looking, it should seem, only at the _Coleoptera_ and _Orthoptera_,
have regarded the _dorsolum_ and _scutellum_ as forming only one
piece[1659], and others have affirmed that the _Lepidoptera_ and
subsequent Orders have no _scutellum_[1660]. But as we proceed in
considering the _scutellum_ in all the Orders, we shall see that
both these opinions are founded on partial views of the subject, and
that all winged insects have a _scutellum_, more or less distinctly
marked out or separated from the _dorsolum_. In the _Coleoptera_
the _scutellum_ is usually the _visible_, mostly triangular,
piece that intervenes between the elytra at their base[1661], and
which terminates the _dorsolum_. Some Lamellicorn beetles, &c.
(_Scarabæidæ_ MacLeay) are stated not to have the part in question
(_exscutellati_): but this is not strictly correct, for in these
cases the _scutellum_ exists as the point of the _dorsolum_ covered
by the _prothorax_, though it does not intervene between the elytra:
in others of this tribe, as _Cetonia chinensis_, _bajula_, &c.,
it separates these organs at their base, though it is covered by
the posterior lobe of the _prothorax_: in _Meloe_ F., the elytra
of which are immoveable, there seems really to be no _scutellum_.
Generally speaking, as was lately observed, but not always, it is
distinguished from the _dorsolum_ by being more elevated: this is
particularly conspicuous in the genus _Elater_, in which it is a
flat plate elevated from the _dorsolum_ by a pedicle; in _Sagra_ the
latter part is horizontal, while the _scutellum_ is vertical: and
even in cases where the distinction is not so striking, these parts
are separated either by a line, or some difference in their sculpture
and substance. In this Order this part varies greatly, and often in
the same tribe or genus, both in size and shape; being sometimes very
large[1662], and sometimes very minute; sometimes very long, and
sometimes very short; sometimes nearly round, at others square; now
oval or ovate, heart-shaped, triangular, acuminate, intire, bifid,
&c. In the _Orthoptera_, though less conspicuous, it still is present
as a triangular elevation of the middle of the posterior part of
the dorsolum, with the vertex either pointing towards the head, as
in _Blatta_, or towards the tail, as in _Locusta_ Leach[1663]. In
the Heteropterous section of the _Hemiptera_ (which, in columns of
_Mandibulata_ and _Haustellata_, appear to bear the same reference
to the _Coleoptera_, that the _Hymenoptera_ do to the _Diptera_,
and the Homopterous _Hemiptera_ to the _Orthoptera_[1664]) the part
we are considering is mostly very large and conspicuous, quite
distinct from the _dorsolum_, and in some (_Tetyra_ F.) covering
the whole abdomen, as well as the _Hemelytra_ and the wings; it is
most commonly, as in the _Coleoptera_, obtriangular[1665], but in
the last-mentioned genus it often approaches to a pentagonal shape.
Though usually so striking a feature in this tribe, in the aquatic
bugs (_Gerris_ &c.) it is covered by the _prothorax_. In some
species of _Reduvius_ F. (_R. biguttatus_, _mutillarius_, _lugens_,
&c.) it is armed with one or more dorsal or terminal spines. In the
Homopterous section, where the _dorsolum_, as in _Tettigonia_ F., is
not covered by the _prothorax_, the _scutellum_, which is merely a
continuation of that part, bears some resemblance to a St. Andrew's
cross, and terminates in a fork[1666]; in _Fulgora_, in which it is
partly covered, it is merely the triangular point of the _dorsolum_:
in the _Cercopidæ_, &c., whose _dorsolum_ is wholly covered, the
triangular scutellum is distinct from it; in _Centrotus_, _Darnis_,
and _Membracis_, in which the _prothorax_ is producted, and covers
the abdomen more or less, the _scutellum_ is a short transverse
distinct piece. In the _Lepidoptera_, from the difficulty of abrading
sufficiently the scales and hairs without injury, it is difficult
to obtain a correct idea of the part in question; in the cabbage
butterfly (_Pieris Brassicæ_) it appears to be triangular: in the
humming-bird hawk-moth (_Macro-glossum Stellatarum_) it approaches
to a rhomboidal shape[1667]; and in the eggar-moth (_Lasiocampa
Quercus_) it is completely rhomboidal. In the _Libellulina_, in the
_Neuroptera_ Order, it seems to be represented by the posterior
point of the _dorsolum_, which terminates in something like a St.
Andrew's cross[1668]. In most of the other tribes of this Order the
_scutellum_ is a triangular piece, with the vertex to the head,
received between two pieces of the dorsolum; in _Psocus_ it is nearly
like that of _Tettigonia_ before described. In the _Hymenoptera_
the _scutellum_ is separated from the _dorsolum_, which it often
embraces posteriorly, as the collar does in front, by a suture; it
varies occasionally in shape in the different tribes, most commonly
it is crescent-shaped, but in many _Ichneumonidæ_ and others it is
triangular[1669]; in the hive bee, &c., it overhangs the succeeding
piece of the alitrunk; in _Melecta_, _Crocisa_, &c., it is armed
with a pair of sharp teeth[1670]; in others (_Oxybelus uniglumis_,
&c.) with one or more spines, and in some with a pair of long
horns[1671]. Before I describe this part in the _Diptera_, it will
be proper to assign my reasons for considering a different piece as
its representative, from what has usually been regarded as such, and
which at first sight seems the analogue of what I admit to be the
_scutellum_ in the _Hymenoptera_. The _dorsolum_, and its concomitant
the _scutellum_, belong to the first pair of the organs of flight,
which are planted usually under the sides of the former, and in the
case of _wings_, by their Anal Area, connected either mediately or
immediately with the latter. Now, if you trace the sides of the piece
that I have considered as the part in question in _Hymenoptera_,
you will find that they lead you not to the base of the _lower_
but to that of the upper _wings_[1672], and in the saw-flies
(_Tenthredo_ L.) you will see clearly that the Anal Area of these
wings is attached to a process of it, a proof that it belongs to
the _mesothorax_, or region of that pair. But in the _Diptera_, the
part that has been usually called the _scutellum_ is not at all
connected, either by situation or as a point of attachment, with
the wing itself, but with the lower valve of the _alula_, which is
with reason thought to be the representative of the secondary wing
of the tetrapterous Orders. You may see this even in the common
crane-fly (_Tipula_), in which there is a real _alula_, connected
by means of a lateral process, terminating in ligament, with this
supposed _scutellum_. If you examine further the same insect, you
will easily find what I regard as the true one in the bilobed piece
which receives the _dorsolum_, situated between the wings, and to
the sides of which they are attached. In _Asilus_, _Tabanus_, &c.,
this part is transverse, and only distinguished on each side by an
oblique impressed line; in the _Muscidæ_ it is square, and marked by
a straight transverse one.
4. _Frænum_[1673]. This appendage to the _scutellum_ and _dorsolum_
varies considerably in the different Orders, and in many cases, as
you will see, is a very important part, being the process by which
the former is mostly connected with the elytra or upper wings. In the
_Coleoptera_, the elytra of which are nearly stationary in flight, and
therefore less require any counteraction to prevent their dislocation,
this part is commonly merely a process or incrassation of the under
margin of the scutellum, which towards the base of the _dorsolum_ is
dilated to form the socket for the elytra. Its use as a countercheck
in this Order is best exemplified in the common water-beetle (_Dytiscus
marginalis_). This at the inner base of the elytra has a membranous
fringed _alula_ resembling those of _Diptera_; to the lower fold of
this the extremity of the _frænum_ is attached, which forms a right
angle with the scutellum, and the upper fold is attached to the base
of the elytrum[1674]. The object of this appendage is probably to
prevent the dislocation of these organs, which seems to indicate that
they are used more in flight than those of other beetles. The _Blattæ_
also, in the next Order, have a winglet attached to the anal area of
the _tegmina_. The _frænum_, as in the preceding Order, lies under the
margin of the _scutellum_ and _dorsolum_, but which here forms one
uninterrupted transverse line; it is nearly vertical, and is attached
to the _alula_. The structure is not very different in the other
_Orthoptera_[1675], but the _frænum_ is surmounted or strengthened
by one or two ridges; in _Mantis_ it runs from the scutellum in an
angular or zigzag direction--but in all it is attached immediately to
the _tegmen_. In the Heteropterous _Hemiptera_ it is represented by the
narrow bead adjacent to the scutellum on each side[1676], which dilates
into a flat plate as it approaches the _Hemelytrum_, with the Anal Area
of which it is connected. But the Homopterous section of the Order in
question furnishes examples of the most remarkable structure of this
countercheck, which proves that it is really, what its name imports, a
_bridle_. If you examine the great lanthorn-fly (_Fulgora laternaria_),
or any species of _Tettigonia_, &c., you will find adjacent to the
scutellum or parallel with it, on each side a flat plate; and
from the angle of that part in the first case, and from one of its
processes in the last, you will further perceive a ridge or nervure
which runs along this plate, in one forming an angle, and in the other
being nearly straight, to the base of the _tegmen_, where it becomes
a marginal nervure to a membrane that is attached to the posterior
part of the base of the Anal and Costal Areas; and that this marginal
nervure, like a _trachea_, consists of a spiral thread, or rather of
a number of cartilaginous rings connected by elastic membrane[1677],
and consequently is capable of considerable tension and relaxation,
as the _tegmen_ rises and falls in flight. In the _Lepidoptera_ it
appears to be a short piece overhung by the scutellum, which as it
approaches the base of the wing is dilated. In the _Libellulina_,
to go to the _Neuroptera_, it has the same kind of elastic nervure
connected with the Anal Area of the wing which I have just described
in the Homopterous _Hemiptera_; another nervure, in _Æshna_ at least,
appears to diverge upwards from the scutellar angle to the Intermediate
Area[1678]: a structure little different distinguishes the rest of the
_Neuroptera_, and even the _Trichoptera_. In the _Hymenoptera_ this
part varies somewhat; in the majority perhaps of the Order, as well
as in the _Diptera_, it appears to be merely the lateral termination
of the scutellum where it joins the wing; but in some tribes, as
in _Tenthredo_ L. (especially _Perga_ Leach), _Sirex_ L., and the
_Ichneumonidæ_, a ridge, and sometimes two, runs from the scutellum to
the wing; the upper one, where there are two, as in _Perga_, being the
stoutest, and connecting with the _Costal_ Area, and the lower one with
the _Anal_.
5. _Pnystega_[1679]. We learn from M. Chabrier, that in the common
dragon-fly, a space, consisting of three triangles, which immediately
succeeds the _frænum_, affords attachment to no muscles, but merely
covers aërial vesicles[1680]. This is the part I have called the
_pnystega_[1681]. An analogous piece may be discovered in _Phasma_
and _Mantis_ in a similar situation; but I cannot trace it in
_Locusta_ Leach, or in the other Orders.
* * * * *
Having considered the parts that constitute the _mesothorax_, we will
next say something upon those, as far as they require notice, that
compose the _medipectus_ or mid-breast. But first I must observe,
in general of the _medipectus_ and _postpectus_ taken together, or
the whole underside of the _alitrunk_, that though usually they
are in the same level with the _antepectus_ or under side of the
_manitrunk_, yet in several instances, as the _Scarabæidæ_ MacLeay,
the _Staphylinidæ_, &c. they are much more elevated than that part;
they are also usually longer, very remarkably so in _Atractocerus_,
but in _Elater sulcatus_ and many others they are shorter. These
parts are also commonly rather more elevated than the abdomen,--much
so in some, as _Molorchus_; but scarcely at all in others, as
_Buprestis_, the Heteropterous _Hemiptera_, &c. In some of the
latter (_Tetyra_ F.) the abdomen seems the most prominent. Another
observation relating to this part must not be omitted, namely, that
though in many cases the _medipectus_ and _postpectus_ are perfectly
distinct and may be separated, yet in others, as for instance the
Lamellicorn beetles, the _Hymenoptera_ and _Diptera_, &c., no suture
separates them; so that though the upper parts, the _mesothorax_ and
_metathorax_, are separable, the lower ones just named are not so.
6. _Peristethium_[1682]. The first piece of the _medipectus_ is
what I have called, after Knoch, the _peristethium_[1683]. This
immediately follows the _antepectus_; on each side it is limited by
the scapulars, and behind by the mid-legs and _mesosternum_. Its
antagonist above is usually the _dorsolum_. In the _Coleoptera_ Order
it varies occasionally, both in form and magnitude, but not so as
to merit particular notice, except that both are regulated by the
scapulars--if these are small, the _peristethium_ is ample; and,
_vice versa_, if they are large it is small. In all the following
Orders, except the _Hymenoptera_, it is equally inconspicuous, but
in them it is often more remarkable. I have a Brazilian species of
_Cimbex_ (_C. mammifera_ K. MS.) which appears undescribed, in which
this part swells into two breast-like protuberances, terminating
posteriorly in membrane, as if it had separate motion: in the
golden-wasps (_Chrysis_ L.) it is anteriorly concave to receive the
_coxæ_ of the mid-legs; and in _Stilbum_, of the same tribe, it is
armed with one or more conical obtuse teeth.
7. _Scapularia_[1684]. The scapulars are situated between the _coxæ_
of the mid-legs and the base or axis of the upper organs of flight,
and they seem to act as a fulcrum to each. In the _Coleoptera_ Order
they are most commonly quadrangular or subquadrangular, often divided
diagonally, and sometimes transversely, by an impressed line; the
posterior part, which is usually the most elevated and often has an
uneven angular surface, is that which intervenes between the coxæ and
elytra: where the former are short, as in the Capricorn beetles, the
scapulars are long; and where they are long, as in the Petalocerous
ones, the latter are short. The anterior part is that which forms
the lateral limit of the _peristethium_, upon which it often
encroaches: this part, in conjunction with the _dorsolum_ above, and
the last-named part below, forms the kind of _rotula_ that plays in
the posterior _acetabulum_ of the manitrunk, as the head does in the
anterior one. In the flower-chafers (_Cetonia_ F.) the scapulars are
very thick and elevated, and interpose between the posterior angles
of the _prothorax_ and the shoulders of the _elytra_, which is one
of the distinguishing characters of that tribe: in this case the
lower angle of the scapular connects with the _coxa_ of the mid-leg,
and the upper angle with the axis of the _elytra_; and the most
elevated and thickest part of the scapular is about midway between
the two. This robust structure seems to indicate that the scapular
has to counteract a powerful action both of the leg and elytrum.
In the _Orthoptera_ the scapulars are usually divided into two
parallel pieces, corresponding probably, though more distinct, with
the two parts lately noticed of those of the _Coleoptera_: the upper
side of the socket of the mid-leg is common to the base of _both_
these pieces, but the articulation of the _tegmen_ is chiefly with
the _anterior_ one. In the grasshoppers, locusts, &c. (_Gryllus_
L.) in which tribe this leg is nearly _opposite_ to that part,
the scapular inclines but little from a vertical position[1685];
but in the praying-insects (_Mantis_), spectres (_Phasma_), and
cockroaches (_Blatta_), in which the insertion of the mid-legs is
_behind_ that of the _tegmina_, it is nearly horizontal. In the
Heteropterous _Hemiptera_ the anterior part of the scapular is
covered by the _antepectus_, and separated by a ridge, more or
less pronounced, from the open part; the whole is of an irregular
shape, and nearly parallel with the _parapleura_. In the Homopterous
section it likewise consists of two pieces, and sometimes of more.
Thus in _Tettigonia_ F. it is bilobed, and between it and the coxa
two small pieces are inserted[1686]. In some others, _Iassus Lanio_
F., &c., it is not very unlike the scapular in _Coleoptera_, being
subquadrangular and divided diagonally. In the _Neuroptera_ this
part and the _parapleura_ are parallel, and placed obliquely[1687].
In the common dragon-fly (_Æshna viatica_) the former forms nearly a
parallelogram[1688], which is not divided by any ridge or channel,
but its lower half is separated into two unequal parts by a black
longitudinal line, opposed to which on the inside is a ridge. The
mid-leg in these is connected with the scapular by the intervention
of a triangular transverse anterior piece, which in fact seems
only marked by a black channel, to which also interiorly a ridge is
opposed[1689]. In the rest of the Order it is divided longitudinally
into _two_ parallel pieces. In _Panorpa_ the posterior piece is
longer than the anterior and props the coxa behind; in _Myrmeleon_
and _Perla_, &c., it appears to consist of _three_ pieces. I have
not been able to obtain a clear idea of them in the _Lepidoptera_,
except that they have more than one piece. _Hymenopterous_ and
_Dipterous_ insects for the most part have no scapular distinct from
the _peristethium_; but in _Cimbex_, _Perga_, and other saw-flies, it
seems represented by its posterior depressed and sometimes membranous
part: in _Vespa_, &c. a small subtriangular piece, just below the
base of the upper wing, is probably its analogue[1690].
8. _Mesosternum_[1691]. The central part of the _medipectus_, or
that which passes between the mid-legs when elevated, protended, or
otherwise remarkable, is called the _mesosternum_ or mid-breast-bone.
In the _Coleoptera_ Order it exhibits the most numerous variations, and
is usually the most strongly marked of any of the three portions of the
_sternum_, affording often important characters for the discrimination
of genera and subgenera. It may be said to be formed upon three
principal types--the first is, where it is a process of the posterior
part of the _peristethium_, and points towards the anus or the
head;--the second, where it is a process of the anterior part of the
_mesostethium_, and points only towards the head: in this case there
is no suture to separate the _medipectus_ from the _postpectus_;--the
last type is where it is a ridge formed by a process both of the
_peristethium_ and _mesostethium_ meeting between the legs; an example
of this you will see in the common dung-chafer (_Geotrupes_ Latr.).
Upon the two first of these cases I shall offer a few remarks;--the
last affording no variation need only be mentioned.
If you examine the terrestrial Predaceous beetles (_Cicindela_ and
_Carabus_ L.) you will find that the _peristethium_ is usually flat,
terminating towards the _postpectus_ in a kind of fork, the sinus
of which receives the anterior point of the _mesostethium_--this
is the _mesosternum_; but in the aquatic insects of this tribe, at
least in _Dytiscus marginalis_, &c. the structure at first sight
seems different, for _apparently_ the _prosternum_ is received by the
anterior fork of the _mesostethium_; but if you proceed to separate the
manitrunk from the alitrunk, you will find that the true _mesosternum_
of the usual form is quite covered by this point, which curves towards
the breast, is longitudinally concave to receive the point of the
_prosternum_, and permit its motion in the groove. In some Heteromerous
beetles, as the _Helopidæ_, &c. this part is anteriorly bilobed, so
as to form a cavity which receives the point of the _prosternum_ when
the head is bent down: in _Helops nitens_ (_Tenebrio_ Oliv.) this
sinus represents a crescent; in _Cistela Ceramboides_ it is shaped
like the Greek letter γ; in the Lady-bird (_Coccinella_ L.) it assumes
nearly the shape of a Saint Andrew's cross; in _Spheniscus_ K.[1692]
the _mesosternum_ is wide, concave and wrinkled, with an anterior and
posterior sinus; while in the analogous genus _Erotylus_[1693] it is
convex anteriorly, and posteriorly more or less rounded; in _Doryphora_
it is a long, robust, subconical horn, often standing at an angle of
about 45°, overhanging the _prosternum_.
In the genus last named, though its _mesosternum_ in its direction
and appearance resembles that of many Petalocerous beetles, yet it is
separated by an evident suture from the _mesostethium_; but in the
last-mentioned tribe its representative is a process of the latter
part: yet as the _peristethium_ and _mesostethium_ are separated by
no suture, though in some cases a transverse channel, and in others
merely a coloured line, marks the point where they may be considered
as soldered together, in these cases the _mesosternum_ may perhaps
be said to be common to both. In this great family, which includes
within its limits some of the most singular and wonderful in their
structure and armour, as well as some of the most brilliant and
beautiful of the beetle tribes,--the part in question, in a vast
number of cases, will enable the Entomologist satisfactorily to
trace its numerous groups, not only where it rises or stretches
out into a horn or ridge, but even often where it is merely a flat
space between the mid-legs. I shall notice some of its most striking
variations in this tribe. In _Phanæus festivus_, and in _Macraspis_
and _Chasmodia_ MacLeay, it is elongated horizontally, with the apex
curving upwards; in _Anoplognathus_ it is horizontal, straight, and
figures an isosceles triangle; in _Cetonia suturalis_, _vitticollis_,
&c. it is very long, passing between the arms and nearly reaching
the head; in _C. marmorea_, _Lanius_, &c. it is a lofty, robust,
conical prominence; and in many _Rutelidæ_, especially those with
striated elytra, it is pyramidal or four-sided; it varies also in its
_termination_, particularly in the _Cetoniadæ_; and even where there
is little or no elevation of it, as in the _Scarabæidæ_ MacLeay, it
is often terminated anteriorly by lines that vary in their angle or
curvature. The genus _Copris_, as restricted by Mr. W. S. MacLeay,
may from an inspection of this circumstance be divided into several
families. Thus in _C. Molossus_ and affinities its _termination_
represents the letter ⅄ reversed, or a triangle surmounted by a
mucro; in _C. orientalis_, &c., it ends in an acute-angled triangle;
in _C. lunaris_, &c. in an obtuse-angled one; and in _C. Iacchus_,
&c., in the segment of a circle.
The part we are considering is not so important in the other Orders.
In the _Orthoptera_, however, it is occasionally remarkable. In
_Acrida viridissima_ (_Locusta_ F.) attached to the anterior margin
of the _peristethium_ are two long triangular pieces which appear to
represent this part; in the kindred subgenus, _Conocephalus_[1694],
it is a single piece bifid at the apex; in _Gryllotalpa_ it is a very
elevated hairy ridge; and in _Locusta_ Leach, it is a flat anterior
process of the _mesostethium_. In the Heteropterous _Hemiptera_ this
part is often merely a portion of the channel in which the _promuscis_
reposes; but sometimes, as in _Edessa_ F., it is an elevated piece
varying in its termination. In the remaining Orders, as far as I have
had an opportunity to examine them, it can scarcely be said to exist
separately from the _medipectus_, except that in _Tipula_ Latr. a
bipartite subtriangular membranous piece seems to be its analogue.
* * * * *
We are now to consider the last segment of the alitrunk, which, as a
whole, may be denominated the _potruncus_; it bears the second pair
of the organs of flight, and the last pair of legs. The upper side of
this is the _metathorax_, and its lower side the _postpectus_.