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.