the internal _prosternum_, branches into the _antefurca_. In the
_Coleoptera_ a plate varying in shape and direction[1755] sends
forth a pair of mostly vertical processes of a cartilaginous
substance[1756], differing in height in different genera. In
_Carabus_ L. there is neither this plate nor its processes; but in
_Dytiscus_ the latter are very visible. A very singular and complex
machine represents the part we are considering in that extraordinary
insect the mole-cricket (_Gryllotalpa_ Latr.). When we look at its
prodigious arms and consider their office[1757], we may imagine
that the requisite apparatus for moving them must be very powerful
and peculiar. Their CREATOR has accordingly provided them with a
machine for this purpose more than usually complex, extending from
the _prothorax_ to the _prosternum_; the former being its _base_,
and the latter its _vertex_. The cavity of the manitrunk is divided
longitudinally by a double cartilaginous partition surmounted by
a bony frame, with an anterior condyle or tuberosity, with which
the inner part of the base of the clavicle of the arm appears to
ginglymate; and the manitrunk is preserved from the injury the
powerful action of the arm might occasion, by the counteraction of
this machine, to describe which fully, would demand more space than I
can afford[1758]. I mentioned under the _mesostethium_, the apertures
visible in the breast of _Locusta_ Leach and _Acrida_ K. Each of
these apertures opens into an internal, tubular, horny, process,
which arching off is attached at the other extremity to the sides
of the trunk--a pair being appropriated to each segment; the first
analogous to the _antefurca_, the second to the _medifurca_, and the
last to the _postfurca_. In the _medipectus_ and _postpectus_ of
_Acrida viridissima_ there is only a single aperture, terminating in
a single tube, which after rising vertically a little way sends off
a branch on either hand to the sides of the trunk. Where there are
_three_ of these holes, as in the _antepectus_ and _medipectus_ of
_Locusta Dux_, there are _three_ of these processes, the intermediate
one being vertical. In the subsequent Orders the processes of the
_endosternum_ are not sufficiently remarkable to require particular
notice: my further observations upon them will therefore be confined
to the _Coleoptera_ Order.
2. _Medifurca_[1759]. This part, which belongs to the mid-legs, is
in many cases more conspicuous than the _antefurca_. In _Copris
Molossus_ the _endosternum_ of the _medipectus_ is represented by a
transverse zigzag ridge[1760] between the sockets of the mid-coxæ,
from which proceeds a pair of branches wide at the base and growing
gradually more slender to the extremity[1761], which is attached to
the sides of the trunk; in _Dytiscus marginalis_ a pair of slender,
vertical, straight processes, fitted with a broad cartilaginous
plate at their apex, rises from the _endosternum_, and sends forth a
lateral one to the side of the _medipectus:_ and lastly, in _Carabus_
the _medifurca_ is represented by a pair of subtriangular _laminæ_
attached to the sides of the trunk.
3. _Postfurca_[1762]. This, which belongs to the hind-legs, is the most
remarkable of the pectoral processes, and has been noticed by more than
one writer[1763]. It is a kind of trident, the branches[1764] of which
are acute, and on their upper surface longitudinally concave, elevated
on a footstalk[1765] inclined towards the _medifurca_, consisting
of two plates, a posterior one supporting the lateral branches, and
an anterior or interior one forming a right angle with the other,
supporting the intermediate one. This footstalk rises from between the
posterior _coxæ_, which appear in the Lamellicorns to ginglymate with
it at its base. The middle branch of the trident dips to the sinus
of the _medifurca_. In _Dytiscus marginalis_ the form is different;
for the intermediate branch consists of two parallel pieces, and the
lateral ones are dilated into broad vertical plates: the stalk of this
is triquetrous, and a triple cartilaginous partition appears to go from
its base anteriorly, the lateral ones diverging to the sides of the
trunk, and the intermediate one running straight to the base of the
_medifurca_.
It may not be without interest to state here some of the several
objects and uses of this structure of the trunk. When our Saviour
says to his disciples, "But even the very hairs of your head are all
numbered"[1766]--he taught them that the attention and care of the
DEITY were not confined to the mighty and the vast, but directed to
every atom of his creation--that he not only decreed the number and
magnitude of the planets and planetary systems, and of their various
inhabitants, but that the most minute and apparently insignificant
part of each individual, both as to its number and form, was
according to the law by him laid down; and whoever studies them with
attention will find that insects furnish a very interesting homily
upon this text; since in various instances I think I have made it
clear, that parts seemingly of the least importance--as a hair, a
pore, or a slight impression--have their appropriate use[1767].
At first, it would seem that the various pieces of which we have
seen the second primary segment of the trunk of these animals to be
composed, would be of little importance; but when we reflect that
this multiplicity of parts is usually not to be found in those that
have no wings, whether they be apterous sexes or tribes[1768], a
suspicion arises in the mind that they must be of more consequence
than their _prima facie_ appearance seems to warrant:--and this is
really the case. The _manitrunk_, which is destined principally to
incase the muscles that move the _arms_, did not require to be so
complex as the part that had to support the action of _wings_ as
well as _legs_. In those that have a large _prothorax_, as the
_Coleoptera_, it may, indeed, be useful in flight as a counterpoise
to the abdomen; and since when the wings descend it rises, and
_vice versa_, it may be of some service by its vibrations[1769];
but for this it required no complexity of structure. But not so
the _alitrunk_: it consists of parts much more numerous, and this
number of parts is of great importance to the animal in its flight.
All of them are so put together, being lined by a common elastic
ligament[1770], as to be capable of a certain degree of tension and
relaxation, which enables the animal to compress or dilate the trunk
as its necessities require. To cause the elevation of the wings, it
must be _compressed_ or have its _longitudinal_ diameter _increased_,
and its _vertical_ and _transverse diminished_: this compression is
produced by the _condensation_ of the internal air, which parts with
some of its caloric, and by the action of the _levator_ muscles. To
cause the _depression_ of the wings, it must be _dilated_, or have
its _longitudinal_ diameter _diminished_, and its _vertical_ and
_transverse increased_, which is effected by the _rarefaction_ of
the internal air, and the action of the _depressor_ muscles[1771].
In some Orders, the _Coleoptera_, &c., this effect is promoted by
the segments of the trunk, which are attached by loose ligamentous
membranes, and received, one or more of them, into each other, which
facilitates the above action[1772]. Thus much for the general use
of these parts. I shall further here mention a partial one of two
of them which seems indicated by a particular circumstance, and
upon which a theory may be built. In some insects the primary and
secondary wings or their analogues are placed _before_ the legs, in
others _over_ the legs, and in others _behind_ the legs: but whatever
their position, the pieces which I have named the _scapularia_
and _parapleuræ_ invariably connect the one with the other; the
former, the _primary_ wings with the _mid-legs_, and the latter,
the _secondary_ wings with the _hind-legs_. This circumstance seems
to prove that the wings by the intervention of these pieces have
an action upon the legs, and the legs upon the wings; and this is
further proved in one case by an observation of M. Chabrier with
regard to _Melolontha vulgaris_,--that the levator muscles of the
wings, by means of a long tendon, are attached to the lower part
of the posterior coxæ[1773]. Now, more than one medical friend
has suggested to me, that what are called the _coxæ_ in insects
are really analogous to the _thighs_ of vertebrate animals[1774]:
consequently these parts must represent the _coxæ_; whence it would
seem that the wings are really appendages of the legs. It must,
however, be observed, that were this opinion admitted, in the
_Aptera_, _Hymenoptera_, and _Diptera_, or even in the _prothorax_
of other insects, there would scarcely be any analogue of the _coxæ_
at all distinct from the trunk itself, of which even in the other
Orders these pieces are component parts. An instance occurs in the
_Strepsiptera_ K., and in which the arms are furnished with an alary
appendage, and the _metathorax_ has none[1775].
VI. _Organs of Motion._ We are next to consider those organs attached
to the trunk of insects which are instruments of _motion_. These are
principally those by which they are transported through the _air_,
and those by which they move on the _earth_ or in the _water_--their
_wings_ and their _legs_. I shall begin with the first, the
_wings_[1776]. These are not formed precisely after any type at present
discovered in vertebrate animals: in some respects they have an analogy
to those of birds[1777]; in others, to the dorsal fins of fishes: but,
perhaps, altogether they approach the nearest to those of the dragon or
flying-lizard (_Draco volans_ L.), which do not, as in birds, replace
the fore-legs, are kept expanded by diverging bony rays, and are
connected with the hind-legs[1778]. As the Divine Creator appears in
his works to proceed gradually from one type of structure to another,
it has been supposed by a learned physiologist of our own country,
that in winged insects, _four_ of the legs of the Decapod _Crustacea_
are represented by the four wings[1779]: this opinion, however, is not
yet fully proved; a remark which may also be applied to a more recent
one of a celebrated French writer, who seems to think their origin and
structure aërostatic, that they are auxiliary to the legs, and borrowed
in part from the respiratory organs[1780]. Were I disposed to enter
into these subtile speculations, I might here recall your attention to
the analogy that, in their metamorphoses, exists between the _Saurian
Reptiles_ or lizard tribe and insects, and conjecture that the wings
of the _Draco_ are really representatives of the mid-legs of Hexapods,
thus preparing to disappear altogether; but I shall content myself with
throwing out this hint, which you are welcome to pursue. The organs of
flight in general may be considered as to their _number_, _kinds_, and
_composition_.
i. _Number._ The most natural number is _four_, for this obtains in
the majority. In almost every Order, indeed, there occur instances of
insects that have solely a single pair or none[1781].
These, however, are only exceptions to the rule; but in the
_Diptera_, unless we consider the _alulæ_, the representatives of
the secondary wings[1782], as a distinct pair, there are never more
than _two_ wings, and one instance is known in which an insect
of this Order has _none_[1783]. Certain genera or individuals of
the Tetrapterous Orders are also furnished with _alulæ_: besides
_Dytiscus_, _Blatta_, _Phalæna hexaptera_, which have been before
noticed[1784], they may be detected in miniature in _Ammophila_ K.
and affinities; these all may be regarded in some slight degree as
insects with _six wings_.
ii. _Kinds._ Under this head we may consider the organs of flight
as to their _situation_ and as to their _substance_. As to their
_situation_, usually the first pair are attached to the _mesothorax_,
and the second to the _metathorax_; but in one instance, as has been
before observed[1785], in the _Strepsiptera_ K., the anterior pair
belong to the _manitrunk_, and the posterior to the _mesothorax_. As
to their substance, they take the several denominations of _elytra_,
_tegmina_, _hemelytra_, and _wings_, for the most part according to its
variations, as will be seen more at large hereafter. Under this head I
shall only further observe, that in many instances the organs of flight
appear to be mere _abortions_ or _rudiments_, which serve to exemplify
what has been more than once stated, that the CREATOR has seen it good
to approach to new organs gradually as well as to new forms. Thus
_elytra_ are mere rudiments that do not serve to protect the wings in
_Atractocerus_; _tegmina_ in some species of _Phasma_, _Acrydium_,
&c.; _hemelytra_ in the bed-bug[1786]; _wings_ in many female moths, in
_Cryptus hemipterus_ a _Hymenopterous_ insect, &c.
iii. _Composition._ The structure of wings has been before explained
to you[1787], and I shall again have occasion to allude to it; but
here I wish to call your attention to a circumstance that has not
hitherto, that I recollect, been adverted to; I mean that all kinds
of organs of flight, and it may be traced as we shall soon see even
in elytra, are divided longitudinally into three _areas_ or folds;
the first or external one I call the _Costal Area_[1788] from its
beginning with the costal nervure; the second is the _Intermediate
Area_[1789]; and the third is the _Anal Area_[1790].
Having made these observations with respect to the organs of flight
in general, I shall now proceed to consider more at large the
_elytra_, _tegmina_, _hemelytra_, and _wings_.
i. _Elytra._ These are the wing-covers of the _Coleoptera_ Order,
distinguished from _tegmina_ by the absence of _nervures_, from
_hemelytra_ by the want of the membrane at the apex, and from both
by their uniting in almost every instance at the suture. I shall
consider them as to their _substance_; _articulation_ with the
_trunk_; _expansion_; _parts_; _shape_; _appendages_; _sculpture_;
_clothing_; _colours_, and _uses_.
1. _Substance._ The firmness of the substance of elytra is usually
regulated by that of the crust of the insect to which they belong;
in hard insects they are hard, and in soft ones they also are soft.
The most impenetrable ones that occur to my recollection are those of
Illiger's genus _Doryphora_, and the softest and most flexile those
of _Telephorus_, _Meloe_ and affinities. With regard to individuals,
they are mostly as hard as the _prothorax_, and harder than the
_back_ of the abdomen. Elytra also, as far as my observation goes,
are never diaphanous.
2. _Articulation with the trunk._ This is by means of a process of the
base of the elytrum which I call the _axis_[1791] or pivot, attached
by elastic ligaments, and certain little bony pieces (_osselets_
Chabr.) in the socket under the side of the anterior angle of the
_dorsolum_[1792]. You may easily remove the elytra attached to the
_mesothorax_ from _Geotrupes stercorarius_, which will enable you to
see the mode of articulation with little trouble[1793].
3. _Expansion._ It is by means of the bony pieces just mentioned that
the organs in question are opened and shut[1794] under the action
of the antagonist muscles. In opening for flight the two elytra
recede from each other, and are elevated so as not to retain their
horizontal position, which would interfere probably with the play of
the wings, but form an angle with the body. When they return to a
state of rest, the sutures usually meet and coincide longitudinally;
but in some cases when closed, as in _Necydalis_, &c., they diverge
from each other at the apex; and in _Meloe_, like the _Orthoptera_,
to which that genus approaches, one laps over the other.
4. _Parts._ The parts to be considered in an elytrum are the
_areas_, the _axis_, the _suture_, the _margin_, the _epipleura_,
the _base_ and _apex_, the _angles_, and the _hypoderma_. At first
it should seem as if an _elytrum_ was not like other wings divided
into areas; but I think upon examination it will be found that,
though often nearly obsolete, these are represented in it; for the
_epipleura_[1795] with the recurved part of the external margin
seems to me analogous to the _Costal_ Area; the inflexed part
adjoining the scutellum and often going beyond it to the _Anal_,
and the rest of the organ to the Intermediate. All this you may see
in the dung-chafer, _Geotrupes stercorarius_. The _axis_[1796] or
pivot by which the elytrum articulates with the trunk is generally
placed about the middle of its base, but nearer the scutellar than
the humeral angle, and varies in length and shape in the different
tribes, but not so as to merit particular notice; it may be regarded
as composed of _three_ parallel pieces, one belonging to each area,
that of the _costal_ being the longest. In many these pieces are
marked by no line of distinction, but in _Macropus_, &c., they may
be readily traced[1797]. The _suture_[1798] is the _internal_ margin
of the elytrum from the point of the scutellum to the end. In many
beetles the right hand suture, looking from the anus to the head, has
a lower ledge or margin, and the other, one more elevated, which when
they are closed lies upon the former; in some _Dynastidæ_ there seems
a kind of ginglymous structure in this part, each suture being fitted
with a kind of ridge which is received by a channel of the other;
in these the suture is generally marked out by an adjacent channel:
but the most remarkable structure of this part distinguishes the
genuine species of the genus _Chlamys_, in which both the sutures,
except at their base, are armed with little teeth, alternating
with each other like the cogs of a mill-wheel. In apterous beetles
the elytra are often _connate_, or have both sutures as it were
soldered together. The margin[1799] or external edge of the elytra
is generally formed by a bead or ridge, which, except in the case
of the truncated ones, in which it is straight, curves more or less
from the base to the apex; this ridge is often recurved so as to form
a kind of channel between it and the disk of the elytrum, as may
be seen in the _Dynastidæ_; in some there are two parallel ridges,
as in _Copris_; in _Silpha_ the margin is dilated; in _Helæus_ and
_Cossyphus_ it is remarkably so and recurved, so that, in conjunction
with those of the _prothorax_ which are similarly circumstanced, they
give the animal some resemblance to a small model of a barge. Though
the margin of elytra is most commonly intire, yet in some beetles,
as _Gymnopleurus_ Illig., a sinus is taken out of it; in _Cetonia_
it often projects at the _base_, and in _Cryptocephalus_ in the
_middle_, into a lobe; in _Phoberus_ MacLeay it is denticulated, and
in many _Buprestes_ more or less serrulated; sometimes it terminates
before it reaches the apex of the elytrum in a tooth, as in many
_Carabi_ Latr. The _epipleura_[1800] or side-cover is that part of
the organ in question, below the margin, with which it usually forms
an angle, being more or less inflexed, that covers the sides of the
body. It varies in different tribes, being sometimes obsolete, as
in the weevils (_Curculio_ L.); in the Capricorn beetles it is very
narrow; in _Carabus_, &c., dilated at the base; in many Heteromerous
beetles, as _Blaps_, _Pimelia_, &c., it is very wide and conspicuous;
in _Cossyphus_ it stands out a little from the abdomen, so as to
form a kind of fence round it. Its _shape_ generally approaches
that of a scythe, being incurved and growing more slender towards
the apex[1801]; but it is sometimes straighter and shorter. In
_Geotrupes_ and many other Lamellicorns, the _base_ of the elytrum
is nearly vertical, forming a right angle with the rest of it; it is
usually transverse and straight; but in _Calandra Palmarum_ and many
_Cassidæ_ it slants to the scutellum; in _Chlamys_ it is sinuate,
and in _Elater_ it has a deep cavity above the axis which receives
the points of the _phragma_ mentioned before[1802]. The _apex_ of
elytra is usually acute, the angle being formed by the confluence
of a curving and straight line: but there are many exceptions; for
instance, in _Mylabris_ it is rounded; in _Hister_ obliquely, and
in _Necrophorus_ transversely, truncated; in many Capricorns it is
emarginate; in others, as _Macropus longimanus_, it is bidentate; in
some _Prioni_, _P. cinnamomeus_, &c., it terminates in a mucro at
the internal angle; and in _Cerambyx Batus_, _horridus_, &c., at the
_external_; and, to name no more, in some species of _Necydalis_ it
ends in a long acumen. The _scutellar angle_ in insects that have a
large scutellum, as _Macruspis_ MacLeay, is obliquely truncated to
admit it, but where it is small it is generally rectangular, with
the angle rounded; in _Buprestis vittata_ it is obtusangular; and in
_Dytiscus marginalis_, &c., it is emarginate. In _Cassida spinifex_,
_perforata_, &c., the _humeral_ angle is producted into an acute lobe
that stretches beyond the head, and in _C. bicornis_ and _Taurus_
it forms a horn at right angles with the elytrum. In general it
is either rectangular or rounded, with a prominence of the elytrum
within it. The _sutural_ and _anal angles_ exist only where the
elytra are truncated at the apex. In this case the _sutural_ is
generally rectangular, and the _anal_ rather obtusangular or rounded.
The _Hypoderma_ is the fine soft membrane before noticed[1803] that
lines the underside of the elytra, the use of which is probably to
prevent injury to the wings from friction with their usually hard
substance; this membrane is commonly of either a pallid or brownish
colour; but in some insects, as _Staphylinus hybridus_, _murinus_,
&c., _Buprestis grandis_, it is of a beautiful green or blue; and
it exhibits the _puncta_, _striæ_, and other modes of sculpture of
the elytra very distinctly, the pores of which usually perforate
this membrane[1804]. Just under the shoulders of these organs you
may observe an oblong and sometimes roundish spot, occasioned by the
_hypoderma_ in that part being particularly tense, and covering a
cavity or pocket which appears to be connected with the axis by the
hollow part, which I regard as representing the Costal Area; this
pocket is evidently the analogue of a part in the _wings_ noticed by
M. Chabrier[1805], and named by me the _phialum_: from its connexion
with the axis by a channel, this part in elytra should also seem
destined to receive a fluid to add to the weight of the margin and
its means of resistance.
5. _Shape._ The shape of elytra is various; taken together, in which
case, in describing insects, they are denominated _coleoptra_, their
most common form is more or less oblong, or forming more or less a
considerable portion of an ellipse; taken separately, it inclines to
that of an isosceles triangle, with the exterior side curvilinear:
truncated elytra are generally quadrangular, sometimes presenting a
trapezium, at others nearly a parallelogram, and at others a square.
With regard to their proportions they vary considerably, but the most
general law seems to be that the length shall exceed _twice_ the
width; in some, as _Buprestis grandis_, it is more than _thrice_; in
many _Staphylinidæ_ they are as wide as they are long and sometimes
wider; they are generally narrower at the apex than at the base, but
in some species of _Lycus_, as _L. fasciatus_, &c., the reverse takes
place; in _Telephorus_ they are nearly of the same width every where:
with regard to their surface they are sometimes very convex, as in
_Moluris_; at others very flat, as in _Eurychora_, _Akis_, &c.