STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 517 
of the median line and between the dorsal cephalo-pharyngeal 
protractors. They are inserted on to the dorsal side of the 
mandibular sclerite (m. s.); by their contraction they elevate 
the sclerite. This sclerite is depressed by the contraction of 
a pair of muscles which control the dentate sclerites (d. s.), 
the latter fitting into a notch on the ventral side of the mandi- 
bular sclerite. The mandibular depressor muscle (m. d.) 18 
attached to the posterior ventral process of the lateral 
pharyngeal sclerite by the three bands into which the posterior 
portion of the muscle is divided; the anterior and single end 
of the muscle is inserted on the ventral process of the dentate 
sclerite. Four pairs of muscles (s. d.) are inserted on the 
hypostomal sclerite (h. s.). Two more dorsal pairs are 
attached to the intersegmental ring between segments three 
and four as shown in fig. 16. The two more ventral pairs are 
attached to the lateral pharyngeal sclerites, one being attached 
to the ventral side of the posterior dorsal process and the 
other to the ventral process beneath the mandibular depressor. 
These muscles, which I call the stomal dilators, are inserted 
on the sides of the hypostomal sclerite. heir function is, I 
believe, to open and close the anterior pharyngeal aperture 
and so control the flow of fluid food into the pharynx and of 
the salivary secretion; the lowest pair of muscles may be more 
directly concerned with the latter. 
The pharyngeal apparatus is controlled, as in the adult fly, 
by a series of muscles. In the larval stadium, however, where 
so large an amount of food is required for the growth and 
building up of the future insect, there is a greater development 
and elaboration of the pharyngeal apparatus, including the 
muscles. In the greater anterior region of the pharynx, that 
is, the part lying within the pharyngeal sclerites (fig. 18), the 
muscular system consists of two bands of oblique muscles 
(0. ph.) arranged in pairs. The muscles are attached dorsally 
to the inside dorsal edges of the lateral plates (J. p.) and 
ventrally to the roof of the pharynx (r. ph.), the ventral attach- 
ment being more posterior than the dorsal. The posterior 
region of the pharynx, which is between the lateral plates and 
