940 



INSECTA. 



phoses in the position of parts of the tegu- 

 mentary skeleton, to which the muscles aie 

 attached. Hence the direction in which these 

 muscles are now required to act is also changed, 

 and from constituting, as in the larva, 01 ly 

 one continuous series of uniform muscles, 

 acting in one direction, in the perfect instct 

 they become muscles that act in several direc- 

 tions, at different angles of the body, and in 

 some parts exceed in importance and size 

 every other division of the muscular system. 

 Thus, in the thoracic region, the dorsal mus- 

 cles, which were parts least employed in the 

 larva, are those which are of the greatest im- 

 portance in the perfect insect, both as regards 

 size and function. In the larva, as we have 

 seen, locomotion depends chiefly upon the ab- 

 dominal recti ; but in the perfect insect, on the 

 contrary, nearly the whole of this power is 

 transferred to the dorsal muscles of the thorax. 

 Hence the arrangement of these muscles is 

 more or less intricate, and differs in different 

 classes, according to the habits of the insects. 

 Thus, in those classes in which the prothorax 

 is short, and almost or entirely anchylosed to 

 the meso-thorax, as in the Hymenoptera, Lepi- 

 doptera, and Diptera, and in which, conse- 

 quently, scarcely any motion of the prothoracic 

 segment is required, the muscles become al- 

 most entiiely atrophied and cease to exist, or, 

 as is sometimes the case, their attachments are 

 transferred to a different part of the tegumen- 

 tary skeleton. 



The most generally developed form of the 

 muscular system of the thorax is found in the 

 Coleoptera, of which Straus has given so admira- 

 ble an illustration in his anatomy of Melolontha. 

 It is from his description of the muscles of that 

 insect that we shall chiefly derive our general 

 description of these parts in perfect insects. 

 We shall, however, for the sake of uniformity, 

 adopt the nomenclature applied to these parts 

 by Burmeister, identifying it with the names 

 originally employed by Straus. 



The muscles that connect the head with the 

 thorax are contained within the prothorax (fig. 

 402,2), and are of three kinds, extensors,jiexors, 

 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 oppo- 

 site side, passes directly forwards and is in- 

 serted 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 pro-sternum, and are attached 



Fig. 402. 



Section of the body of Melolontha. (Straus.) 



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 ante-furca and the other from 

 the posterior jugular plate, and passing up- 

 wards 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 ante-furca, 

 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 (/) 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 



