In the Homopterous Hemiptera there seems to be more than one type on which the ovipositor is constructed. In an insect very common with us, the froth froghopper (Cercopis spumaria), some approach is made to the ovipositors last described, at least the number of pieces is the same—for it has a pair of external valves forming a sheath, which includes three sharp laminæ resembling the blades of a lancet, the middle one of which can be separated into two; this instrument De Geer had reason to think was scored transversely like a file[753]. In the insects of this Order so noted for their song[754] (Cicada), there are only five pieces; namely, two valves forming the sheath, two augers or borers, and an intermediate piece upon which they slide, each being furnished with an internal groove for that purpose, and the middle piece with a ridge to fit; a contrivance of Divine Wisdom, to prevent their dislocation when employed in boring; the augers terminate in a knob which is externally toothed[755]. This structure approaches that of the Hymenoptera, especially the saw-flies. With regard to the Heteropterous section of this Order—as they usually do not introduce their eggs into any substance, they have no call for any remarkable ovipositor, and therefore are not so furnished. A remark which will also apply to the Lepidoptera Order.
In the Libellulina amongst the Neuroptera, an organ of this kind is sometimes discoverable. In Agrion, Reaumur noticed a part which he conjectured to be an ovipositor; it consists of four laminæ or lancets, the interior pair slender, the exterior wider, and all externally serrated[756].
The insects of the Hymenoptera Order have long been celebrated for the organs we are describing, whether used as saws, augers, or darts. I formerly gave you a very general account of the saws,—I shall now give you a very interesting one in detail copied from an admirable little essay of Professor Peck. "This instrument," says he, "is a very curious object; and in order to describe it it will be proper to compare it with the tenon-saw used by cabinet-makers, which being made of a very thin plate of steel, is fitted with a back to prevent its bending. The back is a piece of iron, in which a narrow and deep groove is cut to receive the plate, which is fixed: the saw of the Tenthredo is also furnished with a back, but the groove is in the plate, and receives a prominent ridge of the back, which is not fixed, but permits the saw to slide forward and backward as it is thrown out or retracted. The saw of artificers is single, but that of the Tenthredo is double, and consists of two distinct saws with their backs: the insect in using them, first throws out one, and while it is returning pushes forward the other; and this alternate motion is continued till the incision is effected, when the two saws receding from each other, conduct the egg between them into its place. In the artificial saw the teeth are alternately bent toward the sides, or out of the right line, in order that the fissure or kerf may be made sufficiently wide for the blade to move easily. To answer this purpose in some measure, in that of the Tenthredo the teeth are a little twisted, so as to stand obliquely with respect to the right line, and their point of course projects a little beyond the plane of the blade, without being laterally bent; and all those in each blade thus project a little outwards: but the kerf is more effectually made, and a free range procured for the saws, by small teeth placed on the outer side of each; so that while their vertical effect is that of a saw, their lateral effect is that of a rasp. In the artificial saw the teeth all point outward (towards the end) and are simple; but in the saw of the Tenthredo they point inward, or toward the handle, and their outer edge is beset with smaller teeth which point outwards (towards the end)[757]." Valisnieri, Reaumur, and De Geer describe the groove as being in the back; but in Mr. Peck's insect, if there is no error in his account, it is, as in the Cicadæ, in the saw itself[758]. In the genus Cimbex, belonging to the same tribe, the saw differs in shape, being somewhat sigmoidal or resembling the letter S, while in that of other saw-flies it is cultriform with a concave edge: other minor differences distinguish them, which need not be particularized.
A similar structure, with regard to the organ in question, obtains in the rest of the Hymenoptera, even those that use it as a weapon of offence; but the backs of the saws in them, composed of a single piece, become a sheath for the darts. The valves, however, vary. In most of those with an exerted sting, as Pimpla, they are linear, exerted, and as long as the aculeus itself[759]. In Proctotrupes they appear to be united so as to form a tube for the ovipositor, and are produced by a prolongation of the last abdominal segment. The darts usually run in two grooves of the sheath, and at their apex are retroserrulate[760]. In some cases the sheath itself is serrated[761]. The shanks of the darts are connected with the valves; so that when these open they are pushed out: sometimes on their outer side they have a triangular plate towards the base, which prevents their being pushed out too far[762].
In Sirex and many ichneumons, in which the ovipositor is too long to be withdrawn within the abdomen, it remains always exerted; but in general it is retracted within that part when unemployed. In the gall-fly (Cynips) this instrument is really as long as in Pimpla, &c.; but as it is infinitely more slender, when in repose it is rolled up spirally and concealed within the abdomen. It is the puncture of this minute organ that produces the curious galls formerly described to you[763]. But the most anomalous ovipositor in this Order appears to be that of Chrysis (C. ignita, &c.), which is covered by several demi-tubes or scales enveloping and sliding over each other: when these scales are removed, the true ovipositor appears, which is of a structure similar to that of the rest of the Order, but the valves are long and slender with their summit generally visible without the anus[764].
Though the ovipositor of the majority of Dipterous insects is a tube with retractile joints[765], in the crane-flies this organ is different, and, like that of Acrida above described, consists of what at first sight appear two valves, but each of which is formed of two pieces, the upper ones sharp and longer, and the lower pair blunt. The upper pair forms the auger that bores a hole in the ground, and the lower conducts the eggs into it after it is bored[766].
In the Aptera and Arachnida in general there seems no remarkable instrument of this kind; but Treviranus has described one in spiders for extruding the eggs of a singular construction. It is an oval plate lying between the external genitals and spinning organs, and is composed of a number of small screw-shaped cartilages, connected together in the most wonderful manner. There are few organs, he observes, in the animal kingdom which for their artificial mechanism can be compared with this. Each cartilage inosculates very closely in the adjoining one, and all are besides bound together by a strong skin[767].
The manner in which the eggs of insects are fecundated by the male sperm is one of those mysteries of Nature that are not yet fully elucidated and understood. We can readily conceive that all the eggs may be fertilized by a single intercourse in the case of insects which, like the Ephemeræ and Trichoptera, exclude the whole mass at once; or like many moths and butterflies, in a very short time afterwards; but the subject becomes much more difficult to explain when we advert to the female of the hive-bee, the whole number of whose eggs, deposited in two years, are, as Huber has demonstrated, in like manner fertilized by a single act[768]:—if you bear in mind, however, what I have lately observed with regard to Malpighi's discovery of a sperm-reservoir in insects, you will more readily comprehend how in this case a gradual fecundation may take place. The principal objection to this solution of the difficulty in the case before us, is derived from the very small size of the organ supposed to be destined for this purpose—it being scarcely bigger than the head of a pin[769]: it seems therefore incredible that it should retain any portion of an extraneous fluid at the end of twelve or eighteen months, and still more unlikely that the fluid should in the interval have sufficed for the slightest moistening of not fewer than 30,000 or 40,000 eggs. The only hypothesis that seems at all to square with this fact, is that of Dr. Haighton,—that impregnation is the result not of any actual contact of the sperm with the eggs, but of some unknown sympathetic influence[770], or rather perhaps of some penetrating effluvia or aura seminalis, which, though small in quantity, it may retain the power of emitting for a long period.
Certain female moths, of the species of that family which, from the remarkable cases or sacs the larvæ inhabit, the Germans call sack—träger, before noticed[771], have been supposed to have the faculty of producing fertile eggs without any sexual intercourse; and various observers, after taking great pains, appeared to have satisfactorily proved the fact; so that some doubted whether these insects produced any males at all[772]. The enigma was at length explained by the accurate Von Scheven. At first his experiments were attended with the same result as those of his predecessors; but upon making them more carefully, and separating what he conceived to be the female from the male pupæ, he ascertained not only the existence of a female in the species he examined (Psyche vestita), but that when thus secluded she laid barren eggs; evidently proving that in the contrary instances above alluded to, an unperceived sexual intercourse must have taken place[773]. Though he thus ascertained that these insects do not in this respect deviate from the general rule, he remarked or confirmed several facts in their economy sufficiently anomalous and striking;—as that the female is not only without wings, but with scarcely any feature of a moth, much more closely resembling a caterpillar; and that in ordinary circumstances she never attempts to leave the pupa-case in which she has been disclosed, but that being there impregnated by the male, she there also, apparently after the manner of the female Cocci, deposits her eggs, which hatching produce young larvæ that make their way out of the case, and thus seem to originate without maternal interference[774].
But the most remarkable fact bearing upon this head, though as relating to a viviparous insect it does not strictly belong to it, is the impregnation of the female Aphides, or plant-lice, before alluded to[775]. If you take a young female Aphis at the moment of its birth, and rigorously seclude it from all intercourse with its kind, only providing it with proper food, it will produce a brood of young ones: and not only this; but if one of these be treated in the same way, a similar result will ensue, and so on, at least to the fifth generation!! to which period Bonnet, who first made an accurate series of observations on this almost miraculous fact, successfully carried his experiments, till the approach of winter and the want of proper food forced him to desist[776]; and Lyonet extended it still further[777]. It is now generally admitted as an incontestible fact, that female Aphides have the faculty of giving birth to young ones without having had any intercourse with the other sex. How are we to explain this most extraordinary fact? Are we to suppose with Bonnet that these insects are truly androgynous, as strictly uniting both sexes in one? This supposition, however, is completely overturned by the circumstance, that there are actually male as well as female Aphides, and that these, as was first observed by Lyonet, are united towards the close of the summer in the usual manner[778]. The most likely supposition therefore is, that one conjunction of the sexes suffices for the impregnation of all the females that in a succession of generations spring from that union. It is true that at the first view this supposition appears incredible, contradicting the general laws and course of nature in the production of animals. But the case of the hive-bee, stated above, in which a single intercourse with the male fertilizes all the eggs that are laid for the space of two years, and in the case of a common spider mentioned by Audebert[779], for many years, shows that the sperm preserves its vivifying powers unimpaired for a long period, indeed a longer period than is requisite for the impregnation of all the broods that a female Aphis can produce; and if immediate contact with the fluid be not necessary, who can say that this is impossible? It is, however, one of those mysteries of the Creator that human intellect cannot fully penetrate. But this anomaly in nature is not wholly confined to the Aphides; since Jurine has ascertained that the same thing takes place with Daphnia pennata Müll (Monoculus Pulex L.), one of Branchiopod Crustacea[780]. It is worth observing whether the female Aphides in their natural state, I mean those of the summer or viviparous broods, have intercourse with the male. I think I have noticed males amongst them; but they seem to become most numerous in the autumn, preparatory to the impregnation of the oviparous females. The object of this law of the Creator is probably the more ready multiplication of the species[781].
As to the period of gestation, most insects begin to lay their eggs soon after fecundation has taken place: but in some Arachnida, as the Scorpion, which seems to be both oviparous and ovo-viviparous, nearly a year intervenes, and the eggs increase to four times the size which they had attained at that period, before they are extruded[782]. The time that is required to lay the whole they are to produce, varies also in insects. In this respect they may be divided into two great classes:—those namely which deposit the whole at once, as Ephemerina, Trichoptera, &c., and those which deposit them in succession, occupying in this operation a longer or shorter period. Many in the first class, as the Trichoptera or caseworm-flies, envelope their eggs in a gelatinous substance[783], which renders their extrusion in a mass more easy. Of the second class, which includes by far the greater proportion of insects, some exclude the whole number in a very short period, others require two or three days or a week, as the cockroach[784]; and others, as the queen-bee, not less than two years. The eggs in the ovaries of the last vary infinitely in size; those that have entered the oviduct have arrived at maturity, while the rest grow gradually smaller as they approach the capillary extremity of the tubes, where they become at length invisible to the highest magnifier[785]. In many insects the eggs seem nearly to have reached their full growth previously to the exclusion of the female from the pupa; and this exclusion and the impregnation and laying of the eggs rapidly succeed each other. One moth (Hypogymna dispar), which is remarkable for the number of eggs she contains, sometimes deposits them, even before they are fecundated, in the pupa-case[786]. But in other cases the sexual union is not so immediate, and some time, longer or shorter, is requisite for the due expansion of the eggs; and the ovaries of the animal swell so much, as often to enlarge the abdomen to an extraordinary bulk: this is seen in a very common beetle (Chrysomela Polygoni) that feeds upon the knot-grass; but in no insect is it so striking as in the female of the white ants, whose wonderful increase of size after impregnation I have related to you on a former occasion[787].
I shall conclude this subject with a few observations upon ovo-viviparous insects; supposed neuters, and hybrids, which, though they do not fall in regularly under any of the foregoing heads, may very well have a place in this letter.
1. It has already been observed that there are a few ovo-viviparous insects[788], the young of which exist in the ovaries at first as eggs, but are hatched within the body of the mother, and come forth in the living form of a larva and sometimes even of a pupa. Of the first description are certain Diptera, the Aphides, and the Scorpion.
Reaumur has described two modes in which the larvæ of the first are arranged in the matrix of the mother. In some they are heaped together without much appearance of order, being placed merely parallel to each other[789]; but in others they are arranged in a kind of riband—the length of the little animals, which are also parallel, forming its thickness—rolled up like the mainspring of a watch[790]. These larvæ in general are not divided into two masses corresponding with the pair of ovaries in other insects, but form only a single one[791]. You must not suppose that these little fetuses lie naked in the womb of the mother; each has its own envelope formed of the finest membrane, which, however, is not entirely divided from that of those adjoining to it, but appears to be one tube, which becomes extremely slender between each individual, so as when drawn out to look like a chain[792]. Reaumur seems to have thought that in these flies the larvæ were never confined in any other case or egg[793]; but De Geer sometimes found eggs in the body of Sarcophaga carnaria, though most generally larvæ, from which he conjectures that it is really ovo-viviparous, the eggs being hatched in the body of the mother[794]. As these flies are all carnivorous, and their office is to remove putrescent flesh, you may see at one glance the object of Providence in this law of nature—that no time may be lost, and the animal exercise its function as soon as it is disclosed from the matrix.
The Aphides, so fruitful in singular anomalies, are ovo-viviparous, as I have before hinted[795], at one period of the year, that is during the summer, but strictly oviparous at its close. From the experiments of De Geer, however, upon Aphis Rosæ, it would appear that this faculty is not conferred upon the same individuals, but only upon those of different generations of the same species; all the generations being ovo-viviparous except the last, which is oviparous[796]: nor does it appear, as has been sometimes imagined, that it is common to the whole genus. De Geer observed a species in the fir, which makes curious galls resembling a fir cone (Aphis Abietis), which appeared never to be ovo-viviparous[797].
With regard to scorpions, it does not seem clear that they are always ovo-viviparous: M. Dufour twice found in the midst of the eggs nearly mature, a young scorpion which appeared to him at large in the cavity of the abdomen; it was so large that it was difficult to comprehend how it could possibly be excluded from the animal, without an extraordinary operation[798]. The pupiparous insects (Hippobosca, &c.) have been sufficiently noticed before[799].
2. I have already in several of my former letters stated to you what the modern doctrine of physiologists is with respect to certain individuals, usually forming the most numerous part of the community with insects living in society, that were formerly supposed to be neuters, or as to their sex neither male nor female—that they are in almost every instance a kind of abortive females, fed with a different and less stimulating food than that appropriated to those whose ovaries are to be developed, and in consequence in most instances incapable of conception[800]. Upon these sterile females, you also heard, devolve in general the principal labours of their respective colonies, showing the beneficent design of Providence in exempting them from sexual cares and desires, and meriting for them the more appropriate name, now generally used, of workers. The differences in the structure of the female bee and the workers were also then accounted for; and similar reasoning may be had recourse to with regard to those of ants, in which the worker and the female differ still more materially. My reason for introducing this subject here, is to observe to you that I have some grounds for thinking that this system extends further than is usually supposed, and that to each species in some Coleopterous and other genera there are certain individuals intermediate between the male and female; this I seem to have observed more especially in Copris and Onthophagus. For in almost every British species in my cabinet of these genera I possess such an individual, distinguished particularly by having a horn on the head longer than that of the female, but much shorter than that of the male. I once observed a pair of Pentatoma oleracea, a very pretty bug, in coitu, both sexes being ornamented with white spots, and by them stood a third distinguished from them by red ones. I do not, however, build on this circumstance, though singular; but mention it merely that you may keep it in your eye. It would be curious should it turn up, that, to answer some particular end of Providence, in some tribes of insects there are two kinds of males, as in the gregarious ones two descriptions of females.
I am, &c.
We have seen upon a former occasion the great variety of movements that insects can perform, and of the external organs with which they perform them[801]: but we are now to consider the internal apparatus, by the immediate action of which they take place—their system of muscles. When we reflect upon the wonderful velocity, their size considered, with which many insects move, and the unparalleled degree of muscular force that many exert[802], we feel no small degree of curiosity to know something of that part of their internal structure that produces these almost incredible effects. I shall in the present letter endeavour in some degree to gratify that curiosity, and give you an account of the muscles of these little animals,—first considering them in general; and then, as far as my information goes, adverting to those in particular that move the different parts and organs of an insect's body.
I. The muscles of insects may be considered in general as to their Origin; Substance and Parts; Shape; Colour; Kinds; Attachment; and Motions.
i. Origin. The origin of the muscular fibre in the higher animals is from the blood, the globules of which, by their coagulation in a series, appear to form it[803]; and in insects it is derived from the same universal source of nutrition and accretion, but not till it has been concreted into the adipose tissue or epiploon before noticed[804]. In the pupa of the cabbage-butterfly, Herold observed that this substance first assumed a fine flocky appearance and a blue-green colour, and that from it so changed were produced tender bundles of muscular fibres, extending in various directions, the epiploon itself decreasing in proportion as they were formed[805].
ii. Substance and Parts. The muscular fibre in vertebrate animals appears to consist of globules arranged in a series, and of no larger diameter than those of the blood,—the mean diameter of which in the human subject, when measured under the microscope by a micrometer, is found to be about 1⁄5000th part of an inch[806]. When Cuvier published his immortal work in 1805, the powers of any magnifier then constructed were not sufficient to enable this great physiologist to arrive at the simple fibre[807]; but Mr. Bauer, by the use of improved glasses, amongst other discoveries that will immortalize his name, was the first to detect, under the directions of Sir E. Home, the ultimate thread of which the muscular bundles are composed[808]. Chemists distinguish the substance of which we are speaking, by the name of fibrine. By the abundance of azote or nitrogen that enters into its composition, it possesses a character of animalization more marked than any other animal substance; and its elements are so approximated in the blood, that the slightest stagnation causes them to coagulate: and the muscles are without doubt, in the living subject, the only organs that can separate this matter from the mass of blood, and appropriate it to themselves[809]. The primary bundles of muscles are formed of the simple fibres, and the secondary are the result of an aggregation of the primary. The smaller bundles are not always exactly parallel to each other, but must in many cases diverge more or less, to produce those variations in shape observable in the muscles themselves: there are intervals therefore between the bundles, which in some animals are filled by a cellular substance[810]. Probably much of this statement will apply in most instances to the muscles of insects, but we may conclude that the globules that form them are infinitely smaller[811]. Lyonet has given some interesting observations with regard to those of the caterpillar of the Cossus: he describes them as of a soft transparent substance, capable of great extension, covered and filled by silver tubes of the bronchiæ, penetrated by the nerves, and containing oily particles. Each muscle was enveloped in membrane, and was composed of many parallel bands, consisting of bundles of fibres enveloped likewise in separate membranes. The fibres themselves, (but it is doubtful whether he arrived at the ultimate term of muscular fibre,) in a favourable light and under a good magnifier, appeared to be twisted spirally[812]. In spiders the muscles seemed to him to consist of two substances, the one soft and the other hard, the last forming a kind of stiff twisted filament[813]. A muscle thus composed of different bundles of fibres may be stated as to its parts, in insects, to consist of base, middle, and apex: the base is that part by which they are fixed to any given point of the internal surface of the crust, or of one of its processes, which serves as their fulcrum; the apex is that part by which they are fixed, either mediately or immediately, to the organ to be moved; and the middle is the remainder of the muscle. We usually discover in them no inflation of the middle corresponding with the belly of the muscles in vertebrate animals; they occasionally, however, terminate in a tendon, as those of the thighs and legs; but these tendons are of a different nature from the fibrous ones of warm-blooded animals; for they are hard, elastic, and without apparent fibres: the fleshy ones of the muscle envelope them, and are inserted in their surface[814].
iii. Shape. The muscles of insects are usually linear, with parallel sides; some are cylindrical, as those of the wings of the Libellulina[815]; and others, as those that move the legs in the caterpillar of the Cossus, are triangular[816]. In the suctorious mandibles of the grub of a common water-beetle[817] they are penniform, or shaped like a feather; and some in the Cossus are forked[818]. Under this head I may also observe, that the muscles are sometimes extremely slender threads, crossing each other, and often curiously interwoven in various directions, so as to resemble lace or fine gauze, as may be seen in the alimentary canal of some caterpillars[819]; sometimes also they surround part of this organ, like a series of minute rings[820].
iv. Colour. The most usual colour of the muscles of insects is white: those for flight however, according to Chabrier, differ from the rest, by being of a deeper and reddish colour[821]; and I have observed likewise that those in the head of the stag-beetle, when dried at least, are red, and look something like the flesh of warm-blooded animals.
v. Kinds and Denomination. In general, muscles may be regarded as divided into primary and secondary—the primary being the muscles by which the principal movements of any organs are effected, and the secondary their auxiliaries which are the cause of subordinate movements[822]. Every muscle almost has its antagonist, the action of which is in an opposite direction; so that when it is equal, the organ to which they are attached remains without motion; but when that of one preponderates, a movement in proportion takes place[823]. The principal antagonist muscles that may be found in insects are the following. 1. Levator muscles that raise an organ, and Depressors that depress it. 2. Flexors that bend an organ, and Extensors that unbend or extend it. 3. Abductors that draw an organ back, and Adductors that draw it forwards. 4. Constrictors that contract an opening, and Laxators that relax it. 5. Supinators that turn the underside of an organ upwards, and Pronators that return it to its natural situation. Some of these muscles in insects, like some of their articulations and their spinal chord[824], seem to exercise a double function,—thus the levators and depressors of the wings are constrictors and laxators of the trunk[825]. At first it may seem that insects, not having the power of turning up the hand, cannot have the Supinator and Pronator muscles; but some muscle of this kind must be in the Gryllotalpa, and in those that have a versatile head[826].
v. Attachment and Insertion. The attachment and insertion of the muscles in insects in general is to the interior of the crust, or to some of its internal processes as a fulcrum, and to the organ to be moved. In some cases, however, the muscles act upon the organ by the intervention of other bodies. Thus, those that move the wings are often attached to little bones, as Chabrier calls them[827], which are connected with the base of the wings by ligaments. In the Dynastidæ and other Lamellicorns, and the Libellulina, &c., a remarkable provision is made for giving a vast increment of force to the muscles of the wings, by means of caps or cupules surmounted by a tendon, which receive their extremity; the tendon terminating in a fine point attached to the wing, and thus more muscles are brought to bear upon it[828]. Chabrier seems to think that, in some cases, the back that intervenes between each pair of wings is the medium by which the muscles act upon it[829].
vi. Motions. Irritability is the universal distinction of the muscular fibre,—when put in action by the will or involuntarily, it causes it to contract or become shorter; and the intermediate agents of the will and other causes are the nerves, which, as galvanic experiments seem in some degree to prove, are the conductors of an invisible fluid or power which immediately causes that action. If a nerve is divided, the muscles to which it renders obey it no longer, evidently proving that the nerves cause muscular irritability[830]. How this contraction is immediately effected,—whether the fibre, as some suppose, undergoes any crispation, or becomes zigzag[831], or whether there is any sudden change in their chemical composition that rapidly and strongly augments their cohesion, as Cuvier hints[832], cannot be clearly ascertained, unless a Bauer could submit the living fibre to his glasses. All that we know certainly on the subject is, that muscles alternately contract and relax at the bidding of the will or involuntarily, and so occasion all the movements of animal bodies.
II. Having considered the muscles of insects in general, I must next make a few observations, as far as my means of information will enable me, upon those that move their different parts and organs—at least the principal ones; since to descend to minutiæ would be an endless and unprofitable labour. As larvæ, except those whose metamorphosis is semicomplete[833], differ widely in their system of muscles from perfect insects, I shall begin my observations with them.
We owe by far the most accurate and detailed account of the muscles of larvæ to the illustrious Lyonet, who, with incredible labour and patience without example, dissected the caterpillar of the Cossus, and has described every air-vessel, every nerve, and every muscle that could be detected by the microscope. Cuvier also has given a description of the muscles not only of caterpillars, but of the larvæ of the Lamellicorn beetles, the Hydrophili, and the Capricorn beetles[834]. From these sources are derived what I have now to lay before you. If you look at one of Lyonet's plates[835], the layers of longitudinal muscles look like so many parallel ribands, others run in an oblique, and others again in a transverse direction[836]. He divides them into dorsal, ventral, and lateral muscles[837], terms which sufficiently explain themselves. Of the longitudinal muscles there are four principal rows[838], the others are more numerous. The principal object of these muscles, which are flexors and extensors, is to shorten or lengthen the body, or to act on any particular segment as the circumstances of the animal may require. I shall not here notice the muscles of the head and legs, as they are not remarkably different from those of perfect insects. The prolegs are moved by two muscles—the anterior one covering in part the posterior—of a remarkable structure: one of their points of attachment is by many branches or tails to the sole of the foot, and by several heads to the skin of the animal; so that they can draw the proleg within the body or push it out, and perform other necessary movements[839].
I shall now call your attention to the muscles of the perfect insect, as they move the head and its organs; the Trunk; the Abdomen; and the Viscera.
i. The Head. This part in insects moves upwards, downwards, inwards, to right and left, is pushed forth or drawn in, is often capable in part of a rotatory movement, and is sometimes versatile, turning as it were upon a pivot. All these movements are of course produced by an appropriate apparatus of muscles, which have their attachment in the anterior part of the trunk, mostly in the manitrunk, while their insertion is in the posterior part of the head, in the margin of the occipital cavity. To enumerate and describe them all would be tedious and uninteresting—I shall only mention some of the principal ones. The levators of the head are usually a pair of muscles situated in the manitrunk, to the upper side of which they are attached, and perhaps in Coleoptera and some others to the phragma, which probably Cuvier means by the anterior part of the scutellum[840]; they are inserted in the posterior margin of the upper part of the head, in Coleoptera in a pair of notches (Myoglyphides[841]), or a single one[842]. In Cordylia Palmarum these muscles as they approach the head, to judge from the dead animal, divide into two branches or a fork: thus, as the muscle-notches are wide in this insect, the muscle acts upon each extremity of the sinus—these branches appear to be tendinous[843]. The depressors of the head are the antagonist muscles to the above, and have their attachment to the antepectus and its antefurca[844]. A circumstance distinguishes these muscles in many Coleoptera, that seems hitherto to have been overlooked. If you take the common dung-beetle (Geotrupes stercorarius), and carefully extract the head with its muscles from the trunk, you will see on each side of the depressors a subovate corneous scale, of a pitch colour[845], which is attached only to the muscle, and designed to strengthen it: if you then examine the anterior cavity of the manitrunk, you will perceive on each side, just within the lower margin, a minute triangular scale, of a similar substance; these ligaments, like the pax-wax, or ligamenta nuchæ, in mammalia, though in a lower situation, are doubtless intended to sustain the action of the muscles.
With regard to the moveable organs of the head—the antennæ, maxillæ, palpi, tongue, mandibulæ, &c., have each their appropriate apparatus of muscles: but I shall only notice those of the last, the mandibulæ. These are principally abductors and adductors to open and shut them: from the work that the jaws of some insects have to do, you may conjecture that they must be furnished with powerful muscles. In caterpillars and other larvæ, in which state the action of the mandibles is most in requisition, the muscles are what Cuvier calls penniform[846], and are attached on each side to a tendinous lamina or cartilage. In the grub of Dytiscus the power and magnitude of the adductor muscle is wonderful[847]. In the Orthoptera this structure of the mandibular muscles takes place also in the imago[848]; but in the Coleoptera, at least in the stag-beetle and some others that I have examined, these muscles in this state have no cartilage or tendon. Their attachment is always to the parietes of the head, of the cavity of which the adductors, in some cases, occupy a considerable portion[849]. As to their insertion—these last, in some Orthoptera, enter more or less the interior of the mandible[850]; but commonly they are inserted at or near the interior angle of the mandibular basal cavity, and the abductors at the exterior.
ii. The Trunk. We have little information with regard to the muscles of the parts of the trunk itself, by which, in some insects, the manitrunk is enabled to move independently of the alitrunk: it is more probable that the levators have in part at least their attachment to the anterior surface of the prophragm[851], than that the levators of the head should be there fixed, as Cuvier seems to think; since both the phragma and the ligament that appears in many cases to close the cavity of the manitrunk round the viscera[852], would prevent all communication between those muscles and any part connected with the scutellum: probably the depressors have their attachment partly on the anterior face of the medifurca[853]. These points, however, must be left to future investigators.
With regard to the organs of the trunk, we have more certain and satisfactory information;—the muscles of the legs having been described by Lyonet and Cuvier, and those of the wings most particularly by Chabrier. In caterpillars, the muscles are situated in the interior of the articulations that form the legs: they consist of several bundles appropriated to each, which have their attachment in the parietes of the preceding joint, near the margin, and are inserted in the margin of that they move[854]. Lyonet counted twenty-one muscles in the leg of the caterpillar of the Cossus; but eight of these were appropriated to the claw, or rather formed a pair of semipenniform muscles, having their insertion at the inner angle of its base[855]. In perfect insects, according to Cuvier, each joint of the legs is furnished with a pair of antagonist muscles—a flexor and extensor, the former being the lower, and the latter the upper muscle; and this pair has its insertion in the joint it moves, and its attachment usually in the preceding one: but those of the coxæ—which are rotators, causing it to turn backwards or forwards—and the extensor of the thigh, have their attachment in the parietes of the trunk, and to the endosternum; one of the rotators of the anterior coxa, and the extensor of the anterior thigh to the antefurca; of the intermediate pairs to the medifurca, and of the posterior to the postfurca[856]. Every joint of the tarsus has also its flexor and extensor. In the ground- and water-beetles (Eutrechina and Eunechina), &c., whose posterior coxæ are immoveable, the thigh includes two pair of antagonist muscles[857]. In extracting the posterior leg of Necrophorus Vespillo I observed more than a single pair of muscles that had their attachment in the coxa; and probably many other variations in this respect exist.
Little was known with respect to the most interesting part of the muscular apparatus of insects, that by which such wonderfully rapid and varied motions are imparted to their organs of flight, till Chabrier undertook to elucidate it; which he has done in a manner that will confer a lasting honour upon his name, as one of the most able successors to Swammerdam and Lyonet in their peculiar department. He has given a most admirable account of the internal anatomy of the trunk of insects in general, as far as it relates to their flight; particularly of that of the cockchafer (Melolontha vulgaris), of one of the Libellulina (Æshna grandis), and of a bumble-bee (Bombus); and I believe he has thus illustrated insects of some of the other Orders, but his memoirs on these I have not had an opportunity of consulting. What I have to say on this subject, therefore, will be principally derived from what he has communicated with respect to the above insects.
A considerable difference in the volume of the muscles of the wings takes place in insects according to the force of their flight. Where it is rapid and powerful, the alitrunk is nearly filled by them, and the alimentary canal is much attenuated; but in those whose flight is feeble, they occupy less space, and the alimentary canal is proportionally enlarged[858]. In the Lepidoptera, Hymenoptera and Diptera, the principal muscles of both wings have their attachment in the anterior portion of the alitrunk[859]; in the Coleoptera, in the posterior[860]; and in the Libellulina, those of the anterior wings are confined to the anterior portion, and those of the posterior pair to the posterior[861]. The muscles for flight in general differ from others by their mass, length, and colour; the bundles of fibres are very distinct, strong, and parallel; their direction is uniform, according to the motion they are to produce; their fibres are either attached to the solid parts to be moved, or to cupules, but they never terminate in a tendon; the muscles are perfectly independent of each other, and the wings can be moved by them separately[862]. As to their denomination and kind—the principal ones are the levators and depressors, which with respect to the trunk, as was before observed, are constrictors and laxators. The levator muscles form several distinct bundles in Coleoptera, Lepidoptera, &c.; in the Diptera there are three[863]; in the Libellulina they seem to be single, are all environed with a blackish pellicle, with numerous aërial vesicles, symmetrically arranged, filling the interstices[864]. The most common number is a levator to each wing; there are often, however, as in the cockchafer and the dragon-fly, two depressors[865]: but in the Hemiptera, Lepidoptera, and saw-flies (Serrifera) amongst the Hymenoptera, the secondary wings have distinct levators, but not depressors[866]; the other insects of that Order have only a pair of each[867]. The other wing-muscles are of a secondary description, and auxiliary to the above. Their office is to extend and close the wings: so that though the denomination of extensor will suit the former, that of flexor is not so proper for their antagonists; their office being not so much to bend, as to bring back the wing to its station of repose. The folding of certain wings, as those of Coleoptera, Dermaptera, the Vespidæ, &c., seems more the function of the abdomen than of the wing-muscles; this you may easily see, as I have often done, if you attend to any Staphylinus, when after alighting from flight it proceeds to fold up its wings under the elytra. Perhaps the term retractor might not be inapplicable to the muscles in question. Both these and the extensors are usually small slender muscles, but sometimes numerous[868]. They are larger in the Coleoptera, Lepidoptera, and saw-flies[869]. The muscles that open and shut the elytra of Coleoptera, and probably of Heteropterous Hemiptera, and which also aid their movements during flight, are very slender[870]. With regard to the attachment and insertion of the wing-muscles, it is according to two very distinct types, one of which appertains to insects in general, and the other is peculiar to the Libellulina. In insects in general, the principal muscles for flight have not their insertion in the wings, but act upon their bases by the intervention of small long pieces. The depressors occupy the middle and upper region of the alitrunk, and are inserted anteriorly and posteriorly upon the concave surfaces of two transverse horny semi-partitions, adapted by their elasticity to dilate the trunk—and thus acting the part of both diaphragm and ribs[871]: but in the Libellulina, as in birds, these muscles are placed on each side of the point of support of the humerus[872]; the depressors being attached immediately to the wings without it, and the levators within it, with this sole difference, that they are connected to the internal extremity of the base of the wing by the intervention of a cupule terminating in a tendon; all are disposed perpendicularly to the arms of the levers on which they act, and all incline more or less outwards, the one to dilate, and the other to contract the trunk[873]. It may be observed in general, that in insects formed upon the first type, the great action of these muscles is the dilatation and contraction of the alitrunk, the main tendency of which is to depress and raise the wings[874]. I shall add here a few words upon the attachment of the wing-muscles in the different Orders: but first I must request you to read what I have said on the partitions and chambers of the alitrunk in a former letter[875]. In most insects of the first type, the depressors are longitudinal dorsal muscles that have their posterior point of attachment in the metaphragm (costale Chabr.); but the anterior varies:—in those that have elytra, tegmina, or hemelytra, the muscles for them seem to be contained in the chamber, varying in size, that lies between the prophragm and mesophragm; and the anterior point of attachment of their depressor muscles is the mesophragm: they are also attached in some to the metathorax or back of the posterior portion of the alitrunk[876]. The levator muscles in Coleoptera, at least in the cockchafer, by a long tendon have their posterior attachment in the lower part of the posterior coxæ[877], their anterior attachment to the solid parts to be moved. In the Cockchafer and the Dynastidæ, but not in Geotrupes, on each side of the cavity of the metathorax under the base of the wing is a large and small cupule, which from their lateral situation one would think must receive the levator muscles—apparently unnoticed by M. Chabrier; but as there is a pair of these cupules on each side, there must have been also a pair of muscles attached to them, which does not agree with his statement[878]. In the Hymenoptera and Diptera the anterior attachment of the depressors is to the back of the alitrunk and to the prophragm, and the levators to the breast, and the sides of the back of the trunk[879]. In the Libellulina the depressors and levators that terminate, by a tendon surmounting a cupule, in the base of the wings, have their posterior attachment in the breast. These cylindrical muscles with their cupule and tendon look like so many syringes[880].
Having thus described to you the powerful muscular apparatus by which, either mediately or immediately, the wings of insects are moved, it will not be out of place if I add a few words upon their flight itself. The great object in this is to generate a centrifugal force which may counteract the weight of the body. Its wings are the external organs by which the insect as it were takes hold of the air when they fall, and is impelled by it when they rise; its head makes way for it; its abdomen, as a rudder, steers it; and by alternately increasing and diminishing in volume, and rising and falling, enables it to win an easy way through the fluctuations of the atmospheric sea. The trunk by its elasticity admits the internal action of antagonist muscles, which by turns compress and dilate it; an action promoting the elevation and depression of the wings, and keeping up the elasticity of the internal air, which is thus now rarified and now condensed: in the former state flowing like a tide, accompanied by the blood, into the nervures of the wings[881], and thus increasing their tension and centrifugal force;—in the latter ebbing and receding to the trunk, thus relaxing the one and diminishing the other. The spiracles by which the air enters or is expelled, open and shut at the animal's pleasure[882]; and besides, many insects are furnished, as we have seen[883], with numerous vesicles or reservoirs, which can give out a supply of internal air when wanted: and thus they can vary their aërial motions, diminish or increase the counteracting centrifugal force; rise and fall, and move onwards and in different directions, as their occasions demand.
iii. The Abdomen is perhaps capable of the greatest variety of motions of the three primary sections of the body. Even when the insect is reposing, a constant dilatation and contraction usually takes place in it[884]; and from its annular structure, its parts capable of separate motion are numerous:—it expands and contracts; it rises and falls; it bends in various directions; and its segments can often be lengthened or retracted. Besides all this, its spiracles open and shut, and its reproductive and other anal organs have their appropriate motions. In numerous Coleoptera, however, and some Hemiptera, the upper-side of the abdomen is almost the only part that is moveable, especially near the trunk; the under-side, having its first segments soldered together, is only capable of motion near the tail[885]. The muscles that produce the various motions of this part must be entitled to all the denominations stated above[886]. I have on a former occasion explained to you how, in insects that have a petiolate abdomen, that part is elevated and depressed[887]. In those with a sessile one the base is attached to the metaphragm by strong ligaments[888], and the muscles that move the first piece act from one segment to another. The partial movements of the segments of this part, where they have place, are produced by muscular fibres which extend from the whole anterior margin of one to the whole posterior one of that which precedes it. If those, for example, of the back contract, the abdomen becoming shorter above, bends upwards; and if those of the sides or belly, it bends sideways or downwards[889]: this is a beautiful as well as simple contrivance.
The alternate rush of air from the abdomen into the alitrunk, and from the atmosphere into the abdomen, is attended by the constriction or expansion of that part as it rises or falls in flight[890], which seems to require the action of constrictor and laxator muscles.
iv. The Viscera. Having before had occasion sufficiently to notice the muscles by which the systole and diastole of the dorsal vessel of insects is maintained[891], I shall now only mention those that are woven round their alimentary canal, by which the peristaltic motion of that organ, causing its contractions and the propulsion of its contents, takes place. One would at first think that a view of the intestines of any animal could under no circumstances afford any very pleasing spectacle to the eye of any but a scientific spectator; but any lady who is fond of going to Disons to be tempted with an exhibition of fine lace, would experience an unexpected gratification could she be brought to examine those of a caterpillar under a microscope: with wonder and delight she would survey the innumerable muscular threads that in various directions envelope the gullet, stomach, and lower intestines of one of these little animals; some running longitudinally, others transversely, others crossing each other obliquely, so as to form a pattern of rhomboids or squares; others again, surrounding the intestine like so many rings, and almost all exhibiting the appearance of being woven, and resembling fine lace,—one pattern ornamenting one organ; another, a second; and another, a third. This will suffice to give some idea of this part of the muscular structure of these little animals[892].