THE MUSCULAR SYSTEM
In its general arrangement the muscular system of insects corresponds to the segmented structure of the body. Of the muscles belonging to a single segment, some extend from the front edge of one segment to that of the next behind it, and others to the hinder edge; there are also sets of dorsal and ventral muscles passing in an oblique or vertical course (Figs. 16–18). As Lang observes, “the greater part of the muscles of the body can be traced back to a paired system of dorsal and ventral intersegmental longitudinal muscles.” The muscular system is simplest in larval insects, such as caterpillars, where the musculature is serially repeated in each segment.
In the larva of Cossus Lyonet found on one side of the body 217 dorsal, 154 lateral, 369 ventral, and in the thoracic legs 63, or 803 muscles in all. “Adding to this number the 12 small muscles of the second segment, and 8 others of the third, which he did not describe, there would be for all the muscles on one side of the caterpillar 823. This would make for the entire body 1646, without counting a small single muscle which occurs in the subdivision of the last segment,” and also those of the internal organs as well as those of the head, so that the total number probably amounts to about 2000, not 3000, as usually stated in the books. Lubbock admits that Lyonet was right in his mode of estimating the number. In the larva of Pygærci bucephala he found that “the large muscles scarcely vary at all,” though certain smaller ones are very variable. Lubbock observed that certain of the longitudinal muscles in the caterpillar of Diloba split up into numerous, not less than ten, separate fascicles. “This separation of the fibres composing a muscle into separate fascicles is carried on to a much greater extent in the larvæ of Coleoptera. Of course in the imago the number of thoracic muscles is greatly increased, or at least in Dyticus and the wood-feeding Lamellicorns, which alone I have examined. In these two groups each of the larger muscles is represented by at least twenty separate fascicles, which makes it far more difficult to distinguish the arrangement of the muscles.”
The muscles are whitish or colorless and transparent, those in the thorax being yellowish or pale brown; and of a soft, almost gelatinous consistence. In form they are simply flat and thin, straight, band-like, or in rare cases pyramidal, barrel or feather shaped. They act variously as rotators, elevators, depressors, retractors, protractors, flexors, and extensors.
Fig. 231.—Diagram of the muscles and nerves of the ventral surface of the segments in the larva of Sphinx ligustri: A, A, recti muscles; 1, 2, ventral recti muscles (1, recti majores; 2, recti minores); 3, ridge giving origin to recti muscles of one segment, and insertion to the same of the adjoining segment; 4, ridge for attachment of muscle; 5, retractor ventriculi, connecting the mid-intestine with the outer integument of the body. B, 6, first oblique,—7, second oblique,—9, 10, third oblique, muscles; 11, fourth oblique,—12. third rectus,—13, fifth oblique,—14, triangularis, muscle; 15, transversus medius; 16, transverse ridge; 17, transversi abdominales; 18, abdominales anteriores; 19, 20, abdominales laterales, some (20) longer than others; 21, obliquus posterior; 22, postero-laterales obliqui; 23, transversus lateralis; 24, second transversus lateralis; 25, retractor spiraculi, or constrictor of the spiracles, attached by a long tendon (26); 27, retractor valvulæ.
Nerves: a, ganglion,—c, transverse nerves, of which p is the first, q the second, r the third,
and s the fourth branch; t, the main trunk, which crosses the great longitudinal trachea, receives a
filament from the transverse nerve (n), and divides into two branches (t);—some of these branches
form a small plexus (u); the nerve t divides in two divisions (p and v). The second division ends
in w and x; the branch q divides into y and z. For other explanations, see Newport, art. Insecta.—After
Newport.
Fig. 232.—Musculature of the European cockchafer, Melolontha vulgaris: a, a, levatores capitis; b, depressores capitis; c, rotatores capitis; d, depressors externi; e, retractor or flexor of the jugular plate; f, oblique extensor of the jugular plate; g, the other retractor of the jugular plate; h, retractor prothoracis superior; i, inferior retractor, the proper depressor of the prothorax; k, elevator prothoracis; l, one of the rotatores prothoracis; m, n, o, flexors of the coxa; x, great depressor muscle of the wing; y, y, elevators and protractors attached to the metaphragma and base of the postfurca; z, second flexor of hind leg; a, a, extensors of hind leg; c, c, dorsal recti of abdomen. Q, ejaculatory duct; R, penis; S, its prepuce. M, rectum.—After Straus-Durckheim, from Newport.
Our knowledge of the muscular system of insects is still very imperfect. To work it out thoroughly one should begin first with that of Scolopendrella, then some generalized synapterous form, as Japyx or Lepisma, then passing to that of a caterpillar, and ending with some of the more highly specialized forms, such as a beetle, etc. Thus far our knowledge is confined to that of the caterpillars (Lyonet, Newport, and Lubbock) and the beetle (Straus-Durckheim) and ants (Forel, Lubbock, and Janet).
Musculature of a caterpillar.—Newport’s account of that of the larva of Sphinx ligustri is the most useful (Fig. 231). The muscles here present, he says, great uniformity of size and distribution in every segment, the motions of each of these divisions of the body being almost precisely similar, especially in the 4th to 9th trunk segments. In these segments the first layer seen on removing the fat and viscera are the flat straight recti muscles. They are the most powerful of all the trunk muscles, and are those which are most concerned in shortening the body, in effecting the duplicature of the external teguments during the changes of the insect, and which during the larval state mainly assist in locomotion. There are four sets, two dorsal and two ventral (Fig. 231, A, A). Without entering into farther details, the reader is referred to the works of Newport and to Fig. 231.
Musculature of a beetle.—The best general account of the musculature of a perfect insect is that of Straus-Durckheim in his famous work on the Melolontha. We will copy the summary of Newport, who adopted the nomenclature applied to these parts by Burmeister:—
“The muscles that connect the head with the thorax are contained within the prothorax (Fig. 232, 2), and are of three kinds, extensors, flexors, and retractors. The extensors, levatores capitis (a, a), consist of two pairs, one of which arises from the middle line of the pronotum, and diverging laterally from its fellow of the opposite side, passes directly forwards, and is inserted by a narrow tendon into the anterior superior margin of the occipital foramen. The other arises further back from the prophragma. It is a long, narrow muscle that passes directly forwards through the prothorax, and is inserted by a tendon near the superior median line of the foramen; so that, while this muscle and its fellow of the opposite side elevate the head almost in a straight line, the one first described, when acting alone or singly, draws the head a little on one side; but when the whole of these muscles act in unison, they simply elevate the head upon the prothorax. The depressors or flexors, depressores capitis (b), are exceedingly short muscles, which arise from the jugular plate, or, when that part does not exist, from the border of the prosternum, and are attached to the inferior margin of the occipital foramen. They simply flex the head on the prothorax. The lateral flexors, depressores externi (d), are two little muscles that arise from the same point as the preceding, and are attached to the lateral inferior margin of the occipital foramen. The rotatory muscles, rotatores capitis (c), are two flat muscles like the elevators, which arise, one at the side of the antefurca and the other from the posterior jugular plate, and passing upwards and outwards are attached to the lateral margin of the occipital foramen. The retractor or flexor of the jugular plate is a small muscle (e) that arises from the margin of the antefurca, and passing directly forwards is inserted by a small tendon into the middle of the jugular piece. The oblique extensor of the jugular plate is a long, slender muscle (f) that arises from the external margin of the pronotum, and passing obliquely downwards and forwards traverses the prothorax and is inserted by a narrow tendon to the jugular plate immediately before the retractor. The other retractor (g) arises from the anterior superior boundary of the pronotum, and passing downwards is inserted into the jugular plate between the larger levator and flexor capitis.
“The muscles proper to the prothorax consist of four pairs, by which it is united to the succeeding segments. The first of these, the superior retractor, retractor prothoracis superior (h), arises by a broad, fleshy head from the anterior external margin of the pronotum, and passing directly backwards is inserted by a tendon into the prophragma, a little on one side of the median line. The next muscle of importance, the inferior retractor (i), arises from the anterior border of the medifurca, and is united to the posterior of the antefurca, thus forming with that muscle part of the great recti of the larva. This muscle must be considered as the proper depressor of the prothorax. The elevator prothoracis (k) is narrow, pyramidal, and arises fleshy from the lateral surface of the prophragma. It passes downwards and is attached by a narrow tendon to the superior portion of the antefurca. The rotatores prothoracis are the largest of all the muscles of this segment. They arise, one on each side (l), by a narrow head from the posterior part of the pronotum, and passing beneath the prophragma are considerably enlarged and attached to the tegument between the two segments, and also to the anterior portion of the mesothorax. The remaining muscle proper to the prothorax is the closer of the spiracle, an exceedingly small muscle not shown in the drawing.
“The other muscles of this segment are those of the legs, which are of considerable size. There are three distinct flexors of the coxa (m, n, o). The first of these arises from the superior lateral border of the pronotum, the second from the superior posterior border, the third from the sides of the prothorax, and the fourth a little nearer posteriorly, and the whole of them are attached by narrow tendons to the sides of the coxa. But there is only one extensor muscle to this part. In like manner, the extensor of the trochanter is formed of three portions (Fig. 233, a, b, c); but there is only one flexor (d), and one abductor (e). In the femur, there is one extensor (f),—a long penniform muscle that occupies the superior part of the thigh, and is attached by a tendon to the anterior-posterior margin of the joint formed by the end of the tibia. There is also but one flexor (g) in the femur, which, like the preceding muscle, is penniform, and occupies the inferior portion of the femur, and its tendon is attached to the inferior border of the tibia. In the tibia itself there is also one flexor and one extensor. The flexor (i) occupies the superior portion of the limb, and ends in a long tendon (l) that passes directly through the joints of the tarsus, on their inferior surface, and is attached to the inferior margin of the claw (g). The extensor (h) occupies the inferior portion of the tibia, and is shorter than the preceding muscle, like which it ends in a long tendon that is attached to the upper margin of the claw. Besides these muscles, which are common to the joints of the tarsus, there are two others belonging to the claw, situated in the last joint. The first of these, the extensor (m), is short, and occupies the superior portion of the last phalanx of the tarsus, and the other, the flexor (n), is a much longer penniform muscle, which occupies nearly the whole of the upper and under surface of the posterior part of the phalanx, and is attached, like the long flexor of the tarsus, to the inferior part of the claw.”
Fig. 233.—Muscles of the fore leg of Melolontha vulgaris: a, b, c, three divisions of the extensor of the trochanter; d, flexor,—e, abductor, of the trochanter; f, extensor of the femur; g, flexor of the femur; h, extensor of the tibia; i, flexor of the tibia; l, tendon attached to the lower edge of the claw (g); m, extensor,—n, flexor, of the claw.—After Straus-Durckheim, from Newport.
These are the muscles of the prothorax, and its organs of locomotion. The reader is referred for a further account of the muscles of the hinder thoracic and of the abdominal segments to Straus-Durckheim’s original work.
Minute structure of the muscles.—The muscular fibres of insects are striated (Figs. 235–238), even those of the alimentary canal; the only notable exception being the alary muscles of the pericardial septum, while Lowne states that certain of the thoracic muscles of the blow-fly are not striated (Miall and Denny).
Fig. 234.-Section through the prothorax of Diapheromera femoratum: prov, proventriculus; tr, trachea; n. c, nervous cord; s. gl, salivary gland; hyp, hypodermis; ur. t, urinary tube; ht, heart; m, m″, m‴, muscles for lowering and raising the tergum; m′, another muscle, its use unknown.
Fig. 235.—Striated muscular fibre of Hydrophilus: A and B, two fibrillæ in a state of extension; a, thick disk; b, thin disk; c, intermediate space. C, D, portion of the same fibrillæ seen by moving the objective farther away and using a small diaphragm; n, thick; c, thin disk. × 2000 diam.—After Ranvier, from Perrier. E after Gehuchten, from Lang.
In describing the minute structure of the muscles of ants, wasps, and bees, C. Janet states that each consists of a group of fibres diverging from a tendon, which is an integumentary invagination (Fig. 236). Each fibre may be regarded as a multinucleate cell; the sarcolemma represents the cell-membrane. It forms a resistant and extremely elastic tube. The longitudinal (Fig. 236, E) and radiating filaments or reticulum (spongioplasm of Gehuchten) lie in a nutritive filling substance (the hyaloplasm of Gehuchten). The radiating filaments are formed of an exceedingly elastic substance, and serve to sustain the longitudinal filaments, to transmit the nervous stimulus to them, and to bring them back into position after contraction. Janet’s account agrees on the whole with that of Gehuchten.
Fig. 236.—Preparations from the adductor muscle of the mandible of Vespa crabro, worker, fixed by heat and alcohol several hours after leaving its cell. A to E × 425; F × 212: A, terminal cupule of the tendon of a fibre. B, C, union of the fibres with their tendon. D, branch of the tendon of a muscle sending out tendons of some of the fibres; this branch is accompanied with numerous nervous ramifications (N). E, fragment of a nerve which furnishes the ramifications of Fig. D. F, fragment of the tendon of the adductor muscle of the mandible; at the left are seen the terminal cupules of the fibres (td, c); on the right, on the body of the tendons, some sessile cupules, each of which forms the attachment of a fibre; td, b, tendons of the fibres.—After Janet.
The muscles of flight are said to be penetrated by fine tracheal branches, probably to supply a greater amount of oxygen, as the most energetic movements of the insect are made in moving the wings during flight; while the other muscles of the body are only surrounded by the air-tubes. (Sharp.)
Without entering into tedious details, the reader is referred to figures or references to the more important systems of muscles, such as those of the legs and other appendages, of the wings, of respiration, etc., to the sections treating of those organs or functions; also to Figs. 16, 17, 18, 22, 48, 74, 81, 83, 84, 115, 116, 172, 173, 174, etc.
Muscular power of insects.—The most detailed and careful experiments are those of Plateau. His experiments prove that even the weakest insects pull at least five times their own weight; many of them, however, get the better of a burden twelve to twenty fold as heavy as themselves, while a strong man or a draught horse, for example, is not even able to pull a burden which is equal to the weight of his body. Plateau came to the following results as to the relation of the weight of the body to the load drawn (1 and 2 are to be compared with each other, 1 being the larger, and 2 the smaller insect; it will be seen that the smaller insect is the stronger).
Fig. 237.—Vespa crabro, worker, fixed by heat and alcohol some hours after leaving its cell. A × 425; B to D × 850 times: A, muscular fibre of the motor muscles of the mandibles treated, for ten minutes, by 1 per cent potassium to bring out the reticulum; the nodes of union of the rayed filaments with the longitudinal filaments are indicated by distinct granulations (l.d), and these longitudinal filaments present accessory thickenings (d.a); T, trachea; N, junction of a nervous filament with the muscular fibres. B, fibre of the same muscle, not treated with potassium, stained by hæmatoxylin; C, transverse section of a disk at the level of a layer of rayed filaments; Sarc, sarcolemma. D, transverse section of a disk at the level of the rods; nuc, nucleus.—After Janet.
1. Carabus auratus 17.4.
2. Nebria brevicollis 25.3.
1. Cetonia aurata 15.
2. Trichius fasciatus 41.3.
1. Melolontha vulgaris 14.3.
2. Anomala frischii 24.3.
1. Oryctes nasicornis 4.7.
2. Geotrupes stercorarius 9.8.
3. Onthophagus nuchicornis 14.4.
1. Necrophorus vespillo 15.1.
2. Silpha livida 24.4.
1. Ocypus morio 17.
2. Quedeus fulgidus 29.6.
1. Donacia nymphææ, 42.7.
2. Crioceris merdigera 39.2.
1. Bombus terrestris 16.1.
2. Bombus rupestris 14.5.
3. Apis mellifica 20.2.
As regards the pushing power, the relation of the load to the size of the body in different large beetles, gave the following figures:—
Oryctes nasicornis 3.2.
Geotrupes stercorarius 28.4.
Onthophagus nuchicornis 92.9.
The leaping force of locusts was found by Straus-Dürckheim to be in Œdipoda grossa as 1.6, in Œ. parallela as 3.3 of their weight.
Fig. 238.—Vespa crabro, fixed and stained as in the subjects of the other figures. I, N, P × 1700; H, J, M × 850; the others × 425 times: A-C, motor muscles of the antennal scape. D-P, motor muscles of the 3d coxa. A, B, the two ends, in very different states of contraction, of the same fibre; on one side the transverse striæ are near together, on the other very far apart. C, a crushed and split fibre showing a fibrous appearance, owing to the rupture of the radiated filaments, and the separation of the longitudinal filaments. D, muscular disk seen in section, with two rows of nuclei. E, a muscular fibre with three rows of nuclei. F, a nucleus, accompanied with coagulated protoplasm, oozing from a previous break of the muscular fibre. G, nerve-terminations very near each other on the same muscular fibre. H, longitudinal filaments, evenly covered with the coagulated substance, and forming, throughout the mass of the fibre, continuous filaments. I, filaments widely separated. J, longitudinal filaments showing the beginning of one of the transverse breaks which isolate some of the disks. K, oblique view of a disk obtained by such a break, and of a fibre in circular section, with an axial row of nuclei; this piece comprises three stages of radiated filaments. L, muscular fibre with a row of nuclei; at the lower part, the nuclei have issued from a longitudinal fissure in the fibre, and have remained attached in a chain. M, edge of fibre in which there is quite a large, clear space between the sarcolemma and the rods. N, passage of the trachea, with the spiral thread, into three capillaries with a smooth cuticula. O, elliptical disk from a fibre, with two rows of nuclei, and showing a layer of radiated filaments. P, fragment (highly magnified) of the edge of a disk seen in section.—After Janet.
A humble bee (Bombus terrestris) can carry while flying a load 0.63 of its own weight, and a honey bee 0.78; here, as usual, the smaller insect is the stronger.[[39]]