Part 14

[751] This insect, which connects _Conocephalus_, _Acrida_, &c. with _Locusta_, is also distinguished by antennæ at first filiform and then setaceous.

[752] De Geer iii. _t._ xxiv. _f._ 1, 12.

[753] _Ibid._ 176. _t._ xi. _f._ 19.

[754] VOL. II. p. 397--.

[755] Reaum. v. 177--.

[756] _Ibid._ vi. 435. _t._ xl. _f._ 6, 7.

[757] _Natural History of the Slug-worm_, 12--. _f._ 12, 13.

[758] Valisn. _Esperienz. &c. Musca dé Rosai._ Reaum. v. 100--. De Geer ii. 916--. The last writer thought he saw in the back of the saw itself a longitudinal cavity (918), which applied to the groove would form an open canal.

[759] PLATE XVI. FIG. 1.

[760] Ibid.

[761] Reaum. v. 347. _t._ xlix. _f._ 10. _d, f._

[762] See above, VOL. III. 390. a.

[763] See above, VOL. I. 448--.

[764] De Geer ii. 835. _t._ xxviii. _f._ 20, 21. PLATE XV. FIG. 22. This figure was drawn by a friend--the organ seems more exerted than in De Geer's. I cannot make out the little appendage at the end.

[765] PLATE XVI. FIG. 2, 3.

[766] Reaum. v. 19--. _t._ iii. _f._ 3-6.

[767] _Arachnid._ 40.

[768] Huber _Nouvel. Observ._ i. 106.

[769] Swamm. _Bibl. Nat._ _t._ xix. _f._ 2.

[770] _Philos. Trans._ 1797. 80.

[771] VOL. I. p. 461.

[772] Compare Reaum. iii. 153. Pallas _Act. Nat. Cur._ 1767. iii. 430. _Wien. Verzeich._ 292.

[773] _Naturfor_ St^k. xx. 59--.

[774] It does not appear to be clearly decided whether the eggs are extruded from the female, or whether dying immediately after fecundation they are hatched within her body. As the young larvæ certainly are hatched in the pupa (not merely within the exterior case of bits of grass, &c., which includes it) which the body of the insect must fill, it does not seem easy to conceive how she can find room for oviposition; and yet Von Scheven expressly says that one female of _Ps. vestita_--which being kept from all access to the male actually left the pupa-case and wandered about the glass which contained them--laid unfruitful eggs.

[775] VOL. I. p. 32, 175.

[776] Bonnet i. 19--.

[777] Reaum. vi. 551.

[778] Reaum. vi. 552.

[779] _N. Dict. d'Hist. Nat._ ii. 284.

[780] _N. Dict. d'Hist. Nat._ ix. 125. Bonnet and Jurine both found that the female Aphides and Branchiopods that were fertile without the usual intercourse of the sexes were less fruitful than their mother, and those of the last generation less so than the first. Latr. _Hist. Nat. des Crust. et Ins._ xi. 292.

[781] See more on the subject of fecundation, VOL. II. p. 154--. 169--.

[782] _N. Dict. d'Hist. Nat._ xxx. 426.

[783] VOL. III. p. 68.

[784] De Geer iii. 533.

[785] Swamm. i. 203. b. _t._ xix. _f._ 3.

[786] Reaum. ii. 66.

[787] VOL. II. p. 36.

[788] VOL. III. p. 64--.

[789] PLATE XXII. FIG. 4.

[790] Ibid. FIG. 3.

[791] Reaum. iv. 414.

[792] _Ibid._ _t._ xxviii. _f._ 14, 15.

[793] _Ibid._ 404.

[794] De Geer vi. 63--.

[795] VOL. I. p. 175.

[796] De Geer iii. 70--.

[797] _Ibid._ 128.

[798] _N. Dict. d'Hist. Nat._ xxx. 426--.

[799] VOL. III. p. 64--.

[800] VOL. II. p. 50, 110--, 118--, 125--, 130--. The neuters of the _Termites_, however, (p. 33.) seem to be a distinct sex, if I may so speak--and to merit that name.

LETTER XLIII.

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONCLUDED._

MOTION.

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 (_Eutrech_in_a_ and _Eunech_in_a_), &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.