of M. Marcel de Serres; for though his remarks are confined to
the _Orthoptera_, they may be applied with advantage to the teeth
that arm the mandibles of insects in general. He perceives an
analogy between those of this Order and the teeth of quadrupeds;
and therefore divides them into _incisive_ or cutting, _laniary_
or canine, and _molary_ or grinding teeth. He denominates those
_incisives_ that are broad, having in some degree the shape of a
wedge, their external surface being convex, and their internal
concave; whence they are evidently formed for cutting. The
_laniaries_ are those which have a conical shape, are often very
acute, and in general the longest of any; and in some insects, as
the carnivorous _Orthoptera_ (and the _Libellulina_), they cross
each other. The _molaries_ are the largest of all, and their purpose
is evidently to grind the food. There is never only a single one to
each mandible, while the number of the incisives and laniaries is
very variable. As the molaries act the principal part in mastication,
they are nearer the inner base of the mandible or point of support:
they serve to grind the food, which has been first divided by the
incisives or torn by the laniaries. The carnivorous tribes are
destitute of them; in the omnivorous ones they are very small, and
in the herbivorous ones they are very large[1249]. So that in some
measure you may conjecture the food of the animal from the teeth
that arm its mandibles. Of _incisive_ teeth you may find an example
in those that arm the end of the mandibles of most grasshoppers
(_Locusta_), and of the leaf-cutter-bees (_Megachile_ Latr.)[1250];
of the _laniary_ or canine teeth, you will find good examples in
the mandibles of the dragon-flies (_Libellulina_); the two external
teeth of the apex of those of the leaf-cutter bees may be regarded
as between the incisives and laniaries; and the pointed mandibles
without teeth may be deemed as terminating in a laniary one[1251].
The lower part of the inner or concave surface of the mandibles of
grasshoppers will supply you with instances of the _molary_ teeth,
and the apex, also, of those of some weevils, as _Curculio Hancocki_
K.[1252] But the most remarkable example of a molary organ is
exhibited by many of the Lamellicorn beetles, especially those that
feed upon vegetables, whether flower or leaf.--Knoch, who indeed
was the first who proposed calling mandibles according to their
teeth, incisive, laniary, or molary, but who does not explain his
system clearly, observed that the mandibles of some _Melolonthæ_
have a projection with transverse, deep furrows, resembling a file,
for the purpose of bruising the leaves they feed upon[1253]: and
M. Cuvier, long after, observed that the larvæ of the stag-beetle
have towards their base a flat, striated, molary surface; though he
does not appear to have noticed it in any perfect insect[1254]. This
structure, with the exception of the _Scarabæidæ_ and _Cetoniadæ_,
seems to extend very generally through the above tribe; since it may
be traced even in _Geotrupes_, the common dung-chafer, in which at
the base of one mandible is a concave molary surface, and in the
other a convex one, but without any furrows: a circumstance that
often distinguishes those that have furrows.--In the _Dynastidæ_ the
affinity of structure with the _Melolonthidæ_ &c. is more pronounced,
the furrows to which ridges in the other mandible correspond being
reduced to one or two wide and deep ones; whereas in some of the
latter tribe they are very numerous. These mandibles, in many cases,
at their apex are furnished with incisive teeth to cut off their
food, and with miniature mill-stones to grind it[1255]. The part here
alluded to I call the _Mola_.
Were I to ask you what your idea is with regard to the use of
the organs we are considering, you would perhaps reply without
hesitation, "Of what possible use can the _jaws_ of insects be but
to _masticate_ their food?" But in this you would in many instances
be much mistaken; as you will own directly if you only look at
the mandibles of the stag-beetle--these protended and formidable
weapons, as well as those of several other beetles, cannot be thus
employed. "Of what other use, then, can they be?" you will say.
In the particular instance here named, their use, independent of
mastication, has not been satisfactorily ascertained; but in many
other cases it has. Recollect, for instance, what I told you in a
former letter, of those larvæ that use their unguiform mandibles
as instruments of _motion_[1256]. Again: amongst the Hymenopterous
tribes, whose industry and varied economy have so often amused and
interested you, many have no other tools to aid them in their
various labours and mechanical arts: to some they supply the place of
trowels, spades, and pick-axes; to others that of saws, scissors, and
knives--with many other uses that might be named. In fact, with the
insects of this intire Order mastication seems merely a _secondary_,
if it is at any time their use. Still comprehending in one view all
the mandibulate Orders, though some use their mandibles especially
for purposes connected with their economy, yet their most general
and _primary_ use is the division, laceration, and mastication of
their food; and this more exclusively than can be affirmed of the
under-jaws (_maxillæ_). This will appear evident to you, when you
consider that insects in their larva state, in which universally
their primary business is _feeding_, with very few exceptions use the
organs in question for the purpose of mastication, even in tribes, as
the _Lepidoptera_, that have only rudiments of them in their perfect
state--while the _maxillæ_ ordinarily are altogether unapt for such
use. The exceptions I have just alluded to are chiefly confined to
the instance of suctorious mandibles; or those which, being furnished
at the end with an orifice, the animal inserting them into its prey,
imbibes their juices through it. This is the case with the larvæ of
some _Dytisci_, _Hemerobius_, and _Myrmeleon_[1257]; and spiders have
a similar opening in the claw of their mandibles, which is supposed
to instil venom into their prey[1258].
Under this head I must not pass without notice an appendage of the
mandibles, to be found in some of the rove-beetles (_Staphylinidæ_),
as in _Ocypus_, _Staphylinus_, and _Creophilus_ Kirby. In the first
of these it is a curved, narrow, white, subdiaphanous, submembranous,
or rather cartilaginous piece, proceeding from the upper side of the
base of the mandible[1259]; in the second it is broader, straighter,
and fringed internally and at the end with hairs; and in this at
first it wears the appearance of being attached laterally to the
mandible under the tooth[1260], but if closely examined, you will
find that it is separate: in _Creophilus maxillosus_ it is broader.
This is the part I have named _prostheca_. It is perhaps useful in
preventing the food from working out upwards during mastication.
5. _Maxillæ_[1261]. The antagonist organs to the mandible in the
lower side of the head, are the under-jaws, or _maxillæ_--so
denominated by the illustrious Entomologist of Kiel. Linné appears
to have overlooked them, except in the case of his genus _Apis_, in
which he regards them, and properly, as the sheath of the tongue.
De Geer looked upon them in general as part of the apparatus of the
under-lip or _labium_; and such in fact they are, as will appear when
we consider them more particularly. Fabricius has founded his system
for the most part upon these organs, the principal diagnostic of
ten out of his thirteen Classes (properly Orders) being taken from
them; and in the modern, which may be termed the _eclectic_, system,
although the Orders are not founded upon them, yet the characters of
genera, and sometimes of large tribes, are derived from them: and as
they appear less liable to variation than almost any other organ, as
Mr. W. S. MacLeay has judiciously observed, there seems good reason
for employing them--it is therefore of importance that you should be
well acquainted with them.
Their _situation_ is usually below each mandible, on each side of the
_labium;_ towards which they are often somewhat inclined, so that
their tips meet when closed. In some cases, as in the Predaceous
beetles (_Carabus_ L. &c.), they exactly correspond with the
mandibles; but in others their direction with respect to the head is
more longitudinal, as in the _Hymenoptera_, &c. In _substance_ they
may be generally stated to be less hard than those organs; yet in
some instances, as in the _Libellulina_, _Anoplognathidæ_, &c. they
vie with them, and in the _Scarabæidæ_ and _Cetoniadæ_ exceed them,
in hardness. In the bees, and many other _Hymenoptera_, they are
soft and leathery. Their _articulation_ is usually by means of the
hinge on which they sit: it appears entirely ligamentous, and they
are probably attached to the _labium_ at the base, or _mentum_--at
least this is evidently the case with the _Hymenoptera_, in which
the opening of the _maxillæ_ pushes forth the _labium_ and its
apparatus. In that remarkable genus related to the glow-worms, now
called _Phengodes_ (_Lampyris plumosa_ F.), and in the case-worm
flies (_Trichoptera_ K.), the _maxillæ_ appear to be connate with
the _labium_, or at least at their base.--As to their _composition_,
these organs consist of several pieces or portions. At their base
they articulate with a piece more or less triangular, which I call
the hinge (_Cardo_)[1262]. This on its inner side is often elongated
towards the interior of the base of the _labium_, to which it is, as
I have just observed, probably attached. This elongate process of the
hinge in _Apis_, _Bombus_, &c. appears a separate articulation; and
the two together form an angle upon which the _mentum_ sits[1263],
and by this the _maxilla_ acts upon the labial apparatus.
The next piece is the _stipes_ or stalk of the _maxilla_. This is the
part that articulates with the hinge, and may be regarded in some
cases, as in the _Orthoptera_ &c., as the whole of the _maxilla_ below
the feeler; and in others, as in the _Geotrupidæ_, _Staphylinidæ_ &c.,
as only the back of it, the inside forming the lower lobe. This piece
is often harder and more corneous than the terminal part, is linear,
often longitudinally angular, and in the bee-tribes (_Apis_ L.) is
remarkable on its inner side for a series of bristles parallel to each
other like the teeth of a comb[1264]. In _Pogonophorus_ Latr., a kind
of dor or clock-beetle, it is armed on the back with four jointed
spines, the intermediate one being forked[1265]. M. Latreille has thus
described the stipes of the _maxillæ_ of _Coleoptera_: "Next comes
the stalk," says he, "which consists of three parts: one occupies the
back and bears the feeler; the second forms the middle of the anterior
face, and its figure is triangular; the third fills the posterior
space comprised between the two preceding; and is that which is of
most consequence in the use of the _maxilla_; the anterior feeler,
where there are two, the _galea_, and the other appendages that are of
service in deglutition, are part of that piece[1266]."
The _third_ and terminal portion of the _maxilla_ is formed by the
lobe, or lobes (_Lobi_). This may be called the most important part
of the organ, since it is that which often acts upon the food, when
preparing for deglutition. When armed with teeth or spines at the
end, its substance is as hard as that of the mandibles; but when not
so circumstanced, it is usually softer, resembling leather, or even
membrane[1267]; and sometimes the middle part is coriaceous, and the
margin membranous. This part is either simple, consisting only of
_one_ lobe, as you will find to be the case with the _Hymenoptera_,
_Dynastidæ_, _Nemognatha_, and several other beetles; or it is
compound, consisting of _two_ lobes. In the former case, the lobe is
sometimes very long, as in the bee tribes, and the singular genus
of beetles mentioned above[1268], _Nemognatha_; and at others very
short, as in _Hister_, &c. The bilobed _maxillæ_ present several
different types of form. Nearest to those with one lobe are those
whose lower lobe is attached longitudinally to the inner side of
the stalk of the organ, above which it scarcely rises. Of this
description is the _maxilla_ in the common dung-beetle (_Geotrupes
stercorarius_), and rove-beetle (_Staphylinus olens_).[1269]
Another kind of formation is where the lower lobe is only a little
shorter than the upper: this occurs in a kind of chafer (_Macraspis
tetradactyla_ MacLeay).[1270] A third is where the upper lobe covers
the lower as a shield; as you will find in the _Orthoptera_ order,
and the _Libellulina_, and almost in _Meloe_[1271]. A fourth form is
where the upper lobe somewhat resembles the galeate _maxilla_ just
named; but consists of two joints. This exists in _Staphylinidæ_,
&c.[1272] The last kind I shall notice is when the upper lobe not
only consists of two joints, but is cylindrical, and assumes the
aspect of a feeler or _palpus_[1273]. This is the common character
of almost all the Predaceous beetles (_Entomophagi_ Latr.). This
lobe, which has been regarded as an additional feeler, is strictly
analogous to the upper lobe in other insects, and therefore should
rather be denominated a palpiform lobe than a _palpus_. Where there
are two lobes, the upper one is most commonly the longest; but in
many species of the tribe last mentioned the lower one equals or
exceeds it in length[1274].
The lobes vary in form, clothing, and appendages. The upper palpiform
lobe in those beetles just mentioned, in general varies scarcely
at all in _form_; but the genus _Cychrus_ (which is remarkable
for a retrocession from the general type of form of the _Carabi_
L. making an approach towards that of those _Heteromera_ which,
from their black body and revolting aspect, Latreille has named
_Melosomes_,) affords an exception, the upper joint being rather
flat, linear-lanceolate, incurved, and covering the lower lobe[1275],
which it somewhat resembles. The lower lobe also in this tribe
varies as little as the upper, being shaped like the last joint of
that lobe in _Cychrus_ just described, except that in _Cicindela_ it
is narrowest in the middle[1276]. In other tribes the upper valve is
sometimes linear and rounded at the apex, and the lower truncated, as
in _Staphylinus olens_[1277]; sometimes the upper one is truncated or
obtuse, and the lower acute, as in _Trogosita_ and _Parnus_[1278]. In
_Ptinus_, another tribe of beetles, before noticed as injurious to
our museums[1279], the reverse of this takes place, the upper-lobe,
which is the smallest and shortest, being acute, and the lower
truncated[1280]. In _Blaps_ both are acute[1281]. In _Rhipiphorus_
and _Scolytus_ the lobes are nearly obsolete. The lower lobe is bifid
in _Languria_, a North American genus of beetles, so as to give the
maxilla the appearance of three lobes[1282]; and in _Erotylus_,
a South American one, the upper is triangular[1283]: it is often
oblong, quadrangular, linear, &c. in others.--In those that have
only one lobe the shape also varies. In _Gyrinus_, the beetle that
whirls round and round on the surface of every pool, which, though
it belongs to the Predaceous tribe, has only one lobe, the lobe
represents a mandible in shape of the laniary kind, being trigonal
and acute[1284]; and in the _Anoplognathidæ_, a New Holland tribe
of chafers, in which it is, as it were, broken, the lobe forming an
angle with the stalk, it is concavo-convex and obtuse, and somewhat
figures a molary tooth[1285]. In the first tribe into which the bees
(_Apis_ L.) have been divided (_Melitta_ Kirby), the lobe is often
linear or strap-shaped, and bifid at the apex; and in the second
(_Apis_ K.) lanceolate and intire[1286]. In _Cerocoma_ it is long and
narrow[1287]. More variations in form might be named, but these are
sufficient to give you a general idea of them in this respect. With
regard to their _clothing_, I have not much to observe--in examining
the Predaceous beetles you will observe, that the interior margin of
the lower incurved lobe is fringed with stiff bristles or slender
spines, and in many other beetles either one or both lobes have a
thick coating or brush of stiffish hairs[1288]; but in several cases
only the apex of the lobe is hairy. In the _Orthoptera_ order, and
many of the _Melolonthidæ_ or chafers, the whole _maxilla_ is without
hairs, or nearly so.
The _appendages_ of the _maxillæ_ are next to be noticed. These are
principally their claws, or laniary teeth; for they are seldom armed
with incisive or molary teeth. The whole tribe of Predaceous beetles,
with few exceptions, have the inner lobe of their _maxilla_ armed
with a terminal claw, which in the _Cicindelidæ_ articulates with the
lobe, and is moveable, but in the rest of the tribe is fixed[1289].
In _Phoberus_ MacLeay the lower lobe has two spines[1290]. In
_Locusta_ this lobe has three or four spines or laniary teeth, and
in _Æshna_ there are six, which, like the claw of _Cicindela_, are
moveable[1291]. In others both lobes terminate in a single spine or
claw: this is the case with _Paxillus_ MacLeay[1292]. In _Passalus_,
nearly related to the last genus, the upper lobe is armed with a
single spine, and the lower one with two[1293]. Those _maxillæ_
that terminate in a single lobe are also often distinguished by the
spines or teeth with which it is armed; thus in a nondescript chafer
belonging to the _Dynastidæ_ (_Archon_ K. MS.) it terminates in _two_
short teeth; in that remarkable Petalocerous genus _Hexodon_ Oliv.
in _three_ truncated _incisive_ ones[1294]; in _Dynastes Hercules_
in three _acute_ spines[1295]. _Four_ similar ones arm the apex of
the _maxilla_ in that tribe of _Rutelidæ_ which have striated elytra;
and _five_ that are stout and triquetrous those of _Melolontha
Stigma_ F. Many others have _six_ spines, sometimes arranged in a
triple series[1296]. Besides teeth or spines, in some cases the
lobes of _maxillæ_ terminate in several long and slender _laciniæ_
or lappets fringed with hairs. At least those of a _Leptura_
(_L. quadrifasciata_ L.) described by De Geer, appear to be thus
circumstanced. He conjectures that this beetle uses its _maxillæ_ to
collect the honey from the flowers[1297].
As the principal use of the mandibles is cutting and masticating, so
that of the organs we are considering seems to be primarily that of
_holding_ the food and preventing it from falling while the former
are employed upon it. I say this is their _primary_ use; for I
would by no means deny that they assist occasionally in comminuting
or lacerating it. In fact, were there no organs appropriated to
this use, and if both mandibles and _maxillæ_ were employed at
the same time in comminuting the food, it seems to me that it
must fall from the mouth. In a large proportion of insects the
lobes of the _maxillæ_ are not at all calculated for laceration or
comminution; and in those tribes--as the _Melolonthidæ_, _Rutelidæ_,
_Dynastidæ_--in which they seem most fitted for that purpose, the
mandibles have _incisive_ teeth at their apex, and at their base a
powerful _mola_ or grinder: circumstances which prove, that even in
this case the business of mastication principally devolves upon them.
6. _Palpi Maxillares_[1298]. There is one circumstance that
particularly distinguishes the _maxillæ_ from the mandibles--they
are _palpigerous_, as well as the under-lip. The feelers, or palpi,
emerge usually from a sinus observable on the back of the _maxillæ_
where the upper lobe and stalk meet. Their articulation does not
materially differ from that of the labial palpi. Each _maxilla_ has
properly only _one_ feeler; but, as was lately observed[1299], in
certain tribes the upper lobe is jointed and palpiform, which has
occasioned it to be considered as a feeler, and these tribes have
been regarded as having six feelers. The most general rule with
regard to the _length_ of the _palpi_ is, that the maxillary shall
be longer than the labial; but the reverse often takes place. In
many _bees_ the maxillary consist only of a _single_ joint, and
are very _short_; while the labial consist of _four_, and are very
_long_[1300]: and in some insects (as in _Pogonophorus_ Latr.)
the four palpi are of equal length[1301]. The antennæ are most
commonly longer than the palpi; but in several aquatic beetles, as
_Elophorus_, _Hydrophilus_, &c., whose antennæ in the water are not
in use, the organs we are considering are the longest.--As to the
_number_ of their articulations, it varies from one to six; which
number they are not known to exceed. In each of the Orders a kind
of law seems to have been observed as to the number of joints both
in the maxillary and labial palpi, but which admits of several
exceptions. Thus in the _Coleoptera_, the _natural_ number may
be set at _four_ joints for the _maxillary_, and _three_ for the
_labial_ palpi: yet sometimes, as in _Stenus_, _Notoxus_, &c., the
_former_ have only _three_ joints, and the _latter_, as in _Stenus_
and _Tillus_, only _two_. In the _Orthoptera_ the law enjoins _five_
for the _maxillary_, and _three_ for the _labial_; and to this I
have hitherto observed no exception. In the _Hymenoptera_, the rule
is _six_ and _four_, but with considerable exceptions, especially
as to the _maxillary_ palpi, which vary from _six_ joints to a
_single_ one: thus in the hive-bee and the humble-bee, the labials,
including the two flat joints or elevators, have four joints, while
the maxillaries are not jointed at all[1302]. In _Chrysis_, in which
the latter consist of _five_, the former are reduced to _three_. The
_Libellulina_ may almost be regarded as having no maxillary palpi,
since they exhibit no organ that is distinctly palpiform. It seems to
me that the upper lobe of their maxilla, which articulates with the
stalk in the same manner as a feeler, may be regarded as an instance
in which that lobe and the feeler coalesce into one; and the mucro
that proceeds from the lobe has the aspect of an emerging feeler, and
corresponds somewhat with the labial one above noticed[1303]. In the
remainder of the _Neuroptera_ and the _Trichoptera_, the prevailing
number is _five_ and _three_. In the latter there are exceptions,
which will furnish good characters for genera. In the _Lepidoptera_
we find _two_, and sometimes _three_, the maxillary being very
minute[1304]. The _Diptera_ Order presents two tribes in this
respect quite distinct from each other. The most natural number of
joints in the maxillary palpi of the _Tipulidæ_, _Culicidæ_, &c. is
_four_ or _five_: the last joint, however, in _Tipula_, _Ctenocera_,
&c. like that of the antennæ in _Tabanus_ L., appears to consist
of a number of very minute joints[1305]; but in the _Asilidæ_ and
_Muscidæ_, &c., the number _two_ seems to be most prevalent[1306].
The _labial_ palpi in this order are obsolete.--As to _shape_, the
maxillary palpi, as well as the labial, are usually filiform; but in
the weevil tribes (_Curculio_ L.) they are most commonly very short
and conical[1307]; in the chafers (_Scarabæus_ L.) they usually are
thickest at the apex[1308]; in _Megachile_ and _Euglossa_, wild bees,
they are setaceous, growing gradually more slender from the base
to the summit[1309]: a tribe of small water-beetles (_Haliplus_),
the saw-flies (_Tenthredo_ L.), and several other _Hymenoptera_,
have them thickest in the middle[1310]. Their most important part,
however, and that which varies most in form, is the _terminal_
joint:--of this I have already related some singular instances[1311],
and shall now describe a few more. This joint is sometimes acute,
at others blunt, at others truncated: in figure it is ovate,
oblong, obtriangular, hatchet-shaped, lunate, transverse, conical,
mammillate, subulate, branched, chelate, laciniate, lamellate, &c.
&c.[1312]: terms which I shall more fully explain to you hereafter,
and which I only mention here to show the numerous variations as to
figure, of which this joint exhibits examples. The palpi in general
at their vertex are often rather concave; and this concavity is
formed by a thin papillose membrane, which it is supposed the animal
has the power of pushing out a little, so as to apply it to surfaces.
The _primary_ use of the palpi of insects will be considered when I
treat of their senses; but they probably answer more purposes than
one. For instance, when I was once examining, under a lens, the
proceedings of a species of _Mordella_, which was busily employed
in the blossom of some umbelliferous plant, it appeared to me to
open the anthers with its maxillary palpi, and they often held the
anther between them: when not so employed, they were kept in intense
vibration, more than even its antennæ; and at the same time, as far
as I could judge, an _Elater_ made the same use of them.