_) the hair is slenderer, and more like wool; the body of
two species which I purchased from the collection of Mr. Francillon
is covered with woolly hairs, so long as to give them the appearance
of a shock-dog; and Madam Merian has figured a similar one, which
she could not bring to the perfect state[417]. The hairs of many
_Bombycidæ_, known commonly by the name of _hairy caterpillars_, as
_Arctia erminea_, &c. are stiffer, resembling bristles; sometimes,
as in _Arctia chrysorhea_, mixed with shorter ones. The hairs either
spring immediately from the skin (_Noctua Aceris_, _leporina_), or,
as is more general, proceed only from certain tubercular elevations,
usually subhemispherical, but sometimes conical; of which a number
varying from four to twelve are found on each segment of different
species. They seem to issue from these tubercles, as little diverging
streams from the rose of a watering-pot. In both cases, they form
a coating usually so dense as to conceal the body, but sometimes
more thinly set, and admitting the skin to be seen more or less
between them. In a caterpillar of the beautiful _Arctia ocularia_,
the hairs are set upon tubercles alternately nearer the anterior and
posterior margin of each segment, so as to form a dense band, the
rest being naked; and in the lovely green and black one of _Saturnia
Pavonia_, each tubercle bears but six hairs, diverging like a star,
the central one being the longest and capitate, so that the chief
part of the body appears naked. This diverging position of the
hairs is most common in the thick-clothed larvæ also, but many have
them placed differently: thus, in those of _Callimorpha Caja_ and
_Arctia villica_[418] they are all directed towards the tail, like
the quills of a porcupine: in some others the anterior ones point
towards the head: in that of _Eriogaster Quercus_ half of the tuft
of hairs of each tubercle is directed downwards, the other half
upwards: in that of _Arctia Salicis_ all the hairs point downwards,
so that the belly is thickly covered, while the back is bare. Another
variation is, that the hairs of half the tubercle are sometimes
very long, while those of the other half are very short, and even
of a different colour[419]. In the larva of Tussuck moths (_Laria
pudibunda_, _fascelina_, &c.) the hairs are collected into tufts of a
singular appearance, those on the intermediate segments of the back
being quite level at the top, so as to resemble so many brushes;
while those on the first and last segments are longer, and composed
of feathered hairs converging to a point at their extremity, like a
common camel-hair-pencil[420]. This last mode of arrangement prevails
also in the larva of _Noctua Aceris_; but in this the pencils
are shorter, exactly wedge-shaped, and distinguished by another
particularity, that of springing directly from the skin, and not
from a tubercle. This is also the case with the large caterpillars
of _Odenesis potatoria_, which has a double row of short bundles of
black hairs on the back, intermixed with larger ones: at each end of
the body is a pencil of converging hairs, and the sides are spotted
with bundles of white ones, which with longer tawny ones are bent
downwards, so as to cover the sides of the creature[421]. Some have
the anterior aigrettes disposed like the arms of a cross, of which
the body of the caterpillar is the stem[422]. But not only is there
considerable variety in the general arrangement of the hairs that
clothe our little larvæ, the hairs themselves differ much in their
kind and structure, of which I will now, before I proceed to consider
spines, give you some account. Several of them are feathered like the
plumes of a bird: this is the case with those of _Morpho Idomeneus_,
on each segment of the body of which are three blue tubercles, like
so many little turquois beads, from each of which proceeds a long
black plume[423]. Other hairs terminate in a club; those of the larva
of _Noctua Alni_, a specimen of which I possess taken in England,
are flat and incrassated at the apex, something like the antennæ of
some _Sphingidæ_. Mad. Merian has figured the caterpillar of another
moth which feeds upon the Papaw-tree (_Carica Papaya_) with similar
hairs[424]. But the most remarkable larva for the shape of its hairs
is that of _Anthrenus Musæorum_, the little pest of our cabinets,
which I noticed in a former letter[425]. All the hairs of its body
are rough with minute points; but those of six diverging long tufts
or aigrettes, laid obliquely on the anal extremity of the body, which
the animal when alarmed erects as a porcupine does its quills, are
of a most singular structure: every hair is composed of a series
of little conical pieces, placed end to end, the point of which is
directed towards the origin of each hair, which is terminated at the
other extremity by a long and large conical mass, resembling somewhat
the head of a pike[426].
Besides the one lately mentioned, other caterpillars are rendered
striking by the brilliant colour of the tubercles from which their
hairs emerge. A remarkable instance of this is the thick large
caterpillar of a Bombyx, which feeds upon the _Psidium pyriferum_,
or white Guava, figured by Madame Merian. This caterpillar, which
is white, with transverse black stripes, and which has two singular
long converging curved bunches of hairs near the tail, is splendidly
adorned on each side with fifty red tubercles, shining like coral,
from which proceed six or seven long diverging hairs. Leeuwenhoeck
took these tubercles for eyes[427]. Another figured by the same
lady, who mistakes it, with her usual inaccuracy, for the larva of
a _Lygæus_ F., and which seems by her description to be between
the _onisciform_ and _limaciform_ types, has the apparently fleshy
mamillæ that project from its sides and back crowned with little
hairy red globes, which give the animal a most singular and unique
appearance[428]. Having thus described some of the principal modes in
which the All-wise Creator has decked and defended these creatures
with _hairs_, I shall next give you a short account of the _spines_
with which he has armed others. The spinous larvæ are principally
_lepidopterous_, and more particularly conspicuous in some tribes of
the genus _Papilio_ L., though some saw-flies and _Diptera_ are also
distinguished by them. _Vanessa Io_[429], _Atalanta_ and _Urtica_,
_Argynnis Paphia_, _Urania Leilus_, and many other Butterflies, &c.
are clothed with long sharp points, which claim the denomination
of spines, rather than that of hairs or bristles; being horny and
hard, and so stiff at the point as readily to pierce the skin. Those
of the last-mentioned species, Madame Merian says, are as stiff
as iron-wire[430]. They are sometimes entirely _simple_, and look
like spikes rather than spines, as in the caterpillar of _Nymphalis
Amphinome_ and _Morpho Menelaus_[431]; but ordinarily they are beset
with _hairs_, or more commonly with shorter spines, which often give
them the appearance of plumes, as in _Urania Leilus_ just mentioned:
sometimes these lateral spines are so long as to have the appearance
of a branch of a tree; this is strikingly the case with a small
caterpillar which Captain Hancock brought from Brazil; its body is so
thickly planted with spines of this description, that it absolutely
wears the appearance of a forest or thicket in miniature. A singular
circumstance attends the spines of this species: in many cases a
smaller and very slender hair-like spine issues from them, resembling
a sting; and this accounts for an observation of Abbott's, that many
American caterpillars _sting_ like a nettle, raising little white
blisters on the skin when accidentally or slightly touched[432].
Lewin has described the caterpillar of a moth found in New Holland,
which he names _Bombyx vulnerans_, that, like these Americans, has
also the power of wounding, but in a different way. It darts out,
he says, when alarmed by the approach of any thing, from as many
knobs or protuberances in its back eight bunches of little stings,
with which it inflicts a very painful and venomous wound[433].
The caterpillar of _Papilio Protesilaus_ F., if Madame Merian's
account and figure of it are correct, has its body armed with hairy
spines, the extreme point of which is surmounted by a star-shaped
appendage[434]. Those of a few saw-flies (_Tenthredo Pruni_ L.), and
another figured by Reaumur[435], are covered with a little forest of
spines without lateral branches, but divided into a fork at the apex.
Some spines are merely rough, with very short points, as those round
the head, which give so terrific an appearance to the caterpillar
of the _Bombyx regalis_, of some proceedings of which I gave you an
account in one of my former letters[436].
I must now say something upon the _arrangement_ of these spines.
Though in a few instances so thickly set as entirely to conceal
the body of the animal, as in the case of the Brazil one lately
mentioned, yet generally speaking, even when they are most numerous,
they permit the skin to be distinctly seen. Their arrangement is
various, though always orderly: in the majority they are planted
singly, but in some caterpillars in bundles. In that of _Saturnia
Io_, on each segment there are six bundles of longish, quill-shaped,
sharp, slender, diverging spines, which also appear to sheath aculei.
Madame Merian has figured this larva, or one very near it, as the
grub of a _Euglossa_[437], with which, though she affirms she traced
it to the fly, it can have no connection. With regard to _number_,
some larvæ have only four spines on each segment; others five, others
again six, and others seven, or even eight: they are planted on the
sides and back only, never on the belly. They are often more numerous
on the intermediate than on the anterior and posterior segments;
but sometimes the reverse of this takes place; in that of _Attacus
Erythrinæ_ only the head and tail are armed with spines, the rest of
the body being without any[438]; and in that of _Morpho Teucer_ there
is only a single spine on the four intermediate segments[439]. They
are usually all nearly of equal length; but in some cases those of
the head and tail are much longer than the rest, and remarkably so
in the caterpillar of _Urania Leilus_, also beautifully plumose, and
gracefully waved[440]. Those in the second and third segments are
much longer than any of the rest in that of _Bombyx regalis_; which
circumstance gives it the terrific appearance lately alluded to. In
the family to which _Argynnis Paphia_ belongs, the larva is adorned
with two on the back of the first segment twice as long as the rest,
and resembling at first sight two antennæ.
The spines, as well as the hairs of the new skin, are concealed
under the old one, and not incased in its spines; but Bonnet
ascertained, that if cut off very closely, the larva sometimes died
in consequence, whilst no such result followed a similar operation on
hairy larvæ. We learn from Reaumur[441], that some spinous larvæ of
saw-flies (_Tenthredo_ L.) lose their spines at the last change of
their skin; and from Madame Merian, that that of _Attacus Erythrinæ_
before mentioned loses also at the same period the six tremendous
black spikes that arm its black and yellow larvæ. The grubs of ants
that are destined to pass the winter in the larva state are hairy,
but are not so in summer[442]. The spines found in the grubs of some
gad-flies (_Œstrus_ L.) are of a different kind from those above
described, being very minute triangular flat plates, arranged in
different and contrary directions[443], and serving the insect merely
to change its place and fix itself[444].
Two other kinds of clothing, if so they may be called, neither
coming under the description of _hairs_ nor _spines_, are found
in some other larvæ, not only amongst the _Lepidoptera_, but also
in some of the other orders. _Nymphalis Populi_ and others of the
same family have larvæ furnished on the back of each segment with
cylindricoconical processes of a fleshy substance, obtuse at the apex
and surrounded with capitate hairs. In that of _N. Sybilla_, which
has on each segment two fleshy protuberances, they are bifurcate or
trifurcate, and also encircled at the base with a hairy tuft[445].
Others, as those of _Melitæa Artemis_, _Cynthia_, &c. have each
segment beset on the back with from seven to nine fleshy, pubescent,
wedge-shaped protuberances; two larger ones projecting over the
head. Under this head, too, may be noticed, the glutinous secretion
which clothes the grub of _Cionus Scrophulariæ_, a little weevil;
and of _Tenthredo Cerasi_ L. a saw-fly, and that waxy or powdery
substance which transpires through the skin of the larvæ of several
_Aphides_, _Chermes_, _Cocci_, _Hylotoma ovata_ F., &c. The _Aphis_,
whose extensive ravages of our apple-trees (_A. lanata_) were before
described to you[446], is covered and quite concealed by this kind
of substance, so that the crevices in the bark which they inhabit
look as if they were filled, not with animals, but with cotton. The
insect, also, that forms those curious galls produced upon the spruce
fir, and which imitate its cones (_Chermes Abietis_ L., _Aphis_ De
Geer) secretes a similar substance. In these and other cases of the
same kind, this matter seems to be, if I may so speak, wire-drawn
through numerous pores in certain oval plates in the skin, more
depressed than the rest of the back, arranged regularly upon the
segments, and exhibiting minute tuberosities. When young, these
animals have more of this secretion than when more advanced: it then
hangs from their anal extremity in locks[447].
But the insects most remarkable for a covering of this nature are
those _Coccidæ_ of which Bosc has made a genus under the name of
_Dorthesia_. De Geer is the first author that notices them, and has
given a description and figure of one species under the name of
_Coccus floccosus_[448]. It was discovered by Modeer upon some sere
fir-leaves in a thick bed of moss. Panzer has figured a second found
upon _Geranium sanguineum_, which from the figure appears distinct
from De Geer's, under the name of _Coccus dubius_[449]. Fabricius
regards this as synonymous with the _Dorthesia characias_ of Bosc,
inhabiting _Euphorbia characias_ in South Europe[450]. Olivier found
a species upon the bramble[451]. I once took one, which appears to
differ in some respects from the preceding species, upon _Melampyrum
cristatum_, and our indefatigable friend Mr. Sheppard has sent me
another, on what plant found I do not remember, which does not agree
with any that I have mentioned. The body of the animals of this genus
is covered by a number of cottony or waxy laminæ which partly cover
each other, and are arranged usually in a triple series: in De Geer's
figure the series appears quadruple, the lateral ones being placed
obliquely. The anterior one in my specimen covered the head, and
they are all canaliculate. Above the anus are four diverging ones:
the whole are of the most dazzling whiteness. When these laminæ are
removed, the body appears divided into segments.
With respect to those larvæ which imitate slugs by the viscid
covering that besmears them and issues from their pores, we learn
from Professor Peck that this exudation takes place as soon as they
are hatched; that the animal retains its humidity although exposed
to the fiercest heat of the sun, and that at the last moult the
skin becomes quite clean, and free from all viscidity[452]. It is
probable that the other limaciform larvæ are similarly circumstanced.
Madame Merian has figured an _onisciform_ one, the legs of which, she
says, are covered with a viscid skin: this produced a _Noctua_. Those
of _Papilio Anchises_ also are slimy, and adhere to each other[453].
v. Amongst other qualities which attach to larvæ, we must not
omit to say something concerning their _Colour_. For though those
which live in darkness, in the earth, in wood, in fruits, &c. are,
with few exceptions[454], of an uniform whitish colour, yet such
as are exposed to the influence of the light are usually adorned
with a vast variety of tints, sometimes the most vivid that can be
imagined. That the white colour of the former may be attributed to
the absence of light is proved by an experiment of M. Dorthes, who
having forced some to live under glasses, exposed to the light, found
that they gradually became brown[455]. To attempt any classification
of coloured larvæ would be in vain, since they are tinged with
almost every possible shade that can be conceived, of many of which
it would be difficult to find examples elsewhere; and infinitely
diversified as to the arrangement and figure of their multiform
markings and spots. A few general remarks, therefore, are all that
you will expect on this head. Many are of one uniform colour; while
a variety of tints, very different, and very vivid and distinct,
ornament others. Sometimes they are distributed in longitudinal rays
or bands, at others in transverse ones. Sometimes they are waved or
spotted, regularly or irregularly; at others they are sprinkled in
dots, or minute streaks, in every possible way. Various larvæ are
of the colour of the plant on which they feed, whence they are with
difficulty discovered by their enemies. Thus, a large proportion of
_Lepidoptera_ are green of different shades, sometimes beautifully
contrasted with black bands; a circumstance which renders the
caterpillars of two of our finest insects of this order as lovely as
the fly: I mean that of _Papilio Machaon_ and _Saturnia Pavonia_.
Very frequently the larvæ of quite different species resemble each
other so exactly, in colour as well as shape, as scarcely to be
distinguishable: this sometimes takes place even where they belong
to different genera, as in those of _Bombyx versicolor_ a moth, and
_Smerinthus Populi_ a hawk-moth. And it sometimes happens, very
fortunately for distinguishing allied species, that where the perfect
insects very nearly resemble each other, the larvæ are altogether
dissimilar. Thus, the female of _Pieris Rapæ_ is so much like the
same sex of _Pieris Brassicæ_, that it might be taken for a variety
of it, did not the green caterpillar of the one, and the spotted one
of the other, evince the complete distinction of these butterflies.
_Noctua Lactuca_, _N. umbratica_, and several other species of the
same tribe, which includes _N. Absinthii_, _Verbasci_, _Chamomillæ_,
_Abrotani_, are so extremely alike, that the most practised eye
can scarcely discover a shade of difference between them, though
their larvæ in colour and markings are constantly distinct[456].
The markings of species belonging to the same family are usually
different; but in some cases the latter may be prejudged from the
former. The larvæ of many of the genus _Sphinx_ L., for example,
have their sides marked by oblique streaks running from the back
in a direction towards the head; and by this last circumstance
they are distinguished from those of _Bombyx versicolor_, _Attacus
Tau_, and others of the same tribe, which have also lateral oblique
striæ, but running from the back towards the tail[457]. The colours
of individual larvæ of the same species are usually alike, but in
_Sphinx Elpenor_ and some others they vary exceedingly. Many, like
those of _Lasiocampa Rubi_, _Saturnia minor_, &c., are of one colour
when first disclosed, and assume others quite different in riper age.
Just previously to _changing their skin_, the tints of most larvæ
become as dull and obscure, as they are fresh and vivid when the
change has fully taken place; and in some instances the new skin is
quite differently marked from the old one. This is remarkably the
case with the last skin of some of the larvæ of the genus _Tenthredo_
L., which is entirely different from all the preceding ones. As
people when they advance far in years usually become more simple
in their dress than when they were young, so the larvæ in question
change an agreeably variegated skin for one of a uniform and less
brilliant colour[458]. Madame Merian has observed with respect to
_Attacus Erythrinæ_, that its caterpillar is at first yellowish,
with nine black striæ on each side: when arrived at one third of its
size, they become orange; the striæ are obliterated, and in their
place a round black spot appears on each of the eight intermediate
segments[459]. Mr. Sheppard has remarked to me, that the skin of that
of _Sphinx Ligustri_, after being under ground four days, was changed
from a vivid green to a dull red. Very rarely, however, it becomes of
a more brilliant hue just before entering the pupa state: thus, that
of another hawk-moth (_Smerinthus Tiliæ_) changes to a bright violet;
and the yellow hairs of that of _Laria pudibunda_ then become of a
lovely rose colour. And here I may observe, that the hairs and spines
also, of larvæ, vary greatly in colour. They are to be met with
brown, black, red, yellow, violet, white, &c. De Geer found, that
in the larva of _Cimbex nitens_ the two sides of the body were of a
different colour, the left being of a deep green, whilst the right
side and the rest of the body were paler[460]; but as he saw only
a single individual, this was probably an accidental circumstance.
Though the caterpillars, as I lately said, of one of the most
beautiful butterflies and moths that inhabit Britain contend with
the perfect insect in loveliness, yet in general no judgement can be
formed of the beauty of the future fly from the colour of the larva;
and the young Aurelian must not flatter himself always with the hope,
because the caterpillar excites admiration by its colours and their
arrangement, that the butterfly or moth it is to produce will do the
same; nor ought he to despise and overlook a sombre or plain-coloured
individual of the former, under the idea that it will produce one
equally plain of the latter, for it often happens that the splendid
caterpillar gives a plain butterfly or moth, and _vice versâ_. De
Geer, however, gives us two instances of conformity between the
colours of the caterpillar and those of the future moth; the one is
that of the common currant-moth (_Phalæna G. grossulariata_ L.),
the caterpillar of which is white, ornamented with several black
spots varying in size. At the two extremities it is yellowish, with
a longitudinal ray of the same colour on each side, the head and
legs being black. These colours are all to be found in the fly, the
ground of its wings being white ornamented with many black spots of
different sizes. Its upper wings are traversed by a yellowish band;
and towards their base is a spot of the same colour. Its body is
yellowish, with black spots; but the head and legs are black[461].
The other is that of a green caterpillar, which gives a green moth,
figured by Reaumur (_Pyralis prasinaria_ Fab.)[462]. Sometimes, also,
the sex of the future perfect insect may be predicted from the colour
it exhibits in its first state: thus, the brown caterpillars of
_Noctua Pronuba_ produce males, and the green ones females[463]. The
sexes, also, of _N. exoleta_ and _Persicariæ_ differ in that state.
vi. To the full account of the _Food_ of insects given in a former
letter[464], which had reference chiefly to their larva state,
it is only necessary in this place to add a few particulars not
there noticed. Many larvæ when first excluded, as those of _Pieris
Cratægi_, &c. devour the shells of the eggs from which they have
proceeded[465]; and others (_Cerura Vinula_, _Sphinx Euphorbiæ_,
_Noctua Verbasci_), though their usual food is of a vegetable nature,
eat with great apparent satisfaction the skins which they cast from
time to time, not leaving even the horny legs. This strange repast
seems even a stimulating dainty, which speedily restores them to
vigour, after the painful operation by which they are supplied with
it. Under this head it will not be out of place to mention, that some
larvæ of insects, which feed only on the juices of animals, or the
nectar and ambrosia of flowers, have no anal passage, and of course
no feces. This is said to be the case with the grubs of bees, wasps,
the larvæ of _Myrmeleon_, &c.[466]
vii. You will require no stimulus to induce you to attend to the
subject I am next going to enter upon,--the _Moulting_, namely,
of Larvæ; or their changes of skin. This, indeed, is a subject
so replete with interest, and which so fully displays the power,
wisdom, and goodness of the Creator, affording at the same time such
large occasion for nice investigation, that a pious and inquisitive
mind like yours cannot but be taken with it. In the higher orders
of animals, though the _hairs_ of quadrupeds and the _feathers_ of
birds are in many cases annually renewed, the change, or scaling
and increment of the _skin_, is gradual and imperceptible; no
simultaneous rejection of it, in which it is stripped off by the
animal itself like a worn shirt, being observable, till you descend
in the scale to the Serpent tribe[467], which at certain periods
disengage themselves from their old integument, and start forth with
that new and deadly beauty so finely described by the Mantuan bard:--
"So from his den, the winter slept away,
Shoots forth the burnished snake in open day;
Who, fed with every poison of the plain,
Sheds his old spoils and shines in youth again:
Proud of his golden scales rolls tow'ring on,
And darts his forky _tongue_[468], and glitters in the sun."
PITT.