Part 15

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].

Lyonet counted the muscles contained in the body of the caterpillar of the _Cossus_. In the head he found 228; in the body, 1647; and enveloping the intestines, no less than 2186; which, after deducting 20 that are common to the gullet and the head, gives a total of 4061[893]. In the human subject only 529 have been counted[894]: so that this minute animal has 3532 muscles more than the Lord of the creation!

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The muscles of the _Arachnida_ seem less numerous than those of insects. In the _Scorpionidea_ they appear to be robust, formed of simple straight fibres, of a whitish gray colour: a muscular web, rather strong, clothes the _parietes_, but rarely adheres to them, of the abdomen, and envelopes the _viscera_, with the exception of the lungs, and probably of the heart. The dorsal part of this web gives birth to seven pairs of filiform muscles, which traverse the liver, and are attached to a muscular riband which, passing above the lungs, runs the whole length of the ventral _parietes_. These muscles when exposed to view resemble extended cords. The abdominal segment preceding the tail is filled with a powerful muscular mass which moves that organ[895]. Treviranus discovered two longitudinal muscles in _Scorpio europæus_, running from the breast to the tail, which above and below each gill were connected by another running transversely across the heart, thus forming a quadrangular area in which the gills are situate[896]. The heart appears to be moved by muscles not very dissimilar to those of the _Cossus_[897], as is likewise that of the _Araneidea_; in _Clubiona atrox_ the wider part of this organ is muscular, and incloses a considerable cavity[898]. In this tribe the muscles of the abdomen, the skin of which is soft and unfit to act as a lever to them, are attached to a cartilage, and thus their action is better sustained[899].

Having thus laid before you all of importance that I can collect with regard to the apparatus of muscles discoverable in insects, I shall next say something upon a few other points connected with that subject. When I enlarged upon their _motions_, I related a few instances of the extraordinary power of that apparatus[900] in _leaping_ ones; but this power is not confined to that circumstance. The _flea_, not more remarkable for its compressed form, enabling it to glide between the hairs of animals, and its elastic coat of mail, by which it can resist the ordinary pressure of the fingers, than for its muscular strength, has attracted notice on this account from ancient times. Mouffet relates that an ingenious English mechanic, named Mark, made a golden chain of the length of a finger, with a lock and key, which was dragged by a flea;--he had heard of another that was harnessed to a golden chariot, which it drew with the greatest ease[901]. Another English workman made an ivory coach with six horses, a coachman on the seat with a dog between his legs, a postillion, four persons in the coach, and four lacqueys behind--which also was dragged by a single flea. At such a spectacle one would hardly know which most to admire, the strength and agility of the insect, or the patience of the workman. Latreille mentions a flea of a moderate size dragging a silver cannon on wheels, that was twenty-four times its own weight, which being charged with powder, was fired without the flea appearing alarmed[902]. Many caterpillars are accustomed to extend their bodies from a twig, supported merely by the four hind feet, in one fixed attitude, either in an oblique, horizontal, or vertical direction, either upwards or downwards, and that for hours together. We may conceive what prodigious muscular force must be exerted upon this occasion, by reflecting that the most expert rope-dancer, though endued with the power of grasping with his feet like a bird with its claws, could not maintain himself in a horizontal position even for an instant. Bradley asserts that he has seen a stag-beetle carry a wand half a yard long and half an inch thick, and fly with it several yards[903]. Some insects have the faculty of resisting pressure in a wonderful degree. If you take a common dung-chafer (_Geotrupes_) in your hand and press it with all your strength, you will find with what wonderful force it resists you; and that you can scarcely overcome the counteraction, and retain the insect in your hand: was it not for this quality, the grub of the gad-fly must be crushed probably in passing through the anal sphincter of the horse[904]. But that of _Eristalis tenax_ affords a more surprising instance of this power of counteraction:--an inhabitant of muddy pools, it has occasionally been taken up with the water used in paper-making, and strange to say, according to Linné, has resisted without injury the immense pressure given to the surrounding pulp[905]; like _leather-coat Jack_ mentioned by Mr. Bell[906], who, from a similar force of muscle, could suffer carriages to drive over him without receiving any injury. Almost as remarkable is the state of extreme relaxation into which the muscles of some larvæ fall, when their animation is suspended; and the revived tension to which a subsequent resumption of the vital powers restores them. Bonnet having suspended the animation of the caterpillar of _Sphinx Ligustri_ by keeping it submerged, squeezed it between his fingers, until it had wholly lost its cylindrical form and was as flat and supple as the empty finger of a glove; yet in less than an hour the very same caterpillar became as firm, as compact, as cylindrical, and in short, as well, as though it had never been submitted to treatment so rough[907].

It is fortunate that animals of a large size, as has been well remarked, especially noxious ones, have not been endowed with a muscular power proportionable to that of insects. A _cockchafer_, respect being had to their size, would be _six_ times stronger than a _horse_; and if the _elephant_, as Linné has observed, was strong in proportion to the _stag-beetle_, it would be able to pull up rocks by the root, and to level mountains[908]. Were the _lion_ and the _tiger_ as strong and as swift for their magnitude as the _Cicindela_ and the _Carabus_, nothing could have escaped them by precaution, or withstood them by strength. Could the _viper_ and the _rattlesnake_ move with a rapidity and force equivalent to that of the _Iulus_ and _Scolopendra_, who could have avoided their venemous bite? But the CREATOR in these little creatures has manifested his Almighty POWER, in showing what he could have done had he so willed; and his GOODNESS in not creating the higher animals endued with powers and velocity upon the same scale with that of insects, which would probably have caused the early desolation of the world that he has made. From this instance we may conjecture, that after the resurrection, our bodies by a change in the structure and composition of their muscular fibre--for we know that their locomotive powers and organs, as far as the muscle is concerned, will then be of a very different nature[909]--may become fitted for motions and a potent agency of which we have now no conception.

This wonderful strength of insects is doubtless the result of something peculiar in the structure and arrangement of their muscles, and principally their extraordinary power of contraction, excited by the extent of their respiration: for animals that respire but little, as the fœtus in the womb and the pullet in the egg, have very little contractile muscular power[910]. To get some idea from facts of this extraordinary contractile power in insects,--extract the sting of a bee or a wasp, with its muscles, which appear to be attached to powerful cartilaginous plates[911], and you will find it continue for a long time to dart forth its spicula, almost as powerfully as when moved by the will of the animal. A still more extraordinary instance of irritability is exhibited by the _antlia_, or instrument of suction of the butterfly. If this organ, which the insect can roll up spirally like a watchspring or extend in a straight direction, be cut off as soon as the animal is disclosed from the chrysalis, it will continue to roll up and unroll itself as if still attached to its head: and if after having apparently ceased to move for three or four hours it be merely touched, it will again begin to move and resume the same action. This surprising irritability and contractility of muscle doubtless depends upon the peculiar structure of the antlia, which is composed of an infinite number of horny rings, acted upon by muscles, more numerous probably than those which move the trunk of the elephant. The motion only ceases when the muscles become dry and rigid.

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I have already, under another head[912], considered the _annual_ sleep, or winter state of torpidity of insects, during which an intermission for the most part of muscular motion and action takes place. I shall now make a few observations with respect to their _diurnal_ sleep, which may very properly have its place in the present letter. That insects, usually so incessantly busy and moving in every direction, require their intervals of repose, seems to call for no proof. We see some that appear only in the _day_, and others only in the _night_, others again only at certain hours; which leads to the conclusion, that when they withdraw from action and observation, it is to devote themselves to rest and sleep. The cockchafer flies only in the evening; but if you chance to meet with it roosting in a tree in the earlier part of the day, you will find it perfectly still and motionless, with its antennæ folded and applied to the breast:--we cannot indeed say that its eyes are shut; for as insects have no eyelids, that sign of sleep can never be found in them. Again, if a Lepidopterist goes into the wood to capture moths in the day-time, he finds them often perched on the lichens that cover the north side of the trunk of a tree, with their wings and antennæ folded, and themselves without motion, and insensible of his approach and their own danger. Thus it was that I captured that rare insect the lobster-moth (_Stauropus Fagi_) in the New Forest. Some, however, have asserted that the caterpillar of the silkworm, except when they moult, never intermits feeding day or night, and consequently does not sleep: but the accuracy of this statement, both from analogy and observation, admits of great doubt. Malpighi informs us that these caterpillars for an hour and more, twice a day, remain immoveable with their heads bent down as though asleep, and even if disturbed, resume again the same inactive posture[913]; and other larvæ in great numbers certainly seem to have regular intermissions from eating of considerable duration: those called Geometers, for hours together remain motionless projected from a twig, to which they adhere by their posterior prolegs alone; and the processionary caterpillars make only _nightly_ sorties from their nests, passing the day in inaction and repose[914]. Bees have been often seen by Huber, when apparently wearied with exertion, even in the middle of the day, to insert the half of their bodies into an empty cell, and remain there, as if taking a nap, without motion for half an hour or longer[915]; and at night they regularly muster in a state of sleep-like silence. Mr. Brightwell once observed an individual living specimen of _Haltica concinna_, which appeared to remain motionless on the same spot of a wall for three successive days.