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!
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.