Chapter XXII: Letter XXXV (2)

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_.

9. _Postdorsolum_[1695]. The first external piece of the _metathorax_ 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].

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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_.

1. _Antefurca_[1754]. The first portion of the _endosternum_, or 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.