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.