936 



INSECTA. 



segments that follow the head, and form the 

 thorax of the future imago, are more numerous 

 and complex than those of the abdomen, 

 because unto those segments belong the mus- 

 cles of the proper organs of locomotion ; be- 

 sides which they contain also the rudiments of 

 the muscles for the future wings. The muscles 

 in the abdominal segments are fewer and far 

 more simple than in the anterior part of the 

 body, but their number, even in these, very far 

 exceeds what at first might be expected. So 

 numerous are they in every segment that 

 Lyonet, in his immortal work on the anatomy 

 of the larva of Cossus ligniperda, found two 

 hundred and twenty-eight distinct muscles in 

 the head alone, and, by enumerating the fibres 

 in the layers of the different segments, reck- 

 oned one thousand six hundred and forty-seven 

 for the body, and two thousand one hundred 

 and eighteen for the internal organs, thus 

 making together four thousand and sixty-one 

 muscles in a single larva. In the larva of 

 Sphinx ligustri we have found the muscles 

 equally numerous with those discovered by 

 Lyonet in the Cossus, but in attempting to 

 describe them it has appeared preferable, as 

 we have stated, to consider each layer of fibres 

 collectively as a separate muscle. In describ- 

 ing the muscles of the ventral portion of a 

 segment, we formerly* ventured to designate 

 them by names which were indicatory either of 

 their position or use, and we shall continue to 

 do so on the present occasion. A description 

 of the muscles of a portion of a segment will 

 suffice to convey some idea of their multiplicity 

 and use. We may first state generally that 

 those muscles which form distinct layers or act 

 in concert with each other, are inserted into 

 slightly elevated ridges of the tegument, while 

 a single muscle, or the tendon of many mus- 

 cles united together, is attached to an elevated 

 process of the tegument, which at that point is 

 thicker than in other places, and thus affords 

 a means of attachment. There are always 

 three ridges for the attachment of muscles be- 

 tween two abdominal segments. The middle 

 one is the largest, and affords both origin and 

 insertion to the straight or longitudinal muscles, 

 while the others in like manner afford origin and 

 insertion to the oblique ones. 



On removing the fat and viscera from the 

 abdomen of the larva, the first layer that pre- 

 sents itself, and forms the interior parietes of 

 the body, consists of many longitudinal fibres, 

 which extend from the margin of one segment 

 to that of another as flat, straight muscles, 

 resembling the recti abdominales of vertebrated 

 animals. These muscles extend from the an- 

 terior margin of the sternal surface of the 

 second segment to the posterior part of the 

 twelfth ; but it is only at the anterior margin 

 of the sixth segment, which is in reality the 

 commencement of the true abdomen, that they 

 can properly be considered as recti muscles, 

 since it is at this part of the body that they 

 begin to be fully developed. While passing 



* Phil. Trans, part ii. 1836. 



through the thoracic segment they are nar- 

 rower, thinner, and somewhat differently ar- 

 ranged. They are connected anteriorly with 

 the head, and posteriorly with the sphincters. 

 They are the most powerful of all the muscles 

 of the abdomen, and are those which are most 

 concerned in shortening the body, and effecting 

 the duplicature of the external teguments, 

 during the changes of the insect. They are 

 also those which mainly assist in locomotion 

 during the larva state. There are four sets of 

 these longitudinal muscles, two on the dorsal 

 and two on the ventral surface of the body 

 (Jig. 400, A A). Those on the dorsal surface 

 are placed one on each side of the dorsal vessel 

 or heart, and those on the ventral one on each 

 side of the nervous column. The dorsal sets 

 extend from their attachment to the upper part 

 of the head through the thorax and abdomen 

 to the anus, in the thirteenth segment. In the 

 thoracic region they are narrow like the cor- 

 responding muscles of the ventral surface, but 

 when the insect is undergoing its changes they 

 become enormously enlarged in this region, 

 and form the great depressor muscles of the 

 wings (jig. 402, a 1 ), which are some of the 

 most powerful muscles of the thorax, and ex- 

 tend between the meso-andmeta-phragma. The 

 ventral recti consist of four sets of fibres, two 

 on each side of the nervous cord (fig. 400, 

 1 , 2), and between which there is a slight in- 

 terspace. That set which is placed nearest to 

 the nervous cord and median line of the 

 body, is composed of only three narrow fas- 

 ciculi of fibres, and may be called the recti 

 minores (2), while the other set, situated more 

 externally and covering the greater portion of 

 the ventral surface of the segment, is broad 

 and powerful, and consists of from twenty to 

 twenty- five distinct fasciculi or fibres, and may 

 be called the recti majores (J). The origins 

 and insertions of these are different from those 

 of the smaller recti. The recti majores of one 

 segment arise from the middle ridge between 

 two segments (3), and are inserted close to the 

 origin of the corresponding muscles of the 

 next segment, while the recti minores arise 

 from the most posterior of the three ridges, 

 about one-fifth of a segment posterior to the 

 middle ridge, over which they pass, and pro- 

 ceed in a direction parallel to the larger ones 

 to be attached to part of the corresponding 

 ridge in the next segment. There is a small 

 muscle that originates from the same ridge as 

 the greater rectus, between it and the smaller, 

 which, from its passing directly to the alimen- 

 tary canal, and connecting that viscus to the 

 exterior tegument of the body, may be called 

 the retractor ventriculi (5). There is one of 

 these muscles, as shown also by Lyonet in the 

 Cossus, on each side of the nervous cord, from 

 the fourth to the eleventh segment. On re- 

 moving the recti, we expose two layers of very 

 fine thin muscles. The upper layer (B) con- 

 sists of nine distinct fasciculi of fibres, which 

 pass backwards and outwards, in a slightly 

 diagonal direction (6), but less diagonally than 

 the second layer (7), that lies immediately 



