is the _postdorsolum_, which presents itself under very different forms
and circumstances in the different Orders. In the _Coleoptera_ it is
intirely covered by the _dorsolum_ and _scutellum_; it is generally
more or less of a membranous substance, or partly membranous and partly
corneous, which enables it to yield more to the action of the wings in
flight; it is usually an ample transverse piece with tumid sides[1696];
but in the _Scarabæidæ_ MacLeay, it is short though very wide; and
in _Cychrus_, and probably other _apterous_ beetles, it is extremely
minute and almost obsolete. In the _Orthoptera_ Order, I observe once
for all, the part in question, as well as the _postscutellum_ and
_postfrænum_ are mere counterparts of the _dorsolum_, _scutellum_,
and _frænum_, except that in some cases they are larger[1697]. In the
Heteropterous _Hemiptera_ at first sight it would appear that all
the parts of the _metathorax_ were altogether wanting or absorbed
in the ample _scutellum_; but if you remove this with care, you
will find _under_ it their representatives, its lower surface being
hollowed out to receive them. The _postdorsolum_ appears in these as
a transverse obtusangular band; in the _Nepidæ_, _Notonectidæ_, &c.
the three parts of the _metathorax_ seem united into a single plate,
emerging laterally from under the _scutellum_ below the _frænum_;
in which, however, some traces of a distinction between them may be
discovered. In the Homopterous section the _Fulgoridæ_ exhibit these
pieces very distinctly, covered only at the base by the _mesothorax_:
but in _Tettigonia_ they are not so easily detected; they exist
however as a narrow strip or band, almost concealed by that part.
As to the _Lepidoptera_ Order, in _Pieris Brassicæ_ at least, the
_postdorsolum_ is represented by a pair of nearly equilateral triangles
whose vertexes meet in the centre of the _metathorax_, and between
which and the _scutellum_ is a deep cavity; but in _Macroglossum
Stellatarum_ and _Lasiocampa Quercus_, there appears to be also a
central transverse piece between them. In the _Neuroptera_ there is no
material or striking difference between the parts of the _mesothorax_
and _metathorax_[1698]. In the _Hymenoptera_ more variety occurs in
this part. In the saw-flies, &c. (_Tenthredo_ L.) the _postdorsolum_ is
a transverse piece covered by the _scutellum_; in the _Ichneumonidæ_
it is smaller, but not covered; in the _Vespidæ_ it is apparent,
transverse, and with the _postscutellum_ obtusangular[1699]; in
_Apis_ it is overhung by the _scutellum_. The _Diptera_ exhibit some
variations in this part. In _Tipula_ it consists of three pieces placed
transversely, the central one quadrangular, and the lateral ones
roundish; in the _Asilidæ_ and most others of this Order, with the
_postscutellum_, it forms a segment of a circle[1700], sometimes armed
with a pair of spines, as in _Stratyomis_ F., and is what has been
usually regarded as the real _scutellum_, though, as I have endeavoured
to show, not correctly[1701].
10. _Postscutellum_[1702]. The _postscutellum_ bears the same relation
to the _postdorsolum_ that the _scutellum_ does to the _dorsolum_,
but it is seldom, if ever, a distinct piece. In the _Coleoptera_ it
is represented by the longitudinal narrow channel that terminates
the _postdorsolum_ towards the anus[1703]: this usually figures an
isosceles triangle with the vertex truncated or open; but in _Copris_
the triangle is equilateral. In the other Orders it is little more than
the central posterior point of the _postdorsolum_[1704].
11. _Postfrænum_[1705]. The part now mentioned is much more important
than the preceding one, and must not be passed over so cursorily. In
the _Coleoptera_ it usually presents itself under the form of two
large and usually rather square panels, the disk of which is convex,
but the rest of their surface unequal, which are situated one on each
side of the postscutellum[1706]; under the anterior outer angle of
these is the socket or principal attachment of the secondary wings,
and their basal margin is attached to their outer side; posteriorly
behind the vertex of the _postscutellum_ the _postfrænum_ is crowned
with a ridge or bead, below which it descends vertically or obliquely
to the abdomen; this ridge often turns upwards, and proceeds towards
the middle of the basal margin of the wing. In the Petalocerous
beetles the part in question is usually more or less hairy; but in
many others, as the rose-scented Capricorn (_Callichroma moschatum_),
&c. it is naked. At its side you will commonly observe several plates
and tendons (_osselets_ Chabr.) connected _inter se_ and with the base
of the wing by elastic ligaments, which are calculated to facilitate
the play of those organs. In the _Orthoptera_, _Neuroptera_, and
Homopterous _Hemiptera_, the _postfrænum_ does not differ materially
from the _frænum_[1707]. In the Heteropterous section of the last Order
it is usually a transverse ridge terminating the _postdorsolum_, with
a bifurcation where it unites with the wing; but in _Tetyra_ F. (at
least so it is in _Tetyra signata_,) it is a nearly vertical piece,
marked in the centre with an infinity of very minute folds, which
probably by their alternate tension and relaxation let out and pull
in the wings. Amongst the _Lepidoptera_ it is not remarkable. In the
_Hymenoptera_ Order it is mostly represented, I think, by a double
ridge or fork, sometimes however obsolete, but very conspicuous in the
saw-flies, which laterally terminates the _postdorsolum_; the upper
branch, usually the thickest, going to the anterior part of the base
of the underwing, and the lower one to the posterior. You may observe
something similar in the crane-flies (_Tipula_ Latr.) and _Asilidæ_. A
tendon proceeding from the point of the _postscutellum_ forms a fork
near its end, the upper branch of which connects with the anterior and
the lower with the posterior valve of the winglet; the structure is a
little, but not essentially, different in other _Diptera_.
12. _Pleura_[1708]. By this name I would distinguish the part which
laterally connects the _metathorax_ and _postpectus_. It includes in
it the socket of the secondary wings. In the _Coleoptera_ this is a
two-sided piece lying between the _postfrænum_ and the _parapleura_,
with the upper side horizontal and the lower vertical[1709]--a tendon
usually proceeds from its anterior extremity to the base of the wing.
In the _Orthoptera_, _Neuroptera_, and other Orders, it is merely
the longitudinal line of attachment of that part; but in the genus
_Belostoma_ Latreille, related to the water-scorpion, it presents a
peculiar structure, being a deep channel or demitube, filled at its
posterior extremity by a spiracle and its appendages[1710].
13. _Metapnystega_[1711]. This part, although in the table I have
placed it as an appendage of the _pleuræ_, is not always confined
to them, as you will soon see. It either covers aërial vesicles, or
is the seat of a spiracle. In the Order _Coleoptera_ it is of the
former description. If you examine the _metathorax_ of the common
dung-chafer (_Geotrupes stercorarius_), in the horizontal part of
the _pleura_ you will see a sublanceolate or subelliptical rather
membranous silky tense plate, with its point towards the head,--this
is the part we are considering; something similar you will find in
most beetles; but in some, as _Callichroma moschatum_, it is less
conspicuous. This part, as far as I have observed, is not so situated
in any other Order, except in some Heteropterous _Hemiptera_: in
_Belostoma_ the channel lately mentioned is filled up at its posterior
end by a red organ with an anterior vertical fissure, terminating
behind in a conical bag: in _Notonecta_ the _pleura_ has something of
a plate like that of _Coleoptera_, but of a horny substance. In the
_Orthoptera_ and _Neuroptera_ this part changes its situation, if it
be indeed synonymous; and as the _pnystega_ follows the _frænum_, so
the _metapnystega_ succeeds the _postfrænum_. In the _Libellulina_
M. Chabrier found that this as well as the other covered aërial
vesicles[1712], and it probably does the same in the other cases in
which it occurs. In _Mantis_ and _Phasma_ in the _Orthoptera_ it is
very minute; but in _Locusta_ Leach, it is more conspicuous under the
form of a tense membrane, the surface of which is depressed below that
of the abdomen: in _Acrida viridissima_ K. it fills the sinus of the
_postfrænum_, and is vertical, as it is in _Æshna_. It is worthy of
remark that this piece bears some analogy to that below the ridge of
the part just named in _Coleoptera_, which descends either vertically
or obliquely to the abdomen[1713]. A similar space, though often nearly
obsolete, may be seen in the _Hemiptera_ and _Lepidoptera_. But the
Orders in which this part is most conspicuous are the _Hymenoptera_
and _Diptera_, and in these its aërial vessels are connected with
a spiracle. In _Tenthredo_ L. and _Sirex_ L., what Linné named
_grana_, from their situation, should be regarded as belonging to
the _pnystega_, and whether there is any part representing the
_metapnystega_ I am not quite satisfied; perhaps the membrane at the
base of the abdomen in _Tenthredo_, and the bipartite piece, apparently
its first segment, in _Sirex_[1714], may be its analogues: but in the
great majority of the Order, the convex or flat piece that intervenes
between the _postdorsolum_ and its adjuncts and the abdomen, and which
bears a spiracle on each side, is the _metapnystega_[1715]. This part
is often remarkable, not only for its size, but for the elevated ridges
that traverse it, as in _Ichneumon_, _Chlorion_, &c. In the last genus
it is of a pyramidal shape, with the anterior part horizontal and the
posterior vertical; it is altogether vertical in _Vespa_, _Apis_,
&c. Amongst the _Diptera_, in _Tipula_ it is nearly horizontal, and
shaped like a cushion; but in general in this Order it is vertical, and
concealed under the _postdorsolum_[1716].
* * * * *
We are now to consider the parts that constitute the _postpectus_ or
under-side of the _metathorax_, and which bears the posterior pair of
legs.
14. _Mesostethium_[1717]. This part in _Coleoptera_ is terminated
anteriorly by the _peristethium_, _scapulars_, and _mesosternum_,
laterally by the _parapleuræ_[1718], and behind by the coxæ of the
posterior legs[1719], which generally are inserted transversely between
it and the abdomen. It is commonly very wide; but in _Dytiscus_ L.,
_Carabus_ L., &c., in which the _coxæ_ and _parapleuræ_ are dilated, it
is proportionally reduced: its length is regulated by the distance of
the intermediate and posterior legs; where these are far asunder, as in
the rose-scented Capricorn (_Callichroma moschatum_), &c. it is long:
but where they are near each other, as in the _Scarabæidæ_ MacLeay, it
is short; its width, however, generally exceeds its length. In shape
it is generally subquadrangular[1720], though sometimes rhomboidal,
and other forms of it occur. Between the hind-legs it generally
terminates in a notch or bifurcation distinct from the _metasternum_,
as in _Hydrophilus_, &c.; in _Hister_ there is no notch, and in
many _Scarabæidæ_ it projects between the hind-legs in a truncated
or rounded mucro; in the Vesicatory beetles, _Meloe_ L., it is more
elevated than the _medipectus_, towards which it descends almost
vertically; in _Dytiscus_ L., _Carabus_ L., &c., this part is usually
divided into two by a _transverse_ sinuous channel, and in _Elater_
by a _longitudinal_ straight one. In many _Orthopterous_ genera,
_Gryllotalpa_, _Acrida_ K., _Locusta_ Leach, &c., the _mesostethium_
consists of _two_ pieces[1721]. It is remarkable that in many of these
genera, in this part, as likewise in the _medipectus_ and _antepectus_,
are one or more perforations which appear to enter the chest, the use
of which I shall explain hereafter. In the _Libellulina_, as I shall
soon have occasion to shew, there is a peculiar arrangement of the
legs and wings, in consequence of which this part is placed behind the
posterior ones. In the remaining Orders, the _mesostethium_, though it
exists, exhibits no peculiarities worthy of particular notice, except
in some _Aptera_ and _Arachnida_: thus, in _Nirmus Anseris_ it is
terminated posteriorly by a pair of transverse membranous appendages
which cover the base of the posterior _coxæ_; in _Scorpio_ it consists
of two pieces, the _pectines_[1722] being attached to the sides of the
posterior one.
15. _Parapleura_[1723]. The _parapleura_, speaking generally, is
that piece of the _postpectus_ which, intervening between the
_pleura_, _mesostethium_, and _scapulars_, is attached by its
posterior extremity to the _coxæ_ of the hind-legs; by means of
the _pleura_, from which it does not appear to be separated by any
suture, it connects the secondary or under-wings with the hind-legs,
as the scapular does the primary ones with the mid-legs; so that the
direction of the _parapleura_ depends upon the relative situation
of the legs and wings. In Coleopterous insects its direction is
_horizontal_, it being generally a narrow subquadrangular piece that
runs straight from the posterior coxæ to the scapular[1724], and
usually divided into two unequal portions by an elevated or impressed
line. In the palm-weevil (_Calandra Palmarum_) this part is wider
than usual; in _Dytiscus marginalis_,--in which genus, as likewise
in _Carabus_ L., the _coxæ_ are incapable of separate motion,--it is
nearly a right-angled triangle, and is divided longitudinally into
two _unequal_ portions. In the _Orthoptera_ Order this part usually
consists of two _equal_ portions, and its direction is sometimes
nearly horizontal, as in _Mantis_ and _Phasma_; sometimes forming
an angle with the horizon, as in _Blatta_; and sometimes nearly
vertical, as in _Locusta_ Leach. In the two first cases the wings
are _before_ the legs, and in the last their position is _over_
them. In the Heteropterous _Hemiptera_ it is parallel with the
scapular, is divided into two unequal portions, and its direction is
more or less inclined to the horizon[1725]. As to the _Homopterous_
section--in _Fulgora_ it is of a very irregular shape with an
angular surface, and its direction from the leg to the wing is first
nearly vertical and then horizontal: in _Tettigonia_ it is almost
vertical, and consists of two nearly equal portions. To come to the
_Neuroptera_--in the _Libellulina_ it consists of two pieces, like
those of the scapulars, but smaller[1726], and its inclination is
towards the head: in _Panorpa_ also it resembles the scapulars both
in form and other circumstances[1727]. In the remaining Orders it
exhibits no very remarkable features.
16. _Metasternum_[1727]. The central part of the _mesostethium_
when elevated or porrected, or otherwise remarkable, is called the
_metasternum_. In the _Coleoptera_, in those cases, as we have seen
above[1728], in which the _medipectus_ and _postpectus_ form one piece,
its anterior point becomes the _mesosternum_; but in others, as the
Predaceous and Capricorn-beetles, &c., it is received in a sinus or
fork of that part, or meets it. It is usually neither so remarkable nor
important as the _mesosternum_. In _Bolbocerus_ K. it is a rhomboidal
elevation: in _Gyrinus_ a ridge; as also in many _Hydrophili_, in
which it passes between the hind-legs to the abdomen, and terminates
in a sharp point[1729]; and in _Dytiscus_ its two diverging lobes
cover the base of the posterior trochanters[1730]. In the _Orthoptera_
Order this part is not remarkable; but in _Acrida viridissima_ K. it
consists of three triangular pieces, the lateral ones being erect,
and the intermediate one horizontal: in _Locusta_ Leach it resembles
the _mesosternum_[1731]. In the Heteropterous _Hemiptera_ the whole
_mesostethium_ is elevated, and terminates at both ends in a fork,
the anterior one receiving the point of the _promuscis_, and the
posterior one that of the _epigastrium_: in the Homopterous section,
the _Tettigoniæ_ F. have usually a distinct metasternal point between
their hind-legs. In the remaining Orders there is no _metasternum_,
or no remarkable one, except in one singular Hymenopterous genus,
_Evania_, the parasite of the _Blattæ_[1732], in which there is a
forked posterior process of the _mesostethium_ with recurved points.
17. _Opercula_[1733]. By this term I distinguish those plates, before
largely described[1734], which cover the drums of male _Tettigoniæ_ F.;
and likewise those called also by the same name by M. Chabrier[1735],
which cover, in many cases, the _vocal apparatus_ of the trunk of
insects: those of _Melolontha vulgaris_ he describes as situated below
the wings, and between the two segments of the _alitrunk_[1736]; and
if you take this insect and remove the elytra, the _mesothorax_ and
scapulars, under the latter and below the wing you will find an oval
convex plate, which is probably the part he is speaking of;--but it is
better exemplified, I think, in the common _Dytiscus marginalis_, in
which it is very distinct as a convex subtriangular plate connected
with the _metathorax_ by membranous ligament, covering a kind of pouch,
and appearing to open and shut at the vertex[1737].
* * * * *
I must here observe, with regard to the _Aptera_ and _Arachnida_,
that the trunk in them is much more simple than in those insects that
are furnished with _wings_. In the hexapods, in the former Orders,
though there are usually three pedigerous segments, there is no
distinction of _dorsolum_, _scutellum_, &c. In the _Scolopendridæ_
and _Scutigera_ amongst the Myriapods, according to the acute
observations of M. Savigny[1738],--on which, however, some doubt at
present rests,--there is a remarkable formation, the whole thorax
being represented by the single plate that follows the head, to
the under-side of which are attached the first and second pair of
_palpi_ or _pedipalpi_, and the first pair of legs, representing the
three pairs of legs of hexapods. In the _Iulidæ_ the three segments
that follow the head, each bear a _single_ pair of legs, while
all the rest bear a _double_ one: from whence it should seem to
follow, that these segments and their legs represent the trunk and
legs of Hexapods. In the Octopod _Aptera_ and the _Arachnida_ the
trunk consists of a single piece, not separated from the head, and
sometimes not distinct from the abdomen.
V. _Internal processes_[1739]. Perhaps you will think that this head
would be better considered when I treat of the _Internal Anatomy of
Insects_; but as the parts included under it are really processes of
the _external integument_ of the trunk, it seemed to me best to treat
of them under that head. They are of two descriptions; processes
of the _thorax_ or upper part of the trunk, and processes of the
_breast_ or its under part.
i. _Processes of the thorax_[1740]. These are the _phragma_,
_prophragma_, _mesophragma_, and _metaphragma_. The first belongs to
the _prothorax_, the second to the _mesothorax_, and the two last to
the _metathorax_; each forming a kind of chamber of the under-side of
each segment of the thorax.
1. _Phragma_. The _phragm_, or _septum_ of the _prothorax_, is most
conspicuous in the mole-cricket (_Gryllotalpa_), in which it is a hairy
ligament attached to the inside of the upper and lateral margins of
the base of that part: inclining inwards, it forms the cavity which
receives the _mesothorax_. It is not, however, without a representative
in many _Coleoptera_, though in these it is less striking, from its
being smaller and taking a horizontal direction. In _Elater_, by means
of some prominent points received by corresponding cavities of the
vertical part of the base of the elytrum, it forms a kind of ginglymous
articulation, which probably keeps them from dislocation in repose,
and, by the sudden disengagement of these points from the cavities,
assists the animal in jumping[1741].
2. _Prophragma_[1742]. This is a piece usually almost vertical,
but in _Elater_ horizontal; of a substance between membrane and
cartilage, descending anteriorly from the _dorsolum_, and forming
the first partition of the chest of the _mesothorax_; it is
generally much shorter than the mesophragm. Though very visible in
_Coleoptera_ and the Heteropterous _Hemiptera_, in the other Orders
it is less easily detected, and is sometimes obsolete. It may be
observed here, that in the _Hymenoptera_, at least in the wasp, the
hive-bee, the humble-bee, and the _Diptera_ mostly, the interior of
the upper-side of the alitrunk, instead of _two_, seems at first
to be divided into _four_ chambers, formed by _septula_: but as
these ridges merely mark out the internal limits of the _dorsolum_,
_scutellum_, _postdorsolum_, and _metapnystega_, the last but one
of these being usually less distinct, they seem not analogous to the
three partitions of the alitrunk in other Orders; so that in these
the mesophragm at least seems to have no representative, and the
prophragm and metaphragm include between them only one ample chamber.
In the _Diptera_, wherever there is an external _depression_ or
_suture_ there is a corresponding internal _ridge_ or seam, so that
the parts seem more distinctly marked out on the _inside_ than on the
_outside_ of the crust.
3. _Mesophragma_[1743]. This piece also, which forms the middle
partition of the upper part of the cavity of the alitrunk, dividing it
into two chambers, is most conspicuous in _Coleoptera_. It is usually
in them a vertical piece, resembling the prophragm in substance, but
twice its height, of a quadrangular shape with a notch in the middle;
it fills the sinus of the _postdorsolum_, the sides of which sometimes
descend below it[1744]. In this Order the chamber that it forms with
the prophragm is very small[1745], the motions of the elytra requiring
no powerful apparatus of muscles; but that which it forms with the
metaphragm, which is appropriated to the muscles moving the wings, is
very large[1746]. In the _Orthoptera_ the anterior chamber is larger
than in the preceding Order, which proves that _tegmina_ are more
moved in flight than _elytra_. In the Heteropterous _Hemiptera_ a
remarkable variation takes place--the anterior being larger than the
posterior chamber; which last, in fact, consists of two, one for each
wing: in these the mesophragm towards the abdomen forms an angle, which
in _Pentatoma_, &c., is acute; in _Belostoma_ a right angle, and in
_Notonecta_ an obtuse one. In the two first the angle of the mesophragm
sends two short diverging ridges to the metaphragm; and in the last
only a single one: in this also the posterior chambers together are
nearly as large as the anterior. From this structure it should seem
that in flight the _Hemelytra_ are more important than the wings. In
the Homopterous section the anterior chamber is the smallest, at least
in _Fulgora candelaria_; and the mesophragm is lofty and bipartite. In
the _Lepidoptera_ the anterior chamber is the largest, and the part in
question conspicuous[1747]. In the _Libellulina_ and _Hymenoptera_ it
is merely represented by a low ridge, and in the _Diptera_ it seems
evanescent.
4. _Metaphragma_[1748]. This, in many cases, is the largest and
most remarkable of the three partitions of the upper portion of
the cavity of the alitrunk, which separates it from that of the
abdomen; it is attached to the posterior margin of the _metathorax_,
and is nearly vertical: in substance it may be stated as rather
firmer than the two preceding partitions. In the _Coleoptera_ it
is commonly of the width of the posterior orifice of the alitrunk;
and its centre is cleft so as to form a deep sinus[1749] for the
transmission of the intestines,--a circumstance which also, though
less conspicuously, distinguishes the mesophragm[1750]: from this
sinus it slopes gradually towards the sides, and is sometimes armed
with an intermediate process on each side[1751]. This structure you
will find exemplified in the common cock-chafer and many others of
the Order. I have not, however, discovered traces of it either in
the _Silphidæ_, _Staphylinidæ_, or the vesicatory beetles (_Meloe_
L.); or even in such species of _Carabus_ L. and _Cicindela_ L.
that I have examined; while in _Dytiscus_ it is very visible. In
the _Orthoptera_ it is nearly obsolete; but in _Locusta_ Leach,
under the _metapnystega_, one on each side, is a pair of seemingly
_pneumatic_ pouches which may be mistaken for it. It is almost
equally inconspicuous in both sections of the _Hemiptera_. As to the
_Lepidoptera_,--in _Pieris Brassicæ_, it resembles in some degree,
though in miniature, the metaphragm of the _Coleoptera_; but in
_Sphinx Stellatarum_ and _Lasiocampa Quercus_ it has a sinus on each
side, but no middle one. In _Panorpa_ it nearly closes the posterior
orifice of the trunk, but in the _Libellulina_ it is a mere ridge. In
some _Hymenoptera_, as _Cimbex sericea_, the drone-bee at least, &c.,
it is a large convex bifid piece. In the wasps, under the spiracle of
the _metapnystega_ on each side, as in the _Locusta_, is what I also
take to be a pneumatic pouch, which might easily be mistaken for a
metaphragm. In the _Diptera_ Order this part is very conspicuous. If
you remove the abdomen of any common _Tipula_, you will find that the
posterior orifice of the trunk is closed above by a pair of oblong,
vertical, convex, diverging plates;--do the same by any fly (_Musca_
L.), and you will detect in the same situation a very large convex or
gibbous one notched below, which occupies almost the whole orifice:
this is the metaphragm.
5. _Septula_[1752]. These are the _smaller_ ridges of the interior of
the alitrunk, which afford a point of attachment to many muscles,
and run in various directions both on the interior of the crust
and of the metaphragm. These little seams are not to be found so
generally in the other Orders; but very frequently, as has been
before observed, where there is an exterior impression of the crust,
or a suture, one of these forms its internal base.
ii. _Processes of the pectus_[1753]. We are next to consider the
internal processes of the _breast_ of insects: these consist for
the most part of the _endosternum_, or _internal_ sternum, and its
branches. As the principal feature of this are the processes which
rising from it serve as points of attachment to the muscles that
move the legs, &c., I shall confine myself to them--they are, the
_antefurca_, the _medifurca_, and the _postfurca_.