518 C. GORDON HEWITT. 
the cesophagus (fig. 17), is controlled by two sets of muscles. 
Two pairs of elongate oblique muscles (e. 0. m.) are attached 
dorsally to the dorsal edges of the lateral plates (J. 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.m.), 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 
cesophagus. 
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 cesophagus. 
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 
