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 STEUCTUKEj 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 (ra. 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.) is 

 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. Their 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 

 (o. ph.) arranged in pairs. The muscles are attached dorsally 

 to the inside dorsal edges of the lateral plates (1. 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 



