to the upper-lip, the mouth is closed by another moveable organ,
concerning the nomenclature and analogies of which Entomologists
have differed considerably. At the first view of it, this organ
seems a very complex machine, since the under-jaws or _maxillæ_ are
attached to it on each side, and the tongue is often seen to emerge
from it above, so as to appear merely a part of it; but as the
former answer to the upper-jaws, and the latter is the analogue of
the part bearing the same name in the vertebrate animals, I shall
consider these as distinct and _primary_ organs, and treat of the
under-lip (_labium_) of which I am now speaking, by itself. Linné
takes no notice of this part, but his illustrious compatriot and
cotemporary, De Geer, did not so overlook it: he appears to consider
the whole apparatus, including the _maxillæ_, as the _labium_[1190];
but sometimes he distinguishes the middle piece by that name[1191];
and the tongue, in the case of the stag-beetle, he denominates a
proboscis (_trompe_)[1192]. In the _Hymenoptera_ he calls this part
_tongue_, _under-lip_, and _proboscis_: but seems to prefer the
last term[1193]. Fabricius originally regarded the whole middle
piece as a _labium_[1194]; but afterwards (though his definition
is not accurate, since he assigns the _palpi_ to the _ligula_,
which he affirms is covered by the _labium_--circumstances by no
means universal in _Coleoptera_) he considers this as consisting
of _ligula_ and _labium_[1195]. Latreille at first regarded the
_ligula_ of Fabricius as the _labium_, and called the _labium_ of
that author the _mentum_[1196]; but afterwards he gave the name of
_labium_ to the whole middle piece of the lower apparatus of the
mouth--calling the upper piece, with Fabricius, the _ligula_, and
retaining the denomination of _mentum_ for the lower[1197].
If the circumstances of the case are duly considered, I think you will
be convinced that the term under-lip, or _labium_, should be confined
to that part which the learned Dane so named. For I would ask, Which
is the part on the under side of the head that is the antagonist, if I
may so speak, of the upper-lip or _labrum_? Is it not that organ which,
when the mouth is closed, meets that part, and in conjunction with it
shuts all in? Now in numerous insects, particularly the Lamellicorn
beetles (_Scarabæus_ and _Lucanus_ L.), this is precisely the case. In
the Predaceous beetles, indeed, (_Cicindela_, _Carabus_, _Dytiscus_
L. &c.) the under-lip does not meet the upper, to close the mouth and
shut in the tongue; neither can the tongue be said so to do, but only,
from some circumstances connected with its manner of taking its food,
it is requisite that the last-mentioned organ should not be retractile
or covered; but its miscalled _mentum_ is still the analogue of that
part which helps to close the mouth in the former tribe. Should not
this, therefore, which so often performs the _office_, be distinguished
by the _name_, of a lip? Again, is it not rather incongruous to
consider that organ which confessedly more or less performs the office
of a _tongue_, as a part of the _lip_? Though it often wears that
appearance, yet I believe, if the matter is thoroughly and patiently
investigated, it will be found that on their upper side its roots are
attached to the interior of the upper side of the head, as well as on
their lower side to the _labium_; so that it may be regarded as common
to the two lips, and therefore not properly considered as an appendage
of the under-lip alone.
Having assigned my reasons for preferring the name given to the part
in question by Fabricius, rather than that of Latreille, I shall next
make my observations on the part itself. In many cases the _labium_,
or the middle piece of the lower oral apparatus, appears to consist of
_two_ joints: this you may see in the great water-beetle (_Hydrophilus
piceus_), the burying-beetles (_Necrophorus_), the Orthopterous
tribes[1198], the _Hymenoptera_[1199], and others. In this case the
upper or terminal piece is to be regarded as the _labium_, and the
lower or basal one (which Mr. MacLeay calls the _stipes_) as the
_mentum_ or chin--at other times, as in some Lamellicorn beetles, the
only separation is a transverse elevated line, or an obtuse angle
formed by the meeting of the two parts, and very frequently there is no
separation at all, in which case the whole piece, the _mentum_ merging
in it, may be denominated the _labium_.
With respect to its _substance_, the _labium_ in most Coleopterous
insects is hard and horny, in _Necrophorus_ it is membranous,
and the _mentum_ harder; in _Prionus coriarius_, our largest
Capricorn-beetle, both are membranous; in the bee-tribes, _Apis_
L., the _labium_ rather resembles leather, while the _mentum_ is
hard. Its _surface_ is often convex, sometimes plane, and sometimes
even concave; as for instance in _Melolontha Fullo_, a rare chafer
occasionally found on the coast of Kent. In some it is covered with
excavated points; in others it is quite smooth. In numbers, as in
the Predaceous beetles, both _labium_ and _mentum_ are perfectly
naked; in others, as in the common cockchafer, they are hairy;
in _Geniates barbatus_ Kirby, another chafer in the male insect,
the _labium_ is naked, while the _mentum_, which forms a piece
distinct from that part, is covered with a dense rigid beard[1200].
In _shape_ the whole _labium_ varies considerably, much more than
the _labrum_; for in addition to most of the forms I enumerated
when I described that organ, which I shall not here repeat, you
may meet with examples of many others. Thus, to instance in the
Petalocerous tribes (_Scarabæus_ L.), in some, as in the _Rutelidæ_,
the _labium_ is urceolate, or representing in some degree the shape
of a pitcher[1201]; in others it is deeply concave, and not a little
resembles a basin or a bowl[1202]; this form is peculiar to the
_labium_ of _Cremastocheilus_ Knoch, a scarce North American beetle;
in another related to this, but of an African type (_Genuchus_ Kirby
MS. _Cetonia cruenta_ F.), it is a trapezoid plate, which is elevated
from the head, and hangs over the throat like a chin[1203]. In the
_Hymenoptera_ it is extremely narrow and long, and embraces the sides
of the tongue, as well as covering it from below; so that it wears
the appearance of a kind of tube[1204]. Generally speaking, the
length of the _labium_ exceeds its breadth; but in the Predaceous
beetles the reverse of this takes place, it being very short and
wide, and usually terminating towards the tongue in three lobes or
teeth which form two sinuses varying in depth[1205].
The _mentum_ taken by itself affords no very striking characters
to which I need call your attention: I shall only observe, that
in _Hymenoptera_ it is generally of a triangular shape[1206]; but
before I proceed to consider the labial palpi, it will be proper to
notice the remarkable _labium_ of _Orthopterous_ insects, and of the
_Libellulina_, between which there is no little analogy. At first you
would imagine the terminal part of this organ in the former to be the
analogue of the tongue, or _ligula_ F.; as it is indeed generally
regarded by modern Entomologists[1207]. It seems, like the tongue of
the _Carabi_ L., _Dytisci_, &c., to be a distinct piece, which has
below it both _labium_ and _mentum_; but when you look within the
mouth, you will find a linguiform organ[1208], which evidently acts
the part of a tongue, and therefore ought to have the name; and the
piece just alluded to must either be regarded as the termination of
the lip, or as an external accompaniment of the tongue, analogous,
it may be, to the _paraglossæ_ in bees[1209]. In a specimen of
_Acrida viridissima_ (_Locusta_ F.) which I dissected, the tongue is
as long as the appendage of the under-lip, and by its upper surface
seems to apply closely to it. In the _Libellulina_ a similar organ
is discoverable[1210], which on its upper-side terminates in the
_pharynx_, like that of one of the _Harpalidæ_ before mentioned. In
the _Orthoptera_, therefore, I regard the _labium_ as consisting
of three articulations, the upper one divided into two, three, or
more lobes[1211]; the intermediate one more directly answering to
the _labium_ of other insects, and the basal one or _mentum_. This
organ in the _Libellulina_ is of a different structure: it has only
two articulations representing _labium_ and _mentum_; but the former
consists of three parallel pieces, the two exterior ones rising
higher than the intermediate one, and at their inner angle having an
acute sinus from which the palpi emerge; and the intermediate piece,
which is longitudinally channelled, lapping over the inner side of
the lateral pieces. From the angle of the covered part of these
pieces, a subulate short horizontal horn points inwards towards the
tongue, which it must keep from closing with the _labium_[1212].
3. _Palpi Labiales_[1213].--The last-mentioned organs, the _labial
palpi_, next claim our attention; but before I advert particularly
to them, it will be proper to premise a few words upon _palpi_, or
feelers, in general. These are usually jointed moveable organs,
of a corneous or coriaceous substance, attached by ligaments to
the _labium_ and _maxillæ_, and in the _Crustacea_ even to the
_mandibulæ_. Their joints, which are usually more or less obconical,
articulate also in each other by ligaments, with perhaps some
little mixture of the ball and socket. Their ends, the last joint
especially, seem furnished with nervous _papillæ_ which indicate some
peculiar sense, of which they are the instrument. What that sense is
has not been clearly ascertained, and concerning which I shall enter
more into detail in another place. Their motion seems restrained, at
least in some, to two directions, towards and from the mouth. They
were called _palpi_ or feelers, because the insect has been supposed
to use them in exploring substances. There seem to be no organs in
the vertebrate animals directly analogous to the _palpi_ of insects
and _Crustacea_, unless, perhaps, the _cirri_ that emerge from the
lips of some fishes, as the cod, red mullet, &c. which Linné defines
as used in exploring (_prætentantes_). Whether the _vibrissæ_,
miscalled _smellers_, of some quadrupeds and birds have any reference
to them, I will not venture to affirm; but the feline tribe evidently
use their bristles as explorers, and they are planted chiefly in the
vicinity of the mouth.
Having made these general remarks, I shall now confine myself to the
_labial palpi_. I call them _labial_ palpi, because that term is in
general use, and because in many cases they really do emerge from what
I consider as the _labium_, as in most of the _chafers_; but they
might with equal propriety be denominated _lingual_ palpi, since they
sometimes appear to emerge from the _tongue_ as in the stag-beetle
(_Lucanus Cervus_). In some instances, as in the Predaceous beetles,
they seem to be common to both _labium_ and tongue, being attached
at their base on the upper side to the former, and on the under side
to the latter. As to their _situation_: they emerge from the _base_
of the _labium_ in the locusts (_Locusta_ Leach)[1214]; from its
_middle_ in _Hister maximus_[1215]; from its _summit_ in _Amblyterus_
MacLeay[1216]; and from its lateral margin in _Dynastes_ MacLeay, &c.
They consist of from _one_ to _four_ joints; which, I believe, they
never exceed. They vary in _length_; though generally shorter than the
maxillary palpi, yet in the ferocious tiger-beetles (_Cicindela_ L.)
they equal them in length; and in the hive-bee and humble-bees, and
many other bees, they are considerably longer[1217]. The two first
joints of these palpi, however, in these _bees_ are different in their
structure from the two last, being compressed and flat, or concave;
and the two last joints, instead of articulating with the apex of the
second, emerge from it below the apex: so that these two first joints
seem rather elevators of the palpi than really parts of them[1218].
With respect to the relative _proportions_ of their joints to each
other: in some cases the first joint is the longest and thickest,
the rest growing gradually shorter and smaller[1219]; in others, the
second is the longest[1220]; in others, again, the third[1221], and
sometimes the last[1222]; and often all are nearly of the same size
and length[1223]. They are more commonly _naked_, but sometimes either
generally or partially _hairy_. Thus in _Cicindela_, the last joint
but one is usually planted with long snow-white bristles in a double
series, while the rest of the joints have none; and in _Copris_ Latr.
all of them are extremely hairy. In _shape_ they do not vary so much
as the maxillary palpi, being most frequently filiform or subclavate,
and sometimes setaceous; the last joint varies more in shape than the
rest, and is often remarkably large, triangular, and shaped like the
head of a hatchet[1224]; and at others it resembles the moon in her
first quarter[1225]. In the great dragon-fly, or _demoiselle_ if you
prefer the gentler French name (_Æshna_ F.) the labial palpi, which
are without any visible joints, are terminated by a minute mucro
or point[1226]. With regard to their _direction_ and flexure, they
frequently, as in the instance just mentioned, turn towards each other,
and lie horizontally upon the end of the _labium_. Sometimes, as in the
_Cicindelidæ_, they appear to point towards the tail of the insect, the
last joint rising, and forming an angle with the rest of the feeler.
In other instances they diverge laterally from the _labium_, the last
joint turning again towards it at a very obtuse angle.
4. _Mandibulæ_[1227].--Having considered the analogues of the
_lips_ in our little beings, I must next call your attention to the
representatives of the _jaws_. The vertebrate animals, you know,
are mostly furnished with a single pair of jaws, one above and the
other below, in which the teeth are planted and which have a vertical
motion. But insects are furnished with _two_ pair of jaws, a pair
of upper-jaws and a pair of under-jaws, which have no teeth planted
in them, and the motion of which is horizontal.--I shall begin
with an account of the upper-jaws. These by modern Entomologists,
after Fabricius, are denominated _mandibles_ (_mandibulæ_): a term
appropriated by Linné to the beaks of birds. The upper-jaws of
insects this great naturalist named _maxillæ_--and not improperly,
since the office of mastication is more peculiarly their office than
that of the under-jaws, which Fabricius has distinguished by that
name: as the term _mandible_, however, is generally adopted, I shall
not attempt to disturb it.
The _mandibles_ close the mouth on each side under the _labrum_ or
upper-lip. They are generally powerful organs, of a hard _substance_
like horn; but in the Lamellicorn beetles of Mr. MacLeay's families
of _Scarabæidæ_ and _Cetoniadæ_, they are soft, membranous, and unapt
for mastication. In Coleopterous insects they commonly _articulate_
with the head by means of certain _apophyses_ or processes, of
which in many cases there are _three_ discoverable at the exterior
base of the mandibles; one, namely, at each angle, and one in the
middle. That on the lower side is usually the most prominent, and
wears the appearance of the condyle of a bone: it is received by
a corresponding deep socket (or cotyloid cavity) of the cheek, in
which, being perfectly smooth and lubricous, it moves readily, but
without _synovia_, like a _rotula_ in its _acetabulum_. The upper
one projects from the jaw, forms the segment of a circle, and is
concave also on its inner face. A corresponding more shallow, or,
as anatomists speak, glenoid cavity of the cheek, where it meets
the upper-lip, receives it, and the concave part admits a lubricous
ball projecting from the cheek, upon which it turns[1228].
This structure you will find in the stag-beetle, and some other
timber-devourers. Other _Coleoptera_ have only a process of a
similar structure at each of the dorsal angles of the base of the
mandible, the intermediate one being wanting; and the articulation
does not materially differ, as far as I have examined them, in the
_Hymenoptera_ and _Neuroptera_. In the _Orthoptera_, the structure
approaches more nearly to that of the stag-beetle, since there are
_three_ prominences: it is thus well described by M. Marcel de
Serres: "This articulation," says he, "takes place in _two_ ways.
At first, in the upper surface of the mandible, and at its base,
may be observed two small prominences and a glenoid cavity; these
prominences are received in two glenoid cavities excavated in the
shell of the front, as the cavity of the mandible receives a small
prominence of the same part. Below the mandible, and at its base,
there is a kind of condyle, which has its play in a cotyloid cavity
excavated in the shell of the temple, far below the eye, and at the
extremity of the coriaceous integument of the head[1229]." Within
the head in this order, at least in _Locusta_ Leach, is a vertical
_septum_, which divides the head into two chambers, as it were,
an occipital and a frontal, consisting of a concave triangular
stem, terminating in two narrower concave triangular branches, so
as to resemble the letter Y, and forming three openings, an upper
triangular one, and two lateral subquadrangular ones, which last are
the cavities that receive the base of the mandibles. This partition,
which I would name _Cephalophragma_, doubtless affords a point of
attachment to many of the muscles of the head. It does not appear to
have been noticed, unless it be synonymous with the _intermaxillary
arcade_ of Marcel de Serres[1230]. Probably a corresponding support
to the muscles, &c. may exist, as we have seen it does in _Vespa_
L.[1231], in many other heads of the different Orders, which have
not yet fallen under examination. Many mandibles, as those of the
hornet &c., appear to be suspended to the cavity of the head on
the inside by a marginal ligament sufficiently relaxed to admit
of their play: those of the _Orthoptera_, M. Marcel de Serres
informs us, are united to the head by means of two cartilages, the
outermost being much the shortest, to which their moving muscles
are attached. These he considers as prolongations of the substance
of the mandible[1232]. The bottom of mandibles, when cleared of the
muscles &c., inclines almost universally to a triangular form; but
in some cases, as in the stag-beetle, it is nearly a _trapezium_.
I cannot conclude this subject without noticing the _motions_ of
the mandibles. What the author lately quoted has said with regard
to those of the _Orthoptera_, will, I believe, apply equally well
to all the mandibulate orders. "The articulation of mandibles with
the skull appears to take place by two points solely; and as these
parts only execute movements limited to a certain direction, they
may be referred to _ginglymus_[1233].--The movements of mandibles
are limited to those from within outwards, and from without
inwards[1234]." Whether they are restricted from any degree of
vertical motion, has not yet been proved, as the jaws of vertebrate
animals move horizontally as well as vertically--so those of insects
may have some motion vertically as well as horizontally; and it seems
necessary for some of their operations that they should. I am not
anatomist enough to speak with confidence on the subject, but the
ball and socket articulation at the lower part of the mandible, and
the curving one at the upper, though a kind of _ginglymus_, seems to
imply a degree of _rotatory_ movement, however slight.
I must next say something upon the general shape of these organs.
Almost universally they incline to a triquetrous or three-sided
figure, with their external surface convex, sometimes partially
so, and their internal concave. Most frequently they are arched,
curving inwards; but sometimes, as in _Prionus octangularis_[1235], a
Capricorn beetle, and others of that genus, they are nearly straight;
and in _Rhina barbirostris_[1236], a most remarkable Brazilian
weevil, their curvature is outwards. In _Pholidotus lepidotus_
MacLeay, and _Lucanus Elephas_, two insects of the stag-beetle tribe,
they are bent downwards; and in _Lucanus nebulosus_ K. (_Ryssonotus_
MacLeay) they turn upwards[1237]. They are usually widest at the
base, and grow gradually more slender to the apex, but in the hornet
(_Vespa Crabro_) the reverse takes place, and they increase in width
from the base to the apex; and in the hive-bee, and others of that
tribe, they are dilated both at base and apex, being narrowest in the
middle; others are nearly of the same width every where. In those
insects that use their mandibles principally for purposes connected
with their economy, they are often more broad in proportion to
their thickness, than they are in those which use them principally
for mastication. In the locust tribes (_Locusta_ Leach), they are
extremely thick and powerful organs, and fitted for their work
of devastation; but in the glow-worm (_Lampyris_), they are very
slender and minute. In those brilliant beetles, the _Buprestes_,
they are very short; but in the stag-beetles, and those giants in
the Capricorn tribe, the _Prioni_, they are often very long[1238].
They either meet at the summit, lap over each other, cross each
other, or are protended straight from the head; as you have doubtless
observed in the stag-beetle, whose terrific horns are mandibles
of this description. These organs are usually _symmetrical_, but
in some instances they are not: thus in _Hister lævus_, a kind of
dung-beetle, the left hand mandible is longer than the right; in
_Creophilus maxillosus_ K. (_Staphylinus_ L.), a common rove-beetle,
in the left hand mandible the tooth in the middle is bifid, and in
the right hand one intire; and in _Bolbocerus_ K. the mandible of one
side, in some species the dexter, and in others the sinister, has two
teeth, and the other none.
The next circumstance with respect to these organs which demands our
attention, is the _teeth_ with which they are armed. These are merely
processes of the substance of the mandible, and not planted in it by
_gomphosis_[1239], as anatomists speak, as they are in vertebrate
animals. They have, however, in their interior, at the base at least,
in the _Orthoptera_, a coriaceous _lamina_ that separates them in
some sort from the body of the mandible[1240]. Many insects, however,
have mandibles without teeth; some merely tapering to a sharp point,
others obtuse at the end, and others truncated[1241]. Of those that
have teeth, some have them on the inside at the base, as _Manticora_,
an African tiger-beetle[1242]; others in the middle, as _Staphylinus
olens_, a rove-beetle, _Lethrus cephalotes_, &c.[1243]; others at the
end, as many weevils (_Curculio_ L.)[1244]; others again on the back,
as the _Rutelidæ_, a tribe of chafers[1245], and _Lethrus_, a beetle
just named; others once more on the lower side of the base, in the form
of a tooth or spine, as in _Melitta spinigera_, a species of wild-bee,
and some of its affinities[1246]; and lastly, others on the upper side
of the base in the form of a long tortuous horn, as in that singular
wasp _Synagris cornuta_ F. before noticed as a sexual character[1247].
In the stag-beetle tribes (_Lucanus_ L.) these teeth are often
elongated into short lateral branches, or a terminal fork[1248]. They
are sometimes truncated, sometimes obtuse, and sometimes acute.