518 C. GORDON HEWITT. 



the oesophagus (fig. 17), is controlled by two sets of muscles. 

 Two pairs of elongate oblique muscles (e. o. m.) are attached 

 dorsally to the dorsal edges of the lateral plates (1. p.) and 

 inserted ventrally on to the roof of the pharynx ; these muscles 

 assist the previously described oblique pharyngeal muscles in 

 raising and depressing the roof of the pharynx. They are 

 assisted in enlarging and contracting the lumen of the pos- 

 terior part of the pharynx by a number of semi-circular dorsal 

 muscles (s. d. ra.), which by their contraction make the floor of 

 the pharynx more concave, and it is these muscles, I believe, 

 that are chiefly concerned in the maintenance of the peri- 

 staltic contractions of the pharynx, by means of which the 

 fluid food, which has been sucked into the mouth by the 

 pumping action of the pharynx, is carried on to the 

 oesophagus. 



The similarity between the pharyngeal apparatus of the fly, 

 that is, of the fulcrum and that of the larva, is very striking, 

 both with regard to the form of the skeletal structures and 

 the musculature. If the pharynx of the larva were regarded 

 as being homologous to that of the fly it would further support 

 the view that the head of the larva had been permanently with- 

 drawn into the succeeding anterior body- segments. These 

 structures, however, may be merely analogous ; the similarity 

 of structure may have been brought about by similarity of 

 function. Both larva and adult subsist on fluids which are 

 sucked into the mouth and pumped into the oesophagus. 



The series of muscular actions which takes place during 

 locomotion appears to be as follows. By the contraction of 

 the pharyngeal protractors the anterior end of the larva is 

 extended, the mandibular sclerite being extended at the 

 same time by the contraction of the mandibular extensor 

 muscles. The mandibular sclerite is now depressed by the 

 contraction of the mandibular depressors, and anchors the 

 anterior end of the larva to the substance through which it 

 is moving. A series of segmental linear contractions now 

 takes place, initiated by the large cephalic retractor muscles, 

 and carried on posteriorly from segment to segment by the 



