much to say: in Coleopterous insects this part is commonly naked;
but in some genera, as _Byrrhus_, _Anthrenus_, _Dermestes_, and many
weevils (_Curculio_ L.) it is partially or totally covered with hairs
or scales. In the other thoracic Orders it is usually naked, but in
some _Neuroptera_, the _Myrmeleonina_, &c., it is hairy; and in the
_Libellulina_ it is fringed posteriorly with hairs.
7. As to its relative _proportions_, the _prothorax_ is sometimes
rather wider than the rest of the trunk and the head, as in _Onitis_,
_Pasimachus_, &c.; it is considerably narrower in _Collyuris_
and _Odacantha_; and of the same width in those _Scaritidæ_ with
_striated_ elytra[1623]. Again, it is sometimes of the same width
with the elytra, but wider than the head, as in _Hydrophilus_,
_Dytiscus_, &c.; in some instances it is of the same width with the
head, and narrower than the elytra, for instance in _Anthia_ and
_Brachinus_. In most _Coleoptera_ it is longer than the head and
shorter than the elytra; but in _Manticora_, the vesicatory beetles,
&c., it is shorter than either. In _Gnoma longicollis_[1624],
it is nearly as long as the elytra; in many _Staphylinidæ_,
_Atractocerus_, &c., longer; in _Phanæus carnifex_, _bellicosus_,
&c., it is longer than the elytra and the rest of the body. With
regard to itself, it is sometimes very wide in proportion to its
length--_Dytiscus_, _Helæus_; at others very long in proportion to
its width--_Colliuris_, _Brentus_, _Mantis_, &c. In _Flata_, and
many other Homopterous _Hemiptera_, it is extremely short; extremely
long in _Gnoma_; in _Sagra_ and _Donacia_ its width about equals its
length; in _Elater_, _Dytiscus_, and many Heteropterous _Hemiptera_,
it is narrowest before; in _Languria_ it is every where of equal
width; in _Anthia_, _Carabus_, &c., it is widest before; and, lastly,
in the _Scarabæidæ_ MacLeay it is usually widest in the middle.
ii. _Antepectus_[1625].--The _antepectus_, as was before
observed[1626], in some tribes forms one piece, without any kind of
separation, with the _prothorax_; but very often this is not the
case. In _Carabus_ L. it occupies almost the whole under-side of
the manitrunk; but in _Elater_, in which the _ora_ is very wide,
the _antepectus_ is merely the _middle_ portion of that part. In
_Carabus_ F. &c. between the _ora_ and the base of the arms is
a convex triangular piece, distinguished from the rest of the
_antepectus_ by a spurious suture; and in _Pentatoma_ and other
Heteropterous _Hemiptera_ a similar piece is observable, which
terminates in a convex bilobed subtriangular sheath, receiving the
base of the clavicle[1627]. This piece seems a prop to that part,
and analogous to the _scapula_ of the _medipectus_ and _parapleura_
of the postpectus. I shall say no more upon the _antepectus_, as it
is seldom remarkable. In the mole-cricket, however, one peculiarity
distinguishes it: it is in this of an elastic leathery substance,
while the _prosternum_ is hard, resembling a bone. In other instances
these parts are both of the same substance.
1. The _sternum_ or breast-bone of insects consists mostly of
_three_ distinct pieces; in this resembling the human _sternum_,
which is described by anatomists as composed originally of _three_
bones[1628]. Each of these pieces is appropriated to a pair of legs,
and each of them at times has been called the _sternum_: thus in
_Elater_ the _prosternum_, in the _Cetoniadæ_ the _mesosternum_,
and in _Hydrophilus_ the _metasternum_, have been distinguished by
this name. Our business is now with the first of these pieces, the
_sternum_ of the _antepectus_ or _prosternum_[1629]: this is the
middle longitudinal ridge of the _fore-breast_, which passes between
the arms, when elevated, extended, or otherwise remarkable. It is
most important in the _Coleoptera_ Order, to which my remarks upon it
will be chiefly confined. In these it is sometimes an _elevation_,
and sometimes a _horizontal_ process of the fore-breast. If you
examine the great _Hydrophilus_ (_H. piceus_), at first you will
think that there is only a single _sternum_ common to all the legs;
but if you look more closely, you will perceive between the head
and the arms a triangular vertical process, with a longitudinal
cavity on its posterior face, which receives the point of the
_mesosternum_ that passes between the arms[1630]: this vertical piece
is the real _prosternum_, and not the other, which really belongs
to the alitrunk. In this case the elevation of the _prosternum_
is _before_ the arms; in others it is _between_ them, as you may
see in a Chinese chafer (_Mimela_ K.), which imitates the external
appearance of a quite different tribe[1631]; in others again it is
_behind_ them, as in most of the Lamellicorn beetles. In the common
dung-chafer (_Geotrupes stercorarius_), it is a hairy process, which,
when the head is bent downwards, is received by a deep cavity of
the _mesosternum_. The _Dynastidæ_ MacLeay may always be known by
a columnar _prosternum_ rising vertically between the arms and the
_medipectus_. Lastly, in other tribes there is a prosternal elevation
both _before_ and _behind_ the arms, as in _Cerambyx thoracicus_,
_dimidiatus_, and affinities. Of the _second_ description, those that
have a less elevated horizontal _prosternum_, the point in most is
to the _anus_, but in some to the _head_: thus in _Carabus_ L. it
is generally a subspathulate flat piece, the point of which slides
over the _mesosternum_, or covers it; but in _Harpalus megacephalus_
Latr.[1632], one of this tribe, though similarly shaped, its point
is to the _head_. These horizontal _prosterna_ vary in their
termination. In that of _Carabus_ L. the apex is obtuse; in that
of _Elater_, above described[1633], and _Dytiscus_ it is _acute_;
in _Prionus lineatus_, _Spencii_ K., &c., it is _bilobed_; and in
_Buprestis variabilis_, _attenuata_, &c., obsoletely _trilobed_.
With regard to the other Orders no striking features of this part
are observable, except in some _Orthoptera_. In _Acrida viridissima_
K. (_Locusta_ F.) it is represented by two long filiform vertical
processes; and in _Locusta_ Leach by a single conical horn[1634],
mistaken by Lichtenstein for a process of the throat[1635]. In
one instance, _Gryllotalpa_, this part is a long piece between
the arms, shaped like the human thighbone or _tibia_, being more
slender in the middle and widest at the ends, and which is of a much
harder substance than the rest of the _antepectus_, and forms the
lower termination of a singular machine which will before long be
noticed. In many bugs (_Cimicidæ_), instead of being elevated, the
three portions of the _sternum_ are hollowed out into a longitudinal
groove, in which the _promuscis_ when unemployed reposes.
The most conspicuous and remarkable appendages of the manitrunk,
are the _brachia_ or arms. I shall not, however, enter into the
full consideration of these, as they consist numerically of the
same parts, till I treat of the legs in general. Here it will
only be necessary to assign my reasons for calling them by a
distinct denomination. In this I think I am authorized, not only
by the example of Linné, who occasionally found it necessary to do
this[1636], and more particularly by the ancient notion that this
pair of organs in insects were not to be reckoned as _legs_[1637],
but likewise from their different position and functions. They are
so inserted in the _antepectus_ as to point towards the _head_,
whereas the other two pair point to the _anus_. With regard to their
_functions_, besides being _ambulatory_, and supporting the manitrunk
in walking, they are applied to many other purposes independent of
that office,--thus they are eminently the _scansory_ or climbing legs
in almost all insects; in most _Carabi_ L., by means of the notch
and calcar[1638], they are _prehensory_ legs; in _Scarites_ belonging
to that tribe, the Lamellicorn beetles, and the mole-cricket, they
are _fossorious_ legs, or proper for digging[1639]; in _Mantis_,
_Nepa_, and some _Diptera_, they are _raptorious_, or fitted to seize
and dispatch their prey[1640]: they are used also by many insects to
clean their head, eyes, and antennæ, &c. For many of these purposes
they cannot be fit without a structure different from that of the
other legs, which renders it a matter of as great convenience in
descriptions to speak of them and their parts under different names
from those of the legs, as it is of the arms of man; on this account
it is that I propose to give to the fore-leg and its part the names
by which the analogous parts, or what are so esteemed, in the human
species are distinguished;--when spoken of in common with the other
legs, they may still be called the forelegs.
* * * * *
** _Alitruncus._ The alitrunk is the posterior segment of the trunk,
which below bears the four true legs, and above the organs for flight
or their representatives. In treating of this part we may consider
its _insertion_ or _articulation_, its _shape_, _composition_,
_substance_, _motions_, and _organs_.
i. With regard to its _insertion_, or articulation with the manitrunk
and abdomen, it may be observed that it is attached to both by its
whole circumference by means of ligament; in the _Coleoptera_,
_Orthoptera_, and Heteropterous _Hemiptera_ being received by the
posterior cavity of the _prothorax_, the shield of which in these
Orders, especially the last, almost covers and conceals it; but in
the remaining ones it is merely suspended to it. In the former also,
especially in the _Coleoptera_, it seems more separate and distinct
from the manitrunk than from the abdomen, and more independent of its
motions than of those of the latter part: but in the _Hymenoptera_
and _Diptera_ its greatest separation is from the abdomen in both
respects. In many insects, as in the Lamellicorn beetles, the
mole-cricket, &c., the _manitrunk_ terminates posteriorly, drawing
a line from the base of the _prothorax_ to the _antepectus_, in
an oblique section; in other tribes, as in the _Cerambyx_ L., the
Predaceous beetles, &c., the section here is _often_ vertical, but
in the _alitrunk_ the anterior one is _always_ vertical, while the
posterior, by which it articulates with the abdomen, in the Orders
with an ample thoracic shield, is oblique, so that the _pectoral_
portion is more ample than the _dorsal_.
ii. As to its _composition_, the alitrunk is usually much more
complex than the manitrunk; for, besides the instruments of motion,
it consists of numerous pieces. It may be regarded as formed of _two_
greater segments, the first bearing the elytra, or the primary wings,
and the intermediate legs; and the second, the secondary wings and
the hind legs.
1. _Collare_[1641]. The first segment of the alitrunk is the middle
piece of the whole trunk, and therefore, when spoken of _per se_, may
be called the _meditruncus_. It consists primarily of an upper and
lower part, which in the table are denominated the _mesothorax_ and
the _medipectus_. The first piece in the former that requires notice
is the _collar_. I formerly regarded this piece, which is peculiar to
the _Hymenoptera_, _Diptera_, and one tribe of the _Neuroptera_, as the
representative of the _prothorax_ in the other Orders, and this opinion
seems at this time very generally adopted, but subsequent observations
have caused me to entertain considerable doubts of its correctness.
Many other Entomologists have thought it improper to distinguish these
parts by the same name[1642]. Much, however, may be said on both
sides of this question, and I shall now lay before you the principal
arguments that may be adduced in defence of each opinion, beginning
with those that seem to prove that the collar _is_ the analogue of
the _prothorax_. First, then, the collar, like the _prothorax_, is
placed precisely over the _antepectus_, and being placed in the _same_
situation, on that account seems entitled to the _same_ denomination;
especially as in some genera, for instance _Chlorion_ F., it assumes
the very semblance and magnitude of a thoracic shield, and is separated
from the _mesothorax_ by a considerable incisure. Again, in some cases
that have fallen under my own observation, the collar is endued with
some degree of motion distinct from that of the alitrunk, since in
_Pompilus_ and _Chrysis_ the animal can make the former slide over
the latter in a small degree. A third and last argument is, that no
_prophragm_ is formed from the collar: insects that have a thoracic
shield are generally distinguished by having the anterior margin of
the _dorsolum_ deflexed so as to form a _septum_, called in the table
the _prophragm_, which enters the chest and separates the cavity of
the _mesothorax_ from that of the _prothorax_; now in _Hymenoptera_
this septum is a process of the piece behind the collar, and excludes
it from having any share in that cavity. These arguments at first
sight seem to prove satisfactorily the _identity_ of the collar and
_prothorax_. But _audi alteram partem_, and I think you will allow
that the scale containing the claims of the collar to be considered
as a piece _sui generis_, dips much the lowest. And, first, I must
observe, that though in _Hymenoptera_ the collar seems to replace
the _prothorax_ by its situation, yet it is in fact a part of the
alitrunk; for, if the _manitrunk_ be separated from the latter,
the collar remains, in most cases, attached to it[1643], while the
_antepectus_ and arm, with the ligament that covers its cavity above,
the real representative of the prothorax, are easily removed, and this
in recent individuals: as a further proof of this, I must request you
will examine a neuter _Mutilla_; you will see that in this the collar
is not separated from the alitrunk in any respect, but forms one piece
with it, while the _antepectus_ is distinct and capable of separate
motion: further, the action of the collar is upon the alitrunk, it
being of essential importance in flight, whereas the _prothorax_ is
of no other importance than as a counterpoise to that part[1644]. A
further argument to prove the distinction of these parts may be drawn
from the case of _Xylocopa_, a kind of bee. In this genus the collar
forms a complete _annulus_ or segment of the body: now, if it really
represented the prothorax, the under side of the segment, as in those
_Coleoptera_ in which no suture separates the upper from the lower part
of the manitrunk[1645], should represent the _antepectus_, and have
the arms inserted in it; but in the case before us there is a distinct
_antepectus_ bearing the arms received by the socket formed by this
annulus. But the most powerful argument is the fact that some insects
have both the _prothorax_ and _collar_, a circumstance that completely
does away every idea of their identity. If you examine the common
hornet (_Vespa Crabro_), or any saw-fly (_Tenthredo_ L.), you will
find, as was before intimated, that the real covering of the cavity of
the manitrunk is a ligamentous membrane, which properly represents the
_prothorax_. In another genus of the same order (_Xiphydria_ Latr.),
the sides of the _antepectus_ turn upwards and nearly form a horny
covering distinct from the collar[1646], the ligamentous part being
reduced to a very narrow line, and in _Fœnus_ the dorsal fissure is
quite filled up, so that in this the manitrunk is perfectly distinct,
and exhibits both _prothorax_ and _antepectus_ of the usual substance.
In _Nomada_ likewise, _N. Goodeniana_ K. was the species I examined,
there is a short minute _prothorax_ besides the collar. Next let
us turn our attention to the _Diptera_; if you examine the common
crane-fly (_Tipula oleracea_), you will find, first, a regular short
_prothorax_, to which the _antepectus_, with the arms, is attached; and
behind this also is a short collar embracing the alitrunk anteriorly.
The next insects that I shall mention, as exhibiting both _prothorax_
and collar, are the _Libellulina_. These are generally admitted to
have the former of these parts[1647], but besides this they have
also the latter, which is the most ample and conspicuous piece in
the whole trunk[1648]; intervening, as the collar should do, between
the _prothorax_ and those parts of the trunk to which the wings are
attached. There is one circumstance connected with the subject which
should not be overlooked. In the _Hymenoptera_, usually under a
lateral process of the posterior part of the collar, is a spiracle or
respiratory apparatus; in the _Diptera_ there is also one, though not
covered by the part in question, in the same situation; now this you
will find precisely so situated with respect to the second piece in
the thorax of _Tipula oleracea_, proving that this piece is the real
representative of the collar. Enough, I think, has been said to satisfy
you that I have not changed my sentiments on this subject upon slight
grounds. Probably traces of the part in question might be detected
in the thoracic Orders in general, in connexion with some vocal or
respiratory organ[1649]; but having had no opportunity, by an extended
examination of living subjects, to verify or disprove this suspicion,
I shall merely mention it, and conclude this head by observing, that
the collar varies most in the _Hymenoptera_ order, and that its most
remarkable form is in _Vespa_, _Cimbex_, _Dorylus_, &c., in which it
bends into an ample sinus that receives the dorsolum[1650].
2. _Dorsolum_[1651]. Where there is no apparent collar, the
_dorsolum_ (dorslet) is the _first_ piece of the _mesothorax_, and
where there is one, the _second_; it bears the elytra or other
primary organs of flight. It varies in the different Orders,
particularly with respect to its exposure. In Coleopterous insects
it is most commonly, but not invariably[1652], covered entirely by
the shield of the _prothorax_, the _scutellum_ alone being visible;
as it is also in the _Orthoptera_ (with the exception of _Mantis_
and _Phasma_, in the first of which it is partially, and in the
latter intirely exposed), and the Heteropterous, and most of the
Homopterous section of the _Hemiptera_. The scutellum is likewise
covered in _Gerris_, _Hydrometra_, and _Velia_, and the whole of the
back of the alitrunk by a process of the _prothorax_ in _Acrydium_
F., _Centrotus_, &c. But in the remaining Orders, and the tribe of
_Tettigonia_ in the Homopterous _Hemiptera_, the _dorsolum_ is not
hidden by the thoracic shield. It is usually less elevated than the
_scutellum_; in _Necrophorus_, and some other beetles, however,
the latter is most depressed. With regard to its _substance_, it
is generally not so hard and rigid as the _scutellum_, but in
most _Coleoptera_ harder than in the other Orders in which it is
covered; in the _Hemiptera_, except in _Tettigonia_, it approaches
to membrane. As to shape and other circumstances, it varies in the
different Orders. In the beetle tribes it has generally a sinus
taken out of its anterior margin, and it approaches more or less
to a trapezium; in _Blatta_ it is transverse and somewhat arched;
in _Gryllotalpa_ it is nearly square, and distinguished besides on
each side by a minute aperture, fitted with a tense membrane, which
perhaps covers a respiratory apparatus. In the locusts it is more
or less triangular, and in _Mantis_ and _Phasma_ long and slender.
In the _Hemiptera_ the _dorsolum_ appears to consist of several
pieces, variously circumstanced, separated by sutures, corresponding
with which are as many ridges on the inside of the crust[1653].
In the _Libellulina_ it is rhomboidal[1654]; in _Panorpa_ nearly
hexagonal; in the _Ephemerina_ it is ample and oblong; in _Sialis_
and the _Trichoptera_ this part is represented by three subtriangular
pieces, the _scutellum_ constituting a fourth, with the vertices
of the triangles meeting in the centre[1655]; in the _Lepidoptera_
the part in question is large, and receives the _scutellum_ into
its posterior sinus[1656]. The _Hymenoptera_ usually exhibit a very
ample _dorsolum_, mostly subtriangular with the vertex rounded or
truncated, and pointing in some (_Vespa_ L.) to the head[1657], and
in others (_Apis_) to the anus; in the _Diptera_, except in _Tipula_,
the parts of the _mesothorax_ are not separated by any suture, but
only indicated by impressed lines or channels; in the genus last
mentioned, however, the _dorsolum_ is distinct, subrhomboidal, and
received by an angular sinus of the _scutellum_, which last, I think,
is not the part that has usually been regarded as entitled to that
denomination; for this opinion I shall soon assign my reasons.