399 



The muscular system consists of three prominent sets of 

 muscles : the great longitudinal muscles ; the great transverse 

 (oblique) muscles; and the masticatory muscles, including the 

 dilators of the ph'arynx. 



The longitudinal muscles (fig. 1) are in the form of twenty 

 to twenty-two bands of muscles, passing from one end of the 

 body to the other. Posteriorly where the body tapers off, they 

 all tend to converge towards one point. Anteriorly they are 

 inserted upon the walls of the first larval segment; here also 

 they converge, but are less concentrated than at the hinder 

 end. The muscles are arranged similarly on either side of 

 the median line, and are quite absent beneath the ventral 

 nerve cord. 



The transverse (oblique) muscles (fig. 1) are in the form 

 of nine pairs of muscles stretching from the third to the 

 eleventh segments. They pass in hoops round the body of the 

 larva, upwards and backwards; their lower and upper inser- 

 tions are generally at the junctions of the larval segment with 

 the segments immediately before and behind it respectively, 

 i.e., the oblique muscles are generally intra-segmental. This 

 is not, however, entirely the case, as the last oblique muscle 

 is inserted ventrally on the ninth segment, dorsally at the 

 posterior limit of the tenth. Others also stretch over more 

 than one segment. 



The muscles of feeding are in the form of a number of 

 structures which are inserted upon the pharynx at one end, 

 while their other extremity is attached to the walls of the 

 head, or to a specially thickened cuticular portion of it — the 

 tentorium. They are the dilators of the pharynx. To the 

 tentorium are attached also' two very minute muscles 

 which move the minute jaws. Only one muscle is attached 

 to each mandible, the latter evidently swinging backwards, 

 after functioning, as a result of the elasticity of the surround- 

 ing cuticle. There are six pairs of pharyngeal dilators. Of 

 these the lower two are inserted upon the tentorium. Two 

 other pairs, attached to the dorsal side of the pharynx, are 

 inserted upon the dorsal head cuticle ; while two other pairs 

 radiate outwards towards the lateral head walls. The united 

 pull of these muscles during feeding would dilate the pharynx 

 considerably and would permit efficient sucking of the con- 

 tents of the fly pupa, once the mouth was applied to the 

 ruptured cuticle. In yonng larvae the dilators of the pharynx 

 exhibit a thick dilatation along a considerable part of one 

 side. This swelling becomes less prominent as the larva 

 grows, but is recognizable even in adult larvae. Its nature will 

 be explained later. 



