INSECTA. 



939 



Fig. 401. 



Internal view of spiracle of larva of Sphinx ligustri. 



(Neivport, Phil. Trans.) 



a, anterior margin of spiracle with portion of the 

 trachea; b, the valve; c, retractor valvulae ; e, 

 retractor spiraculi ; d, nerve supplying these mus- 

 cles. 



cular orifice, and acts upon the internal valve 

 (b), which is situated a little within the spiracle. 

 This valve is a thick, moveable, dark-coloured 

 duplicature of the lining membrane of the 

 posterior border of the spiracle, and closes 

 on that of the opposite side (a), which is a 

 concave crescent-shaped margin, not acted 

 upon by muscles like a cushion or pad. 



These muscles of the larva of the Sphinx 

 differ but little from those described by Lyonet 

 in the Cossus. In all insects they give pas- 

 sage between them to the ramifications of tra- 

 cheal vessels, which are most extensively dis- 

 tributed throughout the whole body, to every 

 muscle, nerve, or other organ. They are also 

 covered in many places by numerous con- 

 nected vesicles filled with adipose matter, 

 which exist in the greatest abundance in the 

 larva state in all insects, occupying the inter- 

 stices between the muscles and tracheae. The 

 same general structure of the muscular system 

 as that which we have just described in the 

 Sphinx exists in all larvae that undergo a com- 

 plete metamorphosis, whether they belong to 

 the Coleopterous, Hymenopterous, or Lepi- 

 dopterous classes, although in the particular 

 distribution and form of the muscles in each 

 there are necessarily some differences depen- 

 dent upon difference of species and habit. 

 Thus Burmeister found a similar general con- 



formation of parts in the larva of Calosoma 

 iycophantu* (jig. 354), one of the more per- 

 fect Coleoptera, both in the existence of the 

 rudiments of muscles for the wings and in the 

 longitudinal muscles of the dorsal and ventral 

 surfaces of the body. The muscles of the 

 larvae of Coleoptera, as Burmeister has re- 

 marked, bear a greater resemblance to the 

 muscles of the perfect insects than those of the 

 larvae of other classes. It is not difficult to 

 recognise in them the same general arrange- 

 ment of particular muscles which are after- 

 wards found in a more or less developed state 

 in the perfect insects. An admirable exem- 

 plification of the muscular system of Coleop- 

 tera is given by Straus Durckheim in his 

 splendid work on the anatomy of Melolontha 

 vulgaris, in which many of the muscles that 

 exist in the larva state may be distinctly iden- 

 tified, although greatly modified in form and 

 size to fit them for new modes of action, which 

 have been rendered necessary by the changes 

 that have taken place in the habits and modes 

 of life of the insect. Thus, as also remarked 

 by Straus,f the great ventral series of recti 

 muscles which we have just seen in the larva, 

 form successively the retractor muscles of the 

 labium, the depressors of the head, the re- 

 tractors or depressors of the pro-sternum, or 

 those which draw that part to the meso-sternum, 

 and the pretractors of the post-furca or trian- 

 gular process of the metasternum ; and, lastly, 

 the inferior recti muscles of the abdomen. But 

 in each of these instances the size and form of 

 the muscles are greatly altered, more especially 

 in the thoracic region, while in the abdominal 

 region those of the posterior segments exist 

 with less change of form than in the thoracic, 

 but are greatly reduced in size, and those of 

 the anterior abdominal segments, in which the 

 ventral plates of two or more segments have 

 become consolidated together, are atrophied 

 and have almost disappeared. In like manner 

 the dorsal recti of the larva exist in the imago 

 in the new form of elevators of the head, 

 superior retractors or elevators of the prothorax 

 and scutellum, and levators, depressors, and 

 adductors of the wings and dorsal longitudinal 

 recti of the abdomen. In the latter region 

 neither their form nor direction have been 

 changed, but like the ventral recti they have 

 been much reduced in size, because there is less 

 necessity for their active employment in the 

 perfect than in the larva state, in which nearly 

 the whole of the locomotive powers of the in- 

 dividual are entirely dependent upon those 

 muscles. Their form and direction have not 

 been changed because the direction in which 

 they are employed in the perfect state is pre- 

 cisely similar to that in which they are em- 

 ployed in the larva. But this is not the case 

 in the thoracic region, in which not only have 

 they been enormously increased in size and 

 changed in form, but their relative position 

 has also been altered, owing to the changes 

 that have taken place during the metamor- 



* Trans. Entom. Society, Lond. vol. i. p. 335. 

 t Considerat Generales, p. 149. 



