has employed this term to designate the oral instruments, or rather
their sheath, in the _Muscidæ_ and some others, calling the same
organ, when without fleshy lips, _rostrum_ and _haustellum_: but
as the parts of the mouth in all true _Diptera_ (for _Hippobosca_
and its affinities can scarcely be deemed as co-ordinate with the
rest), are analogous to each other; although in some they are stiff
and rigid, in others flexile and soft, and in _Œstrus_ (except the
palpi) mere rudiments,--the same appellation ought to designate them
all. I am happy to find that M. Latreille agrees with me in this
opinion; and to his sensible observations on this head, if you wish
for further information, I refer you[1375]. The mouth of Dipterous
insects appears to vary in the number of pieces that it presents;
but in all, the _theca_ or sheath is present, which represents the
_labium_ (including the _mentum_) of the mandibulate Orders[1376].
It consists of _three_ joints, the last of which is formed by the
liplets (_Labella_). Those in the _Muscidæ_ are large, turgid,
vesiculose, and capable of dilatation; in the _Bombylidæ_ and other
tribes they are small, slender, long and leathery, and sometimes
recurved. The second joint or stalk, which may be said to represent
the _mentum_, the liplets being properly in a restricted sense
the analogue of the _labium_, its sides being turned up, forms a
longitudinal cavity, which contains the _haustellum_. The upper piece
of this, the _valvula_, is long, rigid, and very sharp, representing
the _labrum_[1377]. Beneath this cover, in the above cavity, are the
lancets; which, as far as they are at present known, vary in number
and form: sometimes there are _five_ of them, sometimes _four_,
sometimes _two_, and sometimes, it should seem, only _one_[1378]. In
the gnat (_Culex_) they are finer than a hair, very sharp, and barbed
occasionally on one side[1379]; in the horse-fly (_Tabanus_ L.) they
are flat and sharp like the blade of a knife or lancet[1380]. In
this tribe the upper pair, or the knives (_Cultelli_), represent the
_mandibles_; the lower pair, or the lancets (_Scalpella_), usually
palpigerous, the _maxillæ_; and the central one the _tongue_. In the
horse-fly Reaumur has figured only _four_, exclusive of the _labrum_
and _labium_; but in a specimen I have preserved there appear to be
_five_, one of which, as slender as a hair, I regard as the analogue
of the tongue[1381].--When the lancets are reduced to two, they
probably represent the _maxillæ_, the mandibles being absorbed in the
_labrum_; and where there is only one, the maxillæ also are absorbed
by the _labium_, which then bears the palpi, the lancet representing
the tongue[1382]. The lancets are so constructed in many cases, as
to be able by their union to form a tube proper for suction, or
rather for forcing the fluid by the pressure of the lower parts to
the _pharynx_[1383]. _Labial palpi_ appear not usually present in the
_proboscis_; but M. Savigny thinks he has discovered vestiges of them
in _Tabanus_[1384]. In this genus the maxillary ones are large, and
consist of _two_ joints[1385]. The proboscis is often so folded, as
to form two elbows; the base forming an angle with the stalk, and the
latter with the lips, so as in shape to represent the letter Z, only
that the upper angle points to the breast, and the lower one to the
mouth: this is the case with the flesh-fly and many others. In other
flies, as _Conops_ and _Stomoxys_, whose punctures on our legs so
torment us[1386], there is only a single fold, with its angle to the
breast. The _proboscis_ is received in a large oblong cavity of the
underside of the anterior part of the head.
It may here be observed, that in the _promuscis_ the elongation
of the organs seems to be made chiefly at the expense of _all_ the
palpi, but in the _proboscis_ at that of the _labial_ only; and in
some cases at that also of the _mandibles_ or _maxillæ_;--the former
merging in the _labrum_ and the latter in the _labium_.
iii. _Antlia_[1387].--The _third_ kind of imperfect mouth is that
of the _Lepidoptera_, which I have called _Antlia_. Fabricius
denominates it _lingua_: but as this organ has no analogy with the
real tongue of insects, this is confessedly improper, and it appeared
necessary therefore to exchange it for another denomination: I
have endeavoured to apply a term to it that indicates its use--to
pump up, namely, the nectar of the flowers into the mouth of the
insect. On a former occasion I described to you the structure of
this instrument[1388]; but further discoveries with regard to it
having since been made by MM. Savigny and Latreille, I shall here
give you the result of their observations. The former of these
able physiologists has detected in the mouth of the _Lepidoptera_
rudiments of almost all the parts of a perfect mouth. Of the
correctness of this assertion you may satisfy yourself, if you
consult his admirable elucidatory plates, and compare them with the
insects. Just above the origin of the spiral tongue or pump, the head
is a little prominent and rounded; and immediately below the middle
of this prominence there is a very minute, membranous, triangular or
semicircular piece; which from its position, as covering the base of
the _antlia_, may be regarded as the rudiment of the _upper-lip_
(_labrum_)[1389]. On each side of the outer base of the _antlia_ is
another small immoveable piece, resembling a flattened tubercle,
the end of which is internally hairy or scaly: these pieces appear
to represent the _mandibles_[1390]. Near the base of each half
of the _antlia_, just below a sinus, may be distinctly seen the
minute, usually biarticulate rudiment of a _maxillary palpus_[1391];
demonstrating to a certainty that these spiral organs, at least their
lateral tubes or _Solenaria_, are real maxillæ[1392]. The rudiment
of the _under-lip_ (_Labium_) is the almost horny triangular piece
united by membrane to the two stalks of the maxillæ, and supporting
at its base the recurved labial palpi; which are so well known
that I need not enlarge upon them[1393]. Amongst these parts there
seems at first sight no representative of the _tongue_; but M.
Latreille has advanced some very ingenious, and I think satisfactory
arguments[1394], which go to prove that this part, at least the
tongue of _Hymenoptera_, has its analogue in the intermediate tube or
_Fistula_ formed by the union of the two maxillæ, and which conveys
the fluid aliment of this Order to the _pharynx_. As in _Diptera_
the _maxillæ_ sometimes merge in the _labium_, so here the _tongue_
(as it were divided longitudinally) merges in the _maxillæ_. He
further observes, that in a transverse section of the maxilla of the
death's-head hawk-moth (_Sphinx Atropos_), the lateral tube appeared
to be divided into two by a membranous partition, and to contain
in the upper cavity a small cylindrical tube, which seemed to be a
_trachea_[1395]. To animals that are without lungs, and breathe by
_tracheæ_, suction must be performed in a very different way from
what it is by those that breathe by the _mouth_: and as in the very
extended organs in question the fluid has a long space to pass before
it reaches the _pharynx_, in some way or other these lateral tubes
may have the power of producing a vacuum in the middle tube, and so
facilitate its passage thither. We see, in the _antlia_, that the
maxillæ receive their vast elongation at the expense of all the other
organs, except the _labial palpi_.
iv. _Rostrulum_[1396].--An animal very annoying to us affords the
type of the next kind of imperfect mouth--I mean the _flea_. Its oral
apparatus, which I would name _rostrulum_, appears to consist of
_seven_ pieces. First are a pair of triangular organs, the _laminæ_,
which together somewhat resemble the beak of a bird, and are affixed,
one on each side of the mouth, under the antennæ: these represent
the _mandibles_ of a perfect mouth[1397]. Next, a pair of long sharp
lancets (_Scalpella_), which emerge from the head below the laminæ:
these are analogous to _maxillæ_[1398]: a pair of palpi, consisting
of four joints, are attached to these near their base[1399], which
of course are _maxillary palpi_. And lastly, in the midst of all is
a slender setiform organ (_ligula_), which is the counterpart of
the _tongue_[1400]. Rösel, and after him Latreille, seem to have
overlooked this last piece, since they reckon only _six_ pieces
in the flea's mouth[1401]: but the hand and eye of our friend
Curtis have detected a _seventh_, as you see in his figure. From
this account it appears, that the elongation of the organs of the
_Aphaniptera_ Order is at the expense of the _labium_ and its _palpi_.
v. _Rostellum._--So little is known of the composition of the next
kind of imperfect mouth, that I need not enlarge upon it. It is
peculiar to the louse tribe (_Pediculidæ_), and it consists of the
tubulet (_Tubulus_), and siphuncle (_Siphunculus_). The former is
slenderer in the middle than at the base and apex, the latter being
turgid, rather spherical, and armed with claws which probably lay
hold of the skin while the animal is engaged in suction. When not
used, the whole machine is withdrawn within the head; the siphuncle,
which is the suctorious part, being first retracted within the
tubulet, in the same way as a snail retracts its _tentacula_[1402].
This apparatus seems formed at the expense of all the other organs.
There are some other kinds of imperfect mouth, which, though they
seem not to merit each a distinct denomination, should not be passed
altogether without notice. The first I shall mention is that of the
family of _Pupipara_ Latr. (_Hippobosca_ L.). It consists of a pair
of hairy coriaceous valves, which include a very slender rigid tube
or siphuncle, the instrument of suction, which Latreille describes as
formed by the union of two setiform pieces[1403]. In _Melophagus_,
the sheep-louse, the union of the valves of the sheath is so short,
that they appear like a tube; but if cut off they will separate,
and show the siphuncle, as fine as a hair, between them. This organ
is of a type so dissimilar, as was before observed, to that of the
_Diptera_ in general, and approaches so near to that of the dog-tick
(_Ixodes_), that they may be deemed rather apterous insects with two
wings, than to belong to that Order; and the circumstance that some
of the family are apterous confirms this idea. In fact they are a
_transition_ family that connects the two Orders, but are nearest to
the _Aptera_. In _Nycteribia_ the oral organs differ from those of
the other _Pupipara_ in having _palpi_. This also is the case with
those of the genus _Ixodes_, the palpi of which are placed upon the
same base with the instrument of suction, than which they are longer:
they appear to consist of _two_ joints, the last very long and flat.
The instrument of suction itself is formed by three hard rigid
laminæ; two shorter parallel ones above, that cover the third, which
is longer and broader, and armed on each side with several teeth
like a saw, having their points towards the base[1404]. Many of the
other _Acari_ L. have mandibles, and several have not: but their oral
organs have not yet been sufficiently examined; and from the extreme
minuteness of most of them, this is no easy task; nor to ascertain in
what points they differ or agree.
If you consider the general plan of the organs of manducation in the
vertebrate animals, how few are the variations that it admits! An
upper and a lower jaw planted with teeth, or a beak consisting of an
upper or a lower mandible with a central tongue, form its principal
features. But in the little world of insects, how wonderful and
infinite is the diversity which, as you see, in this respect they
exhibit! Consider the number of the organs, the varying forms of
each in the different tribes, adjusted for nice variations in their
uses:--how gradual, too, the transition from one to another! how one
set of instruments is adapted to prepare the food for deglutition by
mastication; another merely to lacerate it, so that its juices can
be expressed; a third to lap a fluid aliment; a fourth to imbibe it
by suction--and you will see and acknowledge in all the hand of an
almighty and all-bountiful CREATOR, and glorify his wisdom, power,
and goodness, so conspicuously manifested in the structure of the
meanest of his creatures. You will see also, that all things are
created after a pre-conceived plan; in which there is a regular and
measured transition from one form to another, not only with respect
to beings themselves, but also to their organs--no new organ being
produced without a gradual approach to it; so that scarcely any
change takes place that is violent and unexpected, and for which the
way is not prepared by intermediate gradations. And when you further
consider, that every being, with its every organ, is exactly fitted
for its functions; and that every being has an office assigned, upon
the due execution of which the welfare, in certain respects, of this
whole system depends, you will clearly perceive that this whole plan,
intire in all its parts, must have been coeval with the Creation; and
that all the species,--subject to those variations only that climate
and different food produce,--have remained essentially the same, or
they would not have answered the end for which they were made, from
that time to this.
* * * * *
Having given you this particular account of the _trophi_ or organs
of the mouth of insects, I must now make some observations upon the
_other_ parts of the head. I have divided it, as you see in the
Table, into _face_ and _subface_; the former including its _upper_
and the latter its _lower_ surface. Strictly speaking, some parts of
the face, as the temples and cheeks, are common to both surfaces; but
I do not therefore reckon them as belonging to the subface, which,
exclusive of the mouth and its organs, consists only of the _throat_,
and where there is a neck, the _gula_.
i. _Nasus_[1405].--I shall consider the parts of the face in the
order in which they stand in the Table, beginning with the _nasus_
or nose. Fabricius has denominated this part the _clypeus_, in which
he has been followed by most modern Entomologists. You may therefore
think, perhaps, that I have here unnecessarily altered a term so
generally adopted, and expect that I assign some sufficient reasons
for such a change. I have before hinted that there is good ground
for thinking that the sense of _smell_ in insects resides somewhere
in the vicinity of this part; and when I come to treat of their
senses, I shall produce at large those arguments that have induced
me to adopt this opinion: and if I can make out this satisfactorily,
you will readily allow the propriety of the denomination. I shall
here only state those _secondary_ reasons for the term, which, in
my idea, prove that it is much more to the purpose than _clypeus_.
This last word was originally applied by Linné in a metaphorical
sense to the ample covering of the head of the _Scarabæidæ_, and the
thoracic shield of _Silpha_, _Cassida_, _Lampyris_, and _Blatta_: in
all which cases there was a propriety in the figurative use of it,
because of the resemblance of the parts so illustrated to a _shield_.
But when _Fabricius_ (though he sometimes employs the term, as Linné
did, merely for illustration,) admitted it into his orismological
table, as a term to represent universally the anterior part of the
face of insects to which the labrum is attached (though in some
cases he designates the _labrum_ itself by this name), it became
extremely inappropriate; since in every case, except that of the
_Scarabæidæ_, the part has no pretension to be called a _shield_;--so
that the term is rather calculated to mislead than illustrate. This
impropriety seems at length to have struck M. Latreille, since in a
late essay[1406] he has changed the name of this part to _Epistomis_,
a term signifying _the part above the mouth_. But there are reasons,
exclusive of those hereafter to be produced concerning the sense of
smell, which seem to me to prove that _nasus_ is a preferable term;
not to mention its claim of priority, as having been used to signify
this part a century ago[1407]. When we come to consider the terms for
the other parts of the head, as _lips_, _jaws_, _tongue_, _eyes_,
_temples_, _cheeks_, _forehead_, &c. the concinnity, if I may so
speak, and harmony of our technical language, seem to require that
the part analogous in point of situation to the _nose_ of vertebrate
animals should bear the same name. And any person who had never
examined an insect before, if asked to point out the _nose_ of the
animal, would immediately cast his eye upon this part: so that one
of the principal uses of imposing names upon parts--that they might
be more readily known--would be attained. If it is objected, that
calling a part a _nose_ that has not the sense of smell, supposing it
to be so, might lead to mistakes--I would answer, that this objection
is not regarded as valid in other cases: for instance, the _maxillæ_
are not generally used as _jaws_, and yet no one objects to the term;
because, from their situation, they evidently have an analogy to the
organs whose name they bear. But enough on this subject--we will now
consider the part itself.
To enable you to distinguish the nose of insects when it is not
separated from the rest of the face by an impressed line, you must
observe that it is the terminal middle part that sometimes overhangs
the upper-lip, and at others is nearly in the same line with it; that
on each side of it are the cheeks, which run from the anterior half
of the eyes to the base of the mandibles. Just below the antennæ is
sometimes another part distinct from the nose, which I shall soon have
to mention; so that the nose must not be regarded as reaching always
nearly to the base or insertion of the antennæ, since it sometimes
occupies only half the space between them and the upper-lip, which
space is marked out by an impressed line. But you will not always be
left at such uncertainty when you want to ascertain the limits of
the nose; for it is in many cases a distinct piece, separated by an
elevated or impressed line from the rest of the face. This separation
is either partial or universal. Take any species of the genera
_Copris_, _Onitis_, or _Ateuchus_, and you will see the nose marked
out in the centre of the anterior part of the face by two _elevated_
lines, forming nearly a triangle and bounded by the horn[1408]. Or
take a common wasp or hornet, and you will find a similar space,
though approaching to a quadrangular figure, marked out by _impressed_
lines[1409]. In _Rhagio_ and _Sciara_, two Dipterous genera, this
impression is so deep as to look like a suture. Between these lines,
in those cases, is included what I call the _nose_. As to _substance_,
in general it does not differ from the rest of the head; but in the
_Cleridæ_ it is almost membranous. You must observe, that in all these,
what at first sight appears to be the termination of the front, is not
the nose, but the narrow depressed piece that intervenes between it
and the lip. With regard to its _clothing_, it is most commonly naked,
but in some genera it is covered with hair; in _Crabro_ F. often with
golden or silver pile, which imparts a singular brilliance to the mouth
of the insects of that genus: M. Latreille supposes that the brilliant
colours of the golden-wasp (_Chrysis_ L.) may dazzle their enemies,
and so _promote_ their escape[1410]; the brilliance of the mouth of
the _Crabro_ may on the contrary at first dazzle their prey for a
moment, so as to _prevent_ their escape. The _form_ of the nose, where
distinct from the rest of the face, admits of several variations: thus
in the _Staphylinidæ_ and _Cleridæ_ it is transverse and linear; in
_Copris_ it is triangular, with the vertex of the triangle truncated;
in _Vespa Crabro_ it is subquadrate and sinuated. In many Heteromerous
beetles[1411] it is rounded posteriorly: in _Pelecotoma_, a new genus
in this tribe, related to _Asida_, there is a deep anterior sinus; in
_Blaps_ the anterior margin is concave; in _Cetonia_[1412] _Brownii_,
and _atropunctata_ (forming a distinct subgenus), it is bifid: it
varies in the _Scarabæidæ_, in some being bidentate, in others
quadridentate, and in others again sexdentate, including the cheeks:
in _Mylabris_, a kind of blister-beetle, it is transverse and nearly
oval; in _Lamia_, a capricorn-beetle, it represents a parallelogram;
and in most _Orthoptera_ it is subtriangular: in _Tettigonia_ F. it
is prominent, transversely furrowed, and divided by a longitudinal
channel: in _Otiocerus_ K. it presents the longitudinal section of
a cone[1413]: in the _Diptera_ Order, with the exception of the
_Tipulidæ_ and some others, in which it unites with the cheeks, &c. to
form a rostrum, the nose in general, as to _form_, answers to its name,
resembling that of many of the _Mammalia_: in some of the _Asilidæ_
it is very tumid at the end, and terminates in a sinus, to permit
the passage of the proboscis to and fro: in many of the _Syrphidæ_,
&c. it is first flat and depressed, and then is suddenly elevated,
so as to give the animal's head the air of that of a monkey: in some
tribes, as _Rhingia_, _Nemotelus_, _Eristalis_, &c., in conjunction
with the cheeks it forms a conical rostrum: in _Tabanus bovinus_, and
other horse-flies, it terminates in three angles or teeth. Many more
forms might be mentioned, but these will suffice to give you a general
idea of them. In _size_ and _proportions_ the nose also varies. It
is frequently, as in _Tettigonia_, the most conspicuous part of the
face, both for size and characters; but in the _Staphylinidæ_ it is
very small, and often scarcely discernible, being overshadowed by its
ample front: and it may be observed in general, that when the antennæ
approximate the mouth, as in this genus and many others, the _front_
becomes _ample_, and the _nose_ is reduced to its _minimum_: but when
they are distant from the mouth, the reverse takes place; and the
nose is at its _maximum_ and the front at its _minimum_. _Mutilla_,
_Myrmecodes_, _Scolia_, &c. in the _Hymenoptera_, are an example of
the former; and the _Pompilidæ_, _Sphecidæ_, _Vespidæ_, &c. of the
latter. In _Myopa buccata_, &c. its length exceeds its width; but more
commonly the reverse takes place. The _circumscription_ of the _nose_
also deserves attention. It is usually terminated behind by the front
(_frons_), or, where it exists, by the _postnasus_, in the sides by the
cheeks, and anteriorly by the _labrum_. But this is not invariably the
case; for in the _Cimicidæ_, in which the cheeks form the bed of the
_Promuscis_, the front embraces it on each side by means of two lateral
processes, that sometimes meet or lap over each other anteriorly, which
gives the nose the appearance of being insulated; but it really dips
below these lobes to join the _labrum_. This structure you may see in
_Edessa_ F., and many other bugs. This part sometimes has its _arms_.
Thus in _Copris_, and many _Dynastidæ_, the horns of the head seem, in
part at least, to belong to this portion of it; in _Tipula oleracea_
(the crane-fly), &c. it terminates before in a horizontal mucro. In
_Osmia cornuta_, a kind of wild-bee, each side of the nose is armed
with a vertical horn. The _margin_ of the nose in most Lamellicorn
insects, though mostly level, curves upwards.