Lepidopterous larvæ, &c.).
iii. Both legs and prolegs (_Lepidoptera_, _Tenthredinidæ_, and
some _Coleoptera_).
1. Without claws (_Tenthredinidæ_, &c.).
2. With claws (_Lepidoptera_, &c.).
I should next say something upon the spiracles, or breathing-pores,
or any other external apparatus for the purpose of _respiration_,
in larvæ; but I think it will be best to reserve the consideration
of these for a subsequent Letter. We will therefore conclude this
detailed description of their parts in their first state, with some
account of their other.
iii. _Appendages._ The generality of larvæ have no other
external organs than those already described; but in several
of them we observe various kinds of retractile ones and
others--protuberances--horn-like processes--rays, &c.; which, though
not properly coming either under any of the above _parts_, or under
the _clothing_ of these animals, yet require to be noticed. Upon
these I shall now enlarge a little.
You must have observed upon the back of the last segment but one
of the caterpillar of the silk-worm a horn-like process, rising at
first nearly perpendicularly, and then bending forward. A similar
horn, though confined in the genus _Bombyx_ to the silk-worm and a
few others, if we may believe Madame Merian, who, however, often
makes great mistakes, is found in the beautiful caterpillar of one
of the largest and finest moths that we know (_Erebus Strix_[325]),
the glory of the _Noctuidæ_, and in most of those of the hawk-moths
(_Sphinx_ F.) [_S. Porcellus_, _Vitis_, and a few others excepted; in
some of which, as _S. Labruscæ_, &c., this anal horn is replaced by
a gibbosity, and in others, as _S. Œnotheræ_, by a callous eye-like
plate[326]] in the same situation, but much longer[327], and
commonly curving backwards over the tail[328]. Sometimes, however,
as in _S. ocellata_ and _S. Stellatarum_, it is perfectly straight.
These organs towards the apex are horny, and often end in a sharp
point; nearer the base they are fleshy. They are without any true
joint[329], yet the insect can elevate or depress them at pleasure.
Under a lens, they usually appear covered with spinous eminences,
arranged like scales. The use of these horns is quite unknown:
Goedart fancies that they secrete a potent poison, and are intended
as instruments of defence; but both suppositions are altogether
unfounded. It has been remarked, that the body of those caterpillars
which have these horns, is firmer, and yields less to the touch
than that of those which have no such appendages[330]. The larva of
a small timber-devouring beetle (_Lymexylon dermestoides_ F.) has,
like the above caterpillars, a long horn, and in the same situation:
it has also a singular protuberance on the first segment[331]. Upon
some other caterpillars, as in _Bombyx Stigma_ F., a singular pair of
horn-like appendages arises from the back of the second segment of
the body, excluding the head. In a tawny-coloured one from Georgia,
with a transverse row of short black spines on each segment, these
horns are half an inch long, black, covered with spinous eminences,
rather thickest at the base, and terminate in a little knob. They
appear to articulate with the body at the lower extremity. I have
another species, black, with narrow longitudinal yellow stripes, in
which these horns are of equal thickness at base and apex, but with
the same terminal knob. _Danais Archippus_ has a pair of tentacula
at the head, and another pair, but shorter, at the tail; and _D.
Gylippus_ has, besides these, two in the middle of the body[332].
We are equally ignorant of the use of the upright horn found upon the
back of the fourth segment in the larva of some moths (_Noctua Psi_,
and _tridens_ F.) which is of a construction quite different from that
of those last described. It is cylindrical, slightly thinner at the
apex, which is obtuse, fleshy, incapable of motion, of a black colour,
and about two lines long. On the same segment, also, in the case-worms
(_Trichoptera_ K.) are three fleshy conical eminences, which the animal
can inflate or depress, so that they sometimes totally disappear,
and then in an instant swell out again. When retracted, they form a
tunnel-shaped cavity, varying in depth[333]. Reaumur conjectured that
these eminences were connected with respiration, and one circumstance
seems in favour of this conjecture, that this segment has not the
respiratory threads observable in the subsequent ones. Latreille
mentions certain fleshy naked eminences placed upon the ninth and tenth
segments of some hairy caterpillars, which, like those just mentioned,
the animal can elevate more or less. They are often little cones;
but when it would shorten them, the summit is drawn in, and a tunnel
appears where before there was a pyramid[334].
In a former Letter I gave you a short account of the remarkable
Y-shaped, as it should seem, scent-organs (_Osmateria_) of the
beautiful caterpillar of the swallow-tailed butterfly (_Papilio
Machaon_ L.), and others of the _Equites_[335]; I will now speak
of them more fully. That found in the former is situated at the
anterior margin at the back of the first segment, close to the head,
from which at first view it seems to proceed. At the bottom it is
simple, but divides towards the middle, like the letter Y, into two
forks, of a fleshy substance[336], which it can lengthen, as a snail
does its horns, to five times their ordinary extent, or retract
them within the stalk, so as wholly to conceal them. Sometimes it
protrudes one fork, keeping the other retracted; and often withdraws
the whole apparatus for hours together under the skin, and its place
is only marked by two tawny-coloured dots, so that an ordinary
spectator would not suspect the existence of such an instrument[337].
Unfortunately this larva is rare in this country, so that I can
scarcely flatter you with the hope of seeing this curious organ in
a living specimen[338], unless you choose to import a parcel of its
eggs from the south of Europe, where it is common. This you will
think rather a wild proposition; but why should not Entomologists
import the eggs of rare insects, as well as botanists the seeds of
rare plants? But if you will be satisfied with the dissection of a
dead specimen, I have several, done by the ingenious Mr. Abbott of
Georgia, in which this part is well exhibited[339].
Another small caterpillar, as it should seem, of a geometer, prepared
by the same gentleman, exhibits a pair of similar horns on the fifth
and sixth segments: in these the common base from which the fork
proceeds is very short and wide, and each branch grows gradually more
slender from the base to the apex, where it is involute. Whether
these are retractile, or whether they correspond with those of _P.
Machaon_ in their nature and use, cannot be ascertained from a dead
specimen: as they belong to a larva of a quite different tribe of
_Lepidoptera_, the probability is, that they essentially differ. Two
globose retractile vesicles issue from the ninth and tenth segments
of those of _Arctia chrysorrhea_, &c.[340]
A great number of Lepidopterous larvæ, particularly those which are
smooth and of a moderate size, have between the under-lip and fore-legs
a slender transverse opening, containing a teat-like protuberance of
the same construction as the furcate horn of the caterpillar of the
beautiful mountain-butterfly, _Parnassius Apollo_; and, like that, can
either be wholly retracted and concealed, or by pressure be extended
to the length of one of the legs. In some larvæ this part is of a
subhemispherical figure, generally single, but sometimes double. It is
commonly, however, more slender and conical; and when of this shape, it
is sometimes quadruple[341]. The use of this part is not very clearly
known: some have supposed it to be a second spinneret, and to be of use
in fabricating the cocoon; but it is more probable that it secretes
some other kind of fluid, and is connected with defence.
The singular organ in a similar situation, evidently for that
purpose, with which the puss-moth endeavours to annoy its assailants,
has been described in a former Letter, to which I refer you[342].
Bonnet, who was the first that discovered this organ, ascertained
that it might be cut off without injury to either larva or imago. He
also remarked in a caterpillar found in the wild succory (_Cichorium
Intybus_) another short, black, needle-shaped organ between the
conical part just described and the under-lip[343]. De Geer mentions
a remarkable fleshy horn-like style, which issues from the lower
side of the first segment, between the head and the legs of the
case-worms (_Trichoptera_): he does not describe it as retractile, or
it might be regarded as analogous to those of _Lepidoptera_ similarly
situated, that I have just noticed[344]. In that of the emperor-moth
(_Saturnia Pavonia_), there are perforated tubercles, which when the
animal is molested spirt forth a transparent fluid[345].
The horn-like appendage of the puss-moth (_Cerura Vinula_) is
situated at the tail of the insect, and is composed of two distinct
cylindrical diverging branches, each about four lines long, not
united at the base. Each of these is hollow, and includes a smaller
cylindrical piece, which can be protruded at pleasure, and withdrawn
again, as a pencil within its case; or, rather, as the horns of a
snail. The two outer horns are tolerably firm, moveable at their
base, and beset with black spines; the interior tentacula are
fleshy, moveable in every direction, and in full-grown larvæ of a
rose colour. The animal seldom protrudes them, unless in some way
disturbed; and frequently it approximates the two outer cases so
closely that they resemble a single horn. It appears to use these
inner horns, when protruded, as a kind of whip to drive away the
flies, especially the Ichneumons, that alight upon its body. When
touched in any place, it will unsheath one of them, and sometimes
both, and with them strike the place where it is incommoded[346]. A
similar organ is found in some other _Bombycidæ_, as _B. Tau_ and
_Furcula_ F. Reaumur mentions a caterpillar that to this kind of
tail added the resemblance of two ears, or two cylindrical bodies,
terminating in a point, which emerged from the first segment behind
the head[347]. In another observed by the same author, the legs were
replaced by a single horn, but which did not appear to send forth
an internal one: from the back of its fourth segment also emerged a
single conical or pyramidal fleshy eminence or cleft, terminating in
two points[348]. Some of the tropical butterflies also, as may be
seen in the figures of Madame Merian, have two diverging anal horns
instead of anal prolegs; but it does not appear that they incase
tentacula[349]. Wherever these caudal horns are found, the above
prolegs are wanting[350]. Two conical anal horns also distinguish
the caterpillar of one of the moths called _Prominents_, _Notodonta
camelina_; but these are not terminal, but on the back of the last
segment but one[351]. In that of another British moth, _N. ziczac_
F., there are three dorsal prominences, one near the anus, and two
more in the middle[352]. Some Geometers (_G. fuliginosa_, &c.) have
two erect horns on the eleventh segment, and others (_G. syringaria_,
&c.) two recurved ones on the eighth[353]. I must not here omit to
mention the curious hooks emerging from two tubercles on the back
of the eighth segment of the ferocious larva of that beautiful
tiger-beetle, the _Cicindela campestris_ L., not uncommon on warm
sunny banks. This animal with incessant labour, as we are informed
by M. Desmarets, digs a cylindrical burrow, to the enormous depth,
the size of the animal considered, of eighteen inches. To effect
this, it carries out small masses of earth upon its large concave
head; and having often occasion to rest in ascending this height, by
means of these hooks[354] it fixes itself to the sides of its burrow,
and, having finally arrived at its mouth, casts off its burthen.
When these insects lie in wait for their prey, their head, probably
in conjunction with the first segment of the body, accurately stops
the mouth of the burrow, so as to form an exact level with the
surrounding soil; and thus careless insects, walking over it without
perceiving the snare, are seized in a moment and devoured[355].
Another kind of appendage, which is found in some larvæ, is the organ
employed by them to carry the excrement; with which, instead of letting
it fall to the ground, they form a kind of umbrella to shelter and
probably conceal them. All the tortoise-beetles (_Cassida_ L.) have
instruments for this purpose, as well as an Indian genus (_Imatidium_
Latr.) very nearly related to them. This instrument is a kind of fork,
half as long as the body, consisting of two branches, growing gradually
smaller from the base to the summit, where they terminate in a very
fine point, of a substance rather horny, and attached to the body near
the anal orifice. They are armed on the outside with short spines,
from the base for about a third of their length. When this fork, as it
usually is, is laid parallel to the back, with its points towards the
head, the anal aperture points the same way. When the animal walks, the
fork points the other way, and is in the same line with the body, and
the anus assumes a prone position[356].
The larvæ of a genus of flies (_Volucella_ Geoffr.) remarkable for
inhabiting the nests of humble bees, are distinguished on their upper
side by six long, diverging, pointed, membranous radii; placed in a
semicircle round the anus[357]: what the particular use of these organs
may be, has not been conjectured. Another in my collection has only
four upper radii, but below the anus are two fleshy filiform tentacula.
One of a Tipulidan described by Reaumur, has also four upper teeth;
but instead of two subanal tentacula, has six[358]. The singular larva
of another of this tribe (_Chironomus plumosus_) has on the two
last segments four long, fleshy, filiform, flexible tentacula, often
interlaced with each other; which, according to the same illustrious
author, are used by the animal to fix its caudal extremity, like
the geometers, that the other end may be at liberty. Besides these
organs round the anus, it has also four other oval ones, of uncertain
use: not to mention the two prolegs, which M. Latreille thinks are
air-tubes[359]. Jointed anal organs are observable in other larvæ:
those in that of a saw-fly described by De Geer (_Lyda_ F.) consist
of three joints[360]; in that of _Hister cadaverinus_, a carnivorous
beetle, of two[361]. The larva as well as the pupa and imago of
_Ephemera_ is furnished with three long diverging multiarticulate
tails, which are probably useful as a kind of rudder to assist and
direct their motions. That of the smaller dragon flies (_Agrion_ F.)
is furnished with three long vertical laminæ, by moving which, as fish
do their tails, from side to side, the animal makes its way in the
water[362]. That singular one, also, with a hooked head, figured by
Reaumur, has a single swimming lamina, or fin, shaped like a fan, and
placed in a vertical position under the tail[363].
The whole circumference of the body in some coleopterous larvæ,--for
instance, in that of the tortoise-beetle lately mentioned,--is
surrounded with appendages like rays. These are sometimes simple, rough
with very short spinous points[364]; but I have a dipterous larva,
in which these radii themselves are beautifully pinnated by a fringe
of longish spines on each side. Reaumur has described the grub of a
beetle, the genus of which is uncertain, and which feeds upon the larva
of _Aleyrodes Proletella_, whose body is margined on each side by eight
triangular fleshy mammular processes, terminating each in a bristle,
which give it a remarkable aspect[365]. The curious scent-organs with
which the larva of _Chrysomela Populi_ is fringed have been before
fully described; and therefore I shall only mention them here[366].
In the larvæ of the lace-winged flies (_Hemerobius_), and ant-lions
(_Myrmeleon_), the anus is furnished with a small fleshy retractile
cylinder, from which proceeds the silken thread that forms the cocoon
inclosing the pupa[367]. Providence has many different ways of
performing the same operation. From the structure of the oral organs
of these animals, the silk could not conveniently be furnished by the
mouth; the Allwise Creator has therefore instructed and fitted them
to render it by a spinneret at the other extremity of the body.
The respiratory anal appendages of many Dipterous larvæ will be
fully described in a subsequent Letter: I shall therefore now only
further observe upon this subject, that although there is seldom any
alteration in the form of these appendages &c. in the same species,
the caterpillars of two moths (_Cerura Vinula_ and _Attacus Tau_),
however, are exceptions. The former, when young, has two hairy
projecting ear-like protuberances, which it entirely loses, as I have
myself observed, before it assumes the pupa; and the latter, in like
manner, after its third change of skin, is deprived of its bent
thorn-like points which attend it when young[368]. It is remarkable
that these last larvæ, when just excluded from the egg, are also
entirely destitute of these appendages; they soon, however, appear,
from slight elevations which mark their situation, and rapidly
acquire their usual form[369]. Changes of a similar kind, hitherto
unobserved, may probably take place in other species.
iii. _Figure._ I am next to consider the general figure or shape of
larvæ. All of them, with but few exceptions[370], agree in having a
body more or less constricted at intervals into a series of rings
or _segments_; usually in number, _twelve_; often nearly equal in
length, but sometimes in this respect very dissimilar[371]. The general
outline or shape of the body is extremely various: most frequently
it approaches to cylindrical, as in most of the caterpillars of
_Lepidoptera_, and of the Hymenopterous tribe of saw-flies (_Tenthredo_
L.). The next most common figure is that more or less oblong or oval
one, sometimes approaching to conical, found in many of the larvæ
usually called grubs; such as those of the weevil (_Curculio_ L.) and
of the capricorn (_Cerambyx_ L.), and other coleopterous tribes; of
bees, and all Hymenopterous insects but the saw-flies; and also of a
large number of flies (_Diptera_). In some the figure approaches to
fusiform, as in most of the moths of the Fabrician genus _Lithosia_.
In others, as in those of the water-beetles (_Dytiscus_, &c.), it
approaches to an obovate shape, being widest towards the head, and
terminating in a point at the anus. In others, again, it is linear; an
example of which is that of _Staphylinus_. Some are convex, and others
gibbous, above, and flat underneath; as those of _Silpha_, _Chrysomela_
and many other beetles. Others are flat, both above and below, and
depressed like a leaf; a remarkable instance of which has been before
noticed[372]. Some are very long, as those of most _Lepidoptera_;
others very short, as that of the ant-lion (_Myrmeleon_). Many other
peculiarities of form in individuals might be instanced; but a dry
enumeration of these would be of no great use to you. They can only be
advantageously learned by the study of good figures, and by watching
the actual metamorphosis of the singularly-formed larvæ that you meet
with.
Instead, therefore, of any further specification of individual forms,
I shall now endeavour to give you, as far as my own knowledge of them
and the information I can collect from other sources will enable me,
a larger and more general view of the kinds of larvæ; for analytical
inquiries lose half their value and importance unless we proceed to
apply them synthetically, by forming, if possible, into groups the
objects with which we are individually acquainted.
Partial attempts at a synthetical arrangement with regard to the
larvæ of _Lepidoptera_ and the saw-flies (_Tenthredo_ L.) have been
made both by Reaumur and De Geer. M. Latreille also has recently
given a _Tableau méthodique et général_ of articulated animals
furnished with jointed legs, considered in their first state[373].
The former of these is chiefly founded upon the number of the
prolegs, and the latter upon the metamorphosis, prolegs, habits,
head, and parts of the mouth, without any other notice of the
configuration. Mr. Wm. MacLeay, who, though young in years, is old in
science and critical acumen, has started a perfectly new hypothesis
upon this subject. In the progress of his inquiries into the natural
arrangement of animals, particularly of _insects_ in the _Linnean_
sense, he has been the first to observe, that the relation which
organized objects bear to each other is of two kinds; one of real
_affinity_, and the other only of _analogy_, or resemblance. This
important distinction, upon which I shall enlarge in a future Letter,
when I come to treat of Systems of Entomology, he has applied, in
a way quite original, to larvæ in general, but more particularly
to those of the _Coleoptera_ order. For the basis of his system he
assumes a relation of analogy between the _larvæ_ of Insects that
in the progress of their metamorphosis assume wings, and those that
do not, which form his class _Ametabola_, so that the prototypes
of the former shall be found amongst the latter[374]. But though
Mr. MacLeay appears to consider the analogy between these two as
_primary_, he extends it in a _secondary_ sense to the _Crustacea_,
at least in several instances[375]. Upon this occasion he very
judiciously remarks, that "in terming larvæ _Chilognathiformes_
or _Chilopodiformes_, it is not meant that they are _Scolopendræ_
or _Iuli_, or even near to them in affinity; but only that they
are so constructed that certain analogical circumstances attending
them strongly remind us of these _Ametabola_[376]." This remark you
will bear in mind while I am treating of this subject. It should
seem from another part of the same paragraph, that the comparison
which our learned Physiologist recommends, is between the _young_
of the _Ametabola_ and the larvæ of the corresponding groups of
_Coleoptera_. This must be understood to refer chiefly to the young
of the _Chilopoda_ and _Chilognatha_, since they approach nearer
to them in that state, having then only six legs; but the rest of
the _Ametabola_ should certainly be brought to this comparison in
their adult state: and even the two former orders in that state
more strongly resemble numerous coleopterous larvæ, than when they
are young and much shorter. I before called your attention to the
remarkable circumstance that contrasts very many larvæ of Hexapod
insects that become winged in their perfect state with adult
_Myriapoda_: namely, that in one the progress to this state is by
losing their prolegs and shortening their body; while in the other,
the reverse of this takes place, numerous prolegs and additional
segments being gained before they arrive at maturity[377]. As the
multiplication of organs is a sign of imperfection, it may be
affirmed of the former of these tribes, that their progress is
towards greater perfection; while that of the other may be called a
degradation. As larvæ may be regarded as a stepping-stone by which
approach is made from the apterous to the winged tribes of Insects,
it seems most consistent with general analogy that each should
connect with the other in that state in which the resemblance is
greatest. Now the _Myriapoda_ resemble larvæ, as we have just seen,
most when in their _adult_ state; therefore the comparison should be
between larvæ and adult _Myriapoda_.
Mr. MacLeay divides _coleopterous_ larvæ into five tribes thus
characterized:--