Chapter XXI: Letter XXXV (1)

_EXTERNAL ANATOMY OF INSECTS_,
CONTINUED.

THE TRUNK, AND ITS PARTS AND ORGANS.

As the _head_ of insects is the principal seat of the organs of _sensation_, so is the _trunk_ of those of _motion_; and in it are contained the muscles by which they are moved: it may therefore be regarded as the great centre of motion, and as the main support and prop of the two other primary sections of the body--the head and abdomen, between which it is situated--it may be deemed the most important part of the insect, the key-stone of the whole structure. In treating upon it, for the greater clearness, I shall consider its _substance_, _general form_, _proportions_, _composition_, _internal processes_, and _members_. It will first, however, be necessary to assign my reasons for the nomenclature of its parts that I have adopted.

Had the entomological world been universally agreed upon this subject, and there was an established system of Orismology[1585], I should have proposed no alteration without great reluctance, and the fullest conviction of the absolute necessity of some change; but as the standard of language in our science is still unsettled, and different terms are used by different writers, there seems full liberty left to me to select those that appear upon the whole most appropriate; and where proper and significant terms seem wanting, to invent new ones. M. Latreille, in a late Essay[1586], has proposed many changes of this kind, and seems to hesitate concerning the adoption of some of those recently coined in France for the parts of the trunk[1587]; it may therefore, I think, be permitted me to labour a little in this hitherto imperfectly cultured field, and to suggest such improvements as the subject may seem to require or admit.

Linné called the part we are now considering the _trunk_, its upper-side he usually denominated the _thorax_, and its under-side the _breast_: he notices also the _scutellum_ and _sternum_[1588]. As the _prothorax_ and _scutellum_ are the only _apparent_ parts of the back of the trunk in his first Orders (_Coleoptera_, _Hemiptera_ L.), the rest being covered, in noticing these he puts the part for the whole, calling the _prothorax_ the _thorax_, but which strictly was not synonymous with what he called by the same name in the other Orders. Linné's phraseology with regard to the trunk of insects was adopted by Fabricius and other Entomologists, till Illiger employed the term _thorax_ to designate the whole of the trunk[1589], calling the upper part _thorax superior_ and the lower _thorax inferior_. M. De Blainville, M. Latreille, and other French writers, improved upon this, naming the upper part the back (_dorsum_), and the lower the breast (_pectus_); and dividing the trunk, or according to them _thorax_, into three sections, each bearing a pair of legs. But I see no sufficient reason for this alteration--the terms _trunk_, _thorax_, and _breast_, in the common acceptation are well understood, and lead to no confusion or glaring impropriety; I shall therefore adhere to the old phraseology, especially as French Entomologists in popular language still do the same.

As to the division of the trunk into segments by M. Latreille and others, it has been regarded as consisting of _three_ primary ones, which have been called in the order of their occurrence, reckoning from the head--_prothorax_, _mesothorax_, _metathorax_. The first of these segments, however--and the learned Entomologist just named seems to hint as much[1590]--is usually more distinct from the other two, than they are from each other. If this idea be correct, the trunk is properly resolvable into _two_ primary segments, the first bearing the arms or fore-legs, and the other the proper legs and the organs of flight. M. Chabrier calls the latter _tronc alifère_, or wing-trunk;--a happy term, which I have adopted and latinized, calling it the alitrunk (_alitruncus_): the first segment, because it bears the fore-legs, I have named manitrunk (_manitruncus_). I adopt likewise the terms above mentioned, _prothorax_, _mesothorax_, _metathorax_, to signify the three segments into which the thorax of Linné, or the _upper_ side of the trunk, is resolvable; and those of the breast I denominate _antepectus_, _medipectus_, and _postpectus_. If terms be thought necessary to designate the two intire segments into which the alitrunk is resolvable, the first may be the meditrunk (_meditruncus_), and the other the potrunk (_potruncus_).

I. _Substance._--With regard to its _substance_, the trunk in general is softer than the head, and harder than the abdomen, especially as to its upper surface; but in some cases, where it is not protected by the elytra, as in the rove-beetles (_Staphylinus_ L.), the abdomen appears as hard as the trunk. Though usually not very different from the elytra in this respect, in _Meloe_, _Lytta_, and other vesicatory beetles, it is of a firmer consistence.

II. _General Form._--In the _Coleoptera_ Order the only part of the trunk that is visible on its upper-side is the _prothorax_: the _mesothorax_, with the exception of the _scutellum_, and the _metathorax_, being entirely concealed by it and the elytra; so that, with regard to _shape_, it may nearly be considered as merging in the _prothorax_. Below it is more visible, and may be stated as more or less quadrangular; in oblong beetles inclining to a parallelogram, and in shorter or hemispherical ones to a square. In the majority it is more convex below than above, except in the case of the hemispherical or gibbous beetles (_Coccinella_, _Erotylus_, &c.), in which the under-side is flat and the upper-side very convex. In the _Diurnal Lepidoptera_ the trunk approaches to a cubical shape, in the _Nocturnal_ it is more spherical. A similar difference obtains in the _Hymenoptera_ and _Diptera_: in the bees, wasps and flies, the trunk approaching to the figure of a sphere; in the ants, _Scoliæ_, crane-flies, &c. to that of a cube. The upper part of it in many _Ichneumonidæ_, female ants, &c. is very elevated, forming an arch, and sloping towards the abdomen. In general it may be observed with respect to the remaining Orders, that the form of the trunk merges in that of the whole body, the tendency of which is often to a three-sided figure.

III. _Proportions._--The trunk is usually longer and larger than the head and longer than the abdomen, but not wider: but there are exceptions to both these rules. In _Colliuris_, _Mantis_, &c., it is more _slender_; and in _Atta megacephala_ and some neuter ants, it is _shorter_ than the _head_; in _Atractocerus_, many _Staphylinidæ_, _Phasma_, the _Libellulina_, the _Lepidoptera_, and various _Hymenoptera_, it is shorter, and in the _Mantidæ_ more _slender_ than the _abdomen_. The greatest disproportion between it and the last part is exhibited by the genus _Evania_, parasitic upon the _Blattæ_, in which the abdomen appears merely as a minute and insignificant appendage of the trunk. The vertical diameter of this part, almost without exception, is greater than that of either head or abdomen. When we consider that it contains the muscles that move both the organs of flight and the legs, we see clearly the reason why the CREATOR gave it greater volume.

IV. _Composition._--I lately intimated to you that the trunk, though resolvable into _three_ segments, in most cases properly consists of only _two_ primary ones. Whoever examines the perfect insects of every Order, except the _Aptera_[1591], will find this distinction strongly pointed out, not only by the different direction of the first pair of legs from that of the two last, but also in a large proportion by a deep incisure; and in all it is further manifested by the anterior segment having a motion distinct from that of the rest of the trunk, and separating readily from it; and this not only where it is large, as in insects that have a thoracic shield, but also in those in which the _prothorax_ is less apparent: whereas the other two pedigerous segments have little or no distinct motion, will not readily separate from each other, and in some cases exhibit no pectoral suture between them. Sometimes, however, these two last segments are more prominently distinguished: in _Lytta_, _Mylabris_, and other vesicatory beetles, they are separated below by an incisure, or rather the first or mid-leg segment, is not nearly so elevated as that of the hind-legs. In some ants (_Atta_ Latr.), in the neuters, there is no distinction of segments in the trunk; but in others (_Formica_ Latr.) it follows the general law, and consists of three. In the _Arachnida_, with the exception of _Galeodes_, in which the head is distinct, and the three segments of the trunk may be traced, these parts together form only a single segment. Induced by these reasons, I consider the trunk as consisting in general of two _primary_ segments, the _manitrunk_ and _alitrunk_: the latter resolvable into two _secondary_ ones.

* _Manitruncus_[1592].--The manitrunk, then, is the anterior section of the trunk, which bears the arms and contains the muscles that move them. This part has free motion, or a motion independent of that of the rest of the trunk. This indeed seems a necessary result of the direction and uses of the arms. It consists of an upper and lower part--the _prothorax_ and _antepectus_.

i. _Prothorax_[1593].--The upper part of the manitrunk in the _Coleoptera_, _Orthoptera_, and _Hemiptera_, is by far the most conspicuous part of the trunk, but in the other Orders it is less so. With respect to it, insects may be divided into two classes--those that have and those that have not a _prothorax_. In the _Coleoptera_ Order it is remarkable both for size and variations in its shape and sculpture. In the _Orthoptera_, though less various, it is almost equally conspicuous, especially in _Blatta_. In the _Homopterous_ section of the _Hemiptera_, in many genera it has become extremely short; while in the _Heteropterous_ section its dimensions are not much reduced. In the majority of the _Neuroptera_, likewise, it is comparatively large; in the _Libellulina_ much shorter, and in the _Trichoptera_ and _Lepidoptera_ nearly evanescent[1594].--In the _Hymenoptera_ and _Diptera_, with very few exceptions, the thoracic shield altogether disappears, at least if I am correct in an idea, which I shall hereafter explain, that the _collar_ usually regarded as the analogue of the _prothorax_, is really a part of the alitrunk. In these last Orders, though there is no true _prothorax_, the manitrunk still remains under the form of an _antepectus_, bearing the fore-legs, and containing the muscles that move them.

The prothorax of insects may in general be considered with respect to its _parts_, _margin_, _appendages_, _shape_, _sculpture_, _clothing_, and _proportions_.

1. The _prothorax_, regarded as a whole, distinct from the _antepectus_ or fore-breast, consists commonly of _two_ pieces--the _shield_, or upper part[1595], and the _ora_, or under part[1596]. In the shield you are to observe its _apex_[1597], _base_[1598], _sides_[1599], _limb_[1600], and _disk_[1601]. The _apex_ is the part next the _head_; the _base_ that next the _abdomen_; the _limb_ the _circumference_, and the _disk_ the _central_ part. In many _Orthoptera_ and Heteropterous _Hemiptera_, the shield appears further to consist of two pieces, an anterior and posterior one. The _ora_ is a continuation of the shield below the lateral margin, turned downwards and inwards towards the fore-breast and the legs, but separated from the former in most cases by a suture, as in _Carabus_ L.; and in others merely by an impressed line, as in _Blaps_ F.; but in _Curculio_ and _Cerambyx_ L., &c. there is no ora, the shield being without a lateral margin, and forming one piece with the _antepectus_. The part we are now considering varies in different genera. Sometimes it is very _narrow_, as in _Scarites_; at others very _broad_, as in _Buprestis_, _Nepa_, &c. In _Lampyris_, except _L. italica_, and affinities, it projects posteriorly into a _lobe_ or tooth, which forms a right angle with the rest of the _ora_, and becomes the lower part of the cavity that receives the head; and in _Dermestes_ this part is excavated into an anterior and posterior one which admits the antennæ and arms when folded for repose.

2. The _margin_ of the _prothorax_ is a ridge, either defining its sides or whole circumference. In many cases this margin is broad and dilated, but in others it is merely a thread or bead that separates the shield from the _ora_. Though generally terminating the upper surface, it sometimes, as in _Staphylinus_, dips below it. In many insects, however, as I just observed, the thoracic shield has no lateral margin whatever.

3. Various and singular are the _appendages_ with which the _prothorax_ of numerous insects is furnished. Many of these are _sexual_ distinctions, and have been before described to you[1602]; but there are others common to both sexes, the most remarkable of which I shall notice.--Some are distinguished by a long _horn_ which overhangs the head, as _Membracis cultrata_, _ensata_, &c.[1603]; in others it stands upright, as in _Centrotus spinosus_[1604]; _C. Taurus_ has a pair of thoracic horns like those of a bull, only dorsal[1605]; in _Ledra aurita_ they are flat, and represent ears[1606]; in some species of _Tingis_ (_T. Echii_, _Pyri_, &c.) a kind of reticulated hood, resembling lace, is elevated from the anterior part of the _prothorax_, which receives and shelters the head[1607]. In _Centrotus globularis_ and _clavatus_ F., especially the former, the part in question is armed by a most singular and wonderful apparatus of balls and spines,--in one case standing erect[1608], and in the other being horizontal[1609],--which gives these animals a most extraordinary appearance. In many of the species here quoted the _prothorax_ is producted posteriorly into a long scutelliform horizontal horn, which more or less covers the wings and abdomen; a circumstance which also distinguishes the genus _Acrydium_ F. (_Tetrix_ Latr.). This horn seems to have been sometimes regarded by Linné and Fabricius as a real _scutellum_, and sometimes only as a process of the prothorax: but that it is merely the latter will be evident to you, if you examine carefully any insect furnished with this appendage; for if you remove that part, you will discover the true scutellum and other parts of the trunk concealed beneath it. A very remarkable prothoracic appendage is exhibited by some species of _Mantis_. In general the part we are treating of in this tribe is very slender; but in _M. strumaria_, _gongyloides_, &c., it appears dilated to a vast width, and assumes, either partially or generally, a subrhomboidal form; but if it is more closely examined, it will be found that the form of the _prothorax_ is really similar to that of the rest of the tribe, but that this part is furnished on each side, either on its whole length or anteriorly, with a large membranous flat subtriangular appendage resembling parchment[1610]. Perhaps this kind of sail may be useful to the animal in flight. In _Prionus coriarius_ &c. its sides are armed with teeth, and in many _Lamiæ_, _Cerambyces_, and other Capricorn beetles, and often in various bugs (_Pentatoma_ Latr.) with sharp fixed spines. But the _prothorax_ has _moveable_ as well as _fixed_ appendages; of this kind are those spines (_umbones_), whose base is a spherical boss moving in an _acetabulum_ of the thoracic shield of the Capricorn subgenus _Macropus_ Thunb. If I might hazard a conjecture, I should say that these organs were given to this animal by an all-provident CREATOR, to enable it to push itself forward, when in the heart of some tree it emerges from the pupa, that it may escape from its confinement. Another kind of moveable appendages are attached to the thorax of _Lepidoptera_, usually in the form of a pair of concavo-convex scales covered externally with a tuft of hairs[1611]. M. Chabrier, who examined these scales in recent insects, describes them as vesicles, which appeared to him full of a liquid and of air, and capable of being alternately inflated and rendered flaccid; he regards them as accessories to a pair of spiracles, which he looks upon as vocal[1612], opening into the manitrunk just above the insertion of the arms. These organs are quite distinct from the _tegulæ_ that cover the base of the primary wings of insects of this Order[1613], and are what, borrowing a term from Mouffet[1614], I have called in the table _patagia_, or tippets. Under this head I may include the _caruncles_ at the anterior angles of the _prothorax_ of a genus of beetles with soft elytra, named by Fabricius _Malachius_. When pressed, says De Geer of these insects, a red inflated soft vesicle, of an irregular shape, and consisting of three lobes, emerges from the thorax and from each side of the anterior part of the abdomen, which re-enters the body when the pressure is removed[1615]. M. Latreille seems to think that these vesicles have some analogy with the poisers of _Diptera_ and the pectens of scorpions; and that they are connected with the respiration[1616].

4. We are next to say something upon the _shape_ of the _prothorax_. The forms of the thoracic shield, especially in the _Coleoptera_, are so various, that it would be endless to aim at particularizing all; but it may be useful to notice a few of the most remarkable. The prothorax of _Moluris_, a darkling-beetle, approaches the nearest of that of any insect to a spherical form, from its remarkable convexity; in the wheel-bug (_Reduvius serratus_) it is compressed, and longitudinally elevated into a semicircular serrated crest: it is crested, also, in many _Locustæ_ and _Acridæ_, in some having two parallel ridges; but, generally speaking, its surface is more depressed. In _Necrodes_ it is nearly circular, in _Blatta petiveriana_ semicircular, in _Nilion_ and some _Coccinellidæ_ crescent-shaped, in _Carabus_ obcordate, in _Cantharis_ and _Sagra_ approaching to a square, in _Languria_ to a parallelogram; in many _Cimicidæ_, _Belostoma_, &c., it is triangular, with the vertex truncated; it is trapezoidal in _Elater_, in _Ateuchus_ rather pentagonal, and exhibiting an approach to six angles in some other beetles[1617]: but the prothorax most singular in form is that of some species of M. Latreille's genus _Helæus_[1618], as _H. perforatus_, _Brownii_, &c.: in these its anterior angles are producted, and curving inwards, lap at the end one over the other, so as to form a circular orifice for the head, which otherwise would be quite covered by the shield. Thus the upper portion of the eyes can see objects above, as well as their lower portion those below. I might enumerate many other forms, but these are sufficient to give you some notion of the variations of this part.

5. The _prothorax_ is equally various in its _sculpture_; but since in the Orismological table almost every instance of it has its place, I shall here only notice it as far as it is common to the whole tribes, genera, or subgenera. The _Scarabæidæ_ of Mr. W. S. MacLeay are distinguished by a small _excavation_ on each side of this part, which, as has been before remarked[1619], furnishes an elevated base for an internal process with which the anterior _coxæ_ ginglymate. In _Onitis_ and _Phanæus_, to these excavations are superadded a pair impressed in the base of the _prothorax_, just above the scutellum; in _Carabus_ L. a longitudinal channel divides the thoracic shield into two equal portions; and many genera of that great tribe have in addition, at the base on each side, one or two excavations or short furrows. _Elophorus_ F. has on this part several longitudinal channels, alternately straight and undulated. Generally speaking, in _Carabus_ L. the _prothorax_ has no impressed points; but in one or two subgenera of _Harpalidæ_ (_Chlænia_ &c.) it is thickly covered with them. In numbers of _Locusta_ Leach, the part we are considering is what Linné terms _cruciate_, being divided into four longitudinal portions by three elevated lines, the intermediate one being straight, and the lateral ones diverging from it both at their base and apex, so as to form a sinus or angle[1620]. In certain _Acridæ_ K. (_Locusta_ F.) there are only two of these lines or ridges, but notched or toothed; and in some of the genus first named only one[1621]; in _Locusta Dux_ and affinities the _prothorax_ has several transverse channels or rather folds[1622], with corresponding ridges on its internal surface.

6. With respect to the _clothing_ of the _prothorax_, I have not 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.

3. _Scutellum_[1658]. Some writers on the anatomy of insects, 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.