substance, or at least harder than the rest of the body, in most
larvæ; and on this account, though rarely separated from it by any
visible distinct neck[225], is, if the larva be of a tolerable
size, distinguished at the first view. In those of many Dipterous
insects, however, the head is covered with the same flexible
membranous skin with the rest of the body, from which it is often
scarcely to be distinguished. In these, except that it contains the
organs of manducation, it wears no more the appearance of a head
than any other segment of the body, and scarcely so much as the last
or anal one. The head of these larvæ is also remarkable for another
peculiarity,--that it is capable of being extended or contracted,
and assuming different forms at the will of the insect: a property
which the head of no superior animal can boast. It is probable that
there is a considerable variety in the shape and circumstances
of the heads of larvæ; but since, with the exception of those of
_Lepidoptera_, they have had less attention paid to them than they
deserve (indeed in a vast number of cases, from the difficulty of
meeting with them, these variations, except in a few instances,
have not been described), I will here mention a few of the most
remarkable. The head of the young larva at its first exclusion from
the egg is usually the most dilated part of the body, but it does not
often continue so. In that of _Cicindela campestris_, however,--the
beautiful green beetle sometimes found in sandy banks,--and also
in several caterpillars of _Lepidoptera_, it is much larger than
any of the following segments[226], which, in conjunction with the
animal's formidable jaws, gives it a most ferocious appearance. In
some lepidopterous larvæ the head is of the same diameter with the
rest of the body, but in insects in general it may, I think, be
stated as less; and occasionally it bears no proportion whatever
to it. This is the case with the subcortical one from Brazil lately
mentioned. It is more commonly longer than broad; but in some, as in
the larvæ of carrion beetles (_Silphæ_), the reverse of this takes
place. Its shape varies from triangular to orbicular, the mouth of
the animal forming the vertex of the triangle. In some larvæ of
_Hemerobii_, however, the head is narrowest behind. That of the grub
of a gnat noticed above (_Corethra crystallina_) forms a kind of
sharp horn or claw, terminating the body anteriorly[227]. The contour
of the head of larvæ is usually intire and unbroken; but in the
caterpillars of some _Lepidoptera_, as the butterfly called the grand
admiral (_Vanessa Atalanta_), the Glanville fritillary (_Melitæa
Cinxia_), &c. it is divided into two lobes[228]. In the Brazil flat
larvæ it is trilobed, each lateral lobe being divided into three
smaller ones: in which circumstance it somewhat resembles the head
of some subcortical _Cimicidæ_. Although the part we are treating
of is generally without _horns_, yet in some tropical butterflies
of the tribe of _Nymphales_, it is singularly armed with them. Thus
_Papilio Anchises_ is distinguished, according to Madame Merian[229],
by two in the occiput, which it has the power of retracting. In
the purple highflier (_Apatura Iris_), a British species, the two
lobes of the head, I am informed, terminate behind in two horns; as
they do likewise in the brilliant _Morpho Menelaus_[230], the lobes
assuming the form of a pear, and the horn representing the stalk.
In a caterpillar I found amongst Mr. Francillon's larvæ, the head
is bilobed, with a very long recurving subcapitate subramose spine.
In _Satyrus Cassiæ_, the head is armed with three occipital stout
spines[231]. The larva of _Nymphalis Amphinome_ Latr. (_Limenitis_
F.) is crowned with a coronet of eight occipital stout acute spines,
the intermediate ones being the longest[232]; and that of _Morpho
Teucer_ has a similar coronet, consisting of only seven blunt _rays_,
seemingly, rather than spines[233]. With regard to the articulation
of the head with the trunk, it is generally by its _whole_ diameter;
but in some instances, only by a _part_ of it. This is the case with
one of a sphinx figured by Mad. Merian[234]; and I have another,
probably belonging to the nocturnal _Lepidoptera_ (_Phalæna_
L.)[235]. In both these, the head is vertical and triangular; and in
the latter (which is a remarkable creature, the tail itself being
more like a head, and furnished with what resemble two prominent
black eyes) the vertex of the triangle is considerably higher than
the back of the animal. Whatever may be the clothing of the body,
the head is usually _naked_. Sometimes, however, it is itself beset
with very small simple spines, as in the butterfly of the mallow
(_Hesperia Malvæ_); or with longer compound ones, such as are found
on the rest of the body. This is the case with one of a butterfly
named by Rösel _Papilio morsa_. The most common _colour_ of the head
of larvæ, where it differs from the rest of the body, is a darker or
lighter reddish brown, or piceous. This is particularly observable
in those of Coleopterous insects, but it is very commonly in other
tribes of the same hue. Sometimes, amongst the _Lepidoptera_, the
head is of a different colour from the rest of the body; especially
where a contrast renders it striking. I can show the caterpillar of
some insect, probably of the hawk-moth tribe (_Sphingidæ_), from
Georgia, remarkable for the length of its anal spine, in which the
body is black, and the head red: another has a white head and a
brown body. In the larvæ of some _Lepidoptera_, _Coleoptera_, and
_Diptera_, the head can be wholly or nearly withdrawn within the
first segment of the body. This may be readily seen in that of the
common glow-worm; and that of a small gnat (_Tipula replicata_
De Geer) withdraws it so completely that the anterior margin of
that segment closes the orifice, so that the animal appears to
have no head[236].--The parts of the head which require distinct
consideration are, the _eyes_, _antennæ_, and the _mouth_: consisting
of various organs, which will be specified. Some of these parts
and organs are peculiar to larvæ of one order, others to those of
another, and some are furnished with them all.
_Eyes._ The larvæ of many insects have no eyes. Those with antennæ
which terminate in a lamellated clava (_Scarabæus_ L.), and
capricorn beetles also (_Cerambyx_ L.), amongst the _Coleoptera_,
are without them, and probably several others; and amongst the
_Diptera_, all those with a membranous or variable head. Those of the
remaining orders, with the exception, perhaps, of some _Hymenoptera_
and _Lepidoptera_, are furnished with these organs; and in the
_Coleoptera_ all the predaceous tribes, as well as most of those that
are herbivorous or granivorous, and the Gnats and other Tipulidans
(_Tipulariæ_ Latr.) in the _Diptera_, are also distinguished by
them. In the larvæ of the dragon-flies (_Libellula_ L.), and other
_Neuroptera_, they are composed of many facets as in those of the
perfect insect, from which they differ chiefly in being smaller.
But in the other insects of this description they are simple, and
resemble those of the _Arachnida_, and many aptera. These simple
eyes vary in their number, in different genera and tribes, from one
to six on each side of the head. Thus the larva of _Telephorus_,
and the saw-flies, has only _one_[237]; that of _Cicindela three_,
the two posterior ones being large with a red pupil surrounded by
a paler iris, which adds to the fierce aspect of this animal; and
the anterior one very minute. Those of the tortoise-beetles also
(_Cassida_) have _three_[238]; of _Staphylinus, four_; of _Timarcha_
(the bloody-nosed beetle) _five_; of _Carabus_, and the _Lepidoptera_
in general, _six_. In the last they are of different sizes, and
generally arranged in a circle: in that of _Hemerobius_ there are
five in a circle, with one central one[239]. The appearance of
these globules, which are often not visible but under a powerful
lens, is so different from that of the eyes of a butterfly or moth,
or other perfect insect, that it has been doubted whether they
actually perform the office of _eyes_, but without reason. They
occupy the usual station of those organs, being situated in many
instances upon a protuberance which appears to incase them; and seem
of a construction closely analogous to that of the eyes of spiders,
and the _stemmata_ or _ocelli_ of _Hymenoptera_, which have been
satisfactorily proved to be organs of vision. In the larva of a moth
not yet ascertained to exist in this country, _Attacus Tau_, and
probably other species, the eyes, after the skin has been changed a
few times, are no longer to be seen[240].
_Antennæ._ Most larvæ are provided with organs near the base of the
mandibles, which from their situation and figure may be regarded
as antennæ. Fabricius has asserted that the larvæ of the saw-flies
(_Tenthredo_ L.) have no antennæ; but in this he was mistaken, for
though very short, they are discoverable in them, as he might have
learned by consulting De Geer[241]. In the majority of _Neuropterous_
larvæ, they almost precisely resemble those of the perfect insect.
In all the rest they are very different. The antennæ of Coleopterous
larvæ are usually either filiform or setaceous, consisting of four
or five joints, nearly equal in length. Those of Lepidopterous
larvæ are commonly conical, as are those likewise of _Chrysomela_
and _Coccinella_ &c. amongst the _Coleoptera_, and very short,
composed of two or three joints, of which the last is much thinner
than the first, and ends in one or two hairs or bristles. These
antennæ the larva has the power of protruding or retracting at
pleasure. Lyonnet informs us, that the caterpillar of the great
goat-moth (_Cossus ligniperda_) can draw the joints of its antennæ
one within the other, so as nearly to conceal the whole[242]. The
larva of the common gnat has two long incurved setaceous antennæ,
fringed with hairs at some distance from their apex, which consist
only of a single joint[243]. The greater number of Dipterous larvæ,
however, all indeed except the Tipulidans (_Tipulariæ_ Latr.), and
many belonging to the _Coleoptera_ and _Hymenoptera_ orders (as
those of _Curculio_, _Apion_, _Apis_, &c.), are wholly deprived of
antennæ. It is a general rule, that the antennæ of larvæ are shorter
than the same organs in the perfect insect, the tribe _Ephemerina_
perhaps affording the only example in which the reverse of this takes
place[244].
_Mouth._ All larvæ have a mouth situated in the head, by which they
receive their food, and furnished with one or more instruments for
the purpose of mastication and deglutition. These instruments, in all
the orders except _Lepidoptera_, some _Neuroptera_ and _Diptera_,
bear a general resemblance to the same parts in the perfect insect.
In larvæ of the Coleopterous, Lepidopterous, and Hymenopterous
orders, we can distinguish for the most part an upper and under lip;
two pairs of jaws answering to the mandibulæ and maxillæ; and two,
four, or six palpi[245]: and some of these instruments may be found
in most _Diptera_. Each of these parts require separate notice.
_Upper-lip_ (LABRUM). The mouth of almost all larvæ, except some of
the order _Diptera_, are provided with a distinct _upper-lip_, for
retaining their food during mastication. As the construction of this
part does not widely differ from that of the perfect insect, which will
hereafter be more fully described, it is only necessary to observe,
that it is usually a transverse moveable plate, attached posteriorly to
the nasus (_clypeus_ F.), and situated just above the mandibles[246].
_Upper-jaws_ (MANDIBULÆ). The most usual figure of these, which are of
a hard horny consistence[247], is that of two slightly concave, oblong,
or triangular plates, often at their lower extremity of considerable
thickness, and of very irregular form, the base of which is filled
with powerful muscles, and planted in the side of the mouth so as to
move transversely. The other extremity can be made to meet or diverge
like the claws of pincers, and are divided into one or more tooth-like
indentations, by means of which the food of the larva is cut[248].
This is their construction in the larvæ of all _Lepidoptera_, and in
many of those of the other orders. They frequently, however, assume
a different form, though their situation is always the same. Thus
in the larvæ of the capricorn beetles (_Cerambyx_ L.) and of other
wood-boring species, they are shaped like the half of a cone, the inner
sides of which, applying close to each other, form a pair of powerful
grindstones, capable of comminuting the hardest timber[249]. M. Cuvier
has observed, with regard to the mandibulæ of those of stag-beetles
(_Lucanus_), that besides their teeth at the extremity, they have
towards their base a flat striated molary surface; so that they both
cut and grind their ligneous food[250]. It seems to have escaped
him, that a similar structure takes place in many perfect insects of
the lamellicorn tribe, as I shall hereafter show you. In the larvæ
of the water-beetles (_Dytiscus_ L.), ant-lions (_Myrmeleon_ L.),
and lace-winged flies (_Hemerobius_ L.), they resemble somewhat the
forceps at the tail of an ear-wig, being long and incurved; and, what
is more remarkable, hollow and perforated at the end, so as to serve
as a channel for conveying into the larva's mouth the juices of the
prey which by their aid it has seized. Reaumur even asserts, that the
larva of _Myrmeleon_ has no other entrance into its throat than through
these tubular mandibles[251]. That of the rove-beetles (_Staphylinus_
L.), and of many other Coleopterous genera, have these organs of this
forcipate construction, without being perforated[252]. In the larva of
the carnivorous flies, and many other _Diptera_, are two black incurved
subulate parts, connected at the base, and capable of being protruded
out of, and retracted into, the head, through the skin of which they
are usually visible. As I informed you in a former letter[253],
these mandibles are used for _walking_ as well as _feeding_: they
are parallel to each other, and are neither formed for cutting nor
grinding like the mandibles of other insects, but merely detach
particles of food by digging into it and tearing the fibres asunder.
In this operation they are probably assisted by an acutely triangular
dart-like instrument of a horny substance, which in some species
(_Musca vomitoria_) is placed between the two. In others this part is
wanting. Some Dipterous larvæ have two similar mandibles, but instead
of being parallel, they are placed one above the other; others (_Musca
domestica_ and _meridiana_) have but one such mandible, and some have
no perceptible mandible of any kind. The mandibles of the larva of the
crane-flies (_Tipula_), which are transverse and unguiform, do not act
against each other, but against two other fixed, internally concave and
externally convex, and dentated pieces[254].
_Under-jaws_ (MAXILLÆ). These are a pair of organs, usually of a
softer consistence, placed immediately under the upper-jaws; but as
they are usually so formed and situated as not to have any action
upon each other, it is probable that in general they rather assist
in submitting the food to the action of the mandibulæ, than in the
comminution of it. In Lepidopterous larvæ they appear to be conical
or cylindrical (at least in that of the cossus so admirably figured
by Lyonnet[255]), and to consist of two joints; which may, I imagine,
be analogous to the upper and lower portions of which the maxillæ of
perfect insects usually consist. The last of these joints is surmounted
by two smaller jointed palpiform organs. If any part of the maxillæ can
act upon each other, it is these organs or palpi; but it is evident
they are not calculated for mastication, although they may assist in
the retention of the substance to be masticated. In a figure given
by Reaumur of the under side of the head of another lepidopterous
larva (_Erminea Pomonella_), the maxillæ consist of a single joint,
and appear to be crowned by chelate palpi[256]: a circumstance
which is also observable in that of a common species of stag-beetle
(_Lucanus parallelipipedus_), the weevil of the water-hemlock (_Lixus
paraplecticus_[257]), and other insects. In general the maxillæ of
larvæ are without the lobe or lobes discoverable in those of most
perfect insects, this part being usually represented by a kind of
nipple, or palpiform jointed process, strictly analogous to the
interior maxillary palpi of the predaceous coleoptera; but in most
of the lamellicorn beetles the lobe exists in its proper form[258],
as it does likewise in that of the capricorn-beetle before noticed
(_Callidium violaceum_[259]). In the former instance, it is armed with
spines or claws; but in the latter it is unarmed, and rounded at the
end. In the larva of _Cicindela campestris_, the base of the maxilla
runs in a transverse direction from the mentum, to which, as is usually
the case, it is attached. From this at right angles proceeds the lobe,
from the outer side of which the feeler emerges; and the inner part
terminates in an unguiform joint, ending in two or three bristles. The
structure in the larvæ of water-beetles (_Dytiscus_ L.) is different,
for they appear to be without maxillæ[260]; but the case really seems
to be, that these organs are represented by the first joint of what M.
Cuvier calls their _palpi_[261]; from which proceed the real palpi, the
interior one being very short, and consisting only of a single joint.
These maxillæ of larvæ were regarded by Reaumur and other writers as
parts of the under-lip, on each side of which they are situated; and
indeed, as well as those in the perfect insect, they form a part of
the same machine, being connected by their base with the mentum, which
is part of the labium, but they are clearly analogous to the maxillæ
of the imago. They are not to be found in the larvæ of many Dipterous
insects, and perhaps in some species belonging to other orders. In some
Neuropterous larvæ, as those of the _Libellulina_ MacLeay, the maxillæ
are of a substance quite as solid and horny as the mandibles, which in
every respect they resemble[262].
_Under-lip_ (LABIUM). Between the two maxillæ in the larvæ of most
of the insects under consideration is a part termed by Reaumur the
middle division of the under-lip, but which is in fact analogous to
the whole of that organ in the _imago_. This organ varies in shape,
being sometimes quadrangular, often conical, &c. Interiorly it is
frequently connected with a more fleshy protuberance, called the
tongue by Reaumur[263], and supplying the place of the _ligula_ in
the perfect insect. On each side of the apex of the under-lip is a
minute feeler, and in the middle between these in the _Lepidoptera_
and many others, is a filiform organ, which I shall call the
_spinneret_ (_Fusulus_), through which the larva draws the silken
thread employed in fabricating its cocoon, preparatory to assuming
the pupa state, and for other purposes[264]. This organ is found
only in those larvæ which have the power of spinning silk; that
is, in all _Lepidoptera_, most _Hymenoptera_, _Trichoptera_, some
_Neuroptera_, and even a _Dipterous_ insect[265]. This tube, Lyonnet
had reason to believe, is composed of longitudinal slips, alternately
corneous and membranous, so as to give the insect the power of
contracting its diameter, and thus making the thread thicker or
smaller. There is only a single orifice at the end, which is cut
obliquely, somewhat like a pen, only with less obliquity, and without
a point, the opening being below, so as to be conveniently applicable
to the bodies on which the larva is placed. Reaumur conceived that
this spinneret had two orifices; but Lyonnet ascertained this to be
a mistake, the two silk tubes uniting into one before they reach the
orifice. From the contractile nature of the sides and the form of the
orifice, combined with the power the insect has of moving it in every
direction, results the great difference which we see in the breadth
and form of the threads, some being seven or eight times as thick as
others, some cylindrical, others flat, others channelled, and others
of different thickness in different parts[266]. In the larvæ of many
_Diptera_ the under-lip is merely a small tubercle, which can be
protruded from the insect's mouth by pressure[267].