44 MEDICAL ENTOMOLOGY 



other cases the rings develop a median prominence, directed proximally, 

 and so appear T-shaped. The most proximal of these modified rings is 

 then attached to the prestomum. 



At the margin of the prestomum, between the terminations of 

 the pseudotracheae, there is a series of minute tooth-like processes, 

 which, though of little importance in the economy of 

 Musca, are of great interest as being the homologues 

 of the powerful biting teeth of the blood-sucking 

 forms. They differ in number "and arrangement in closely allied 

 species, but preserve a general similarity. Each tooth is an elongate slip 

 of thin chitin, broader at the distal end than proximally, and with 

 a finely serrated but extremely thin margin. The lateral borders are 

 thickened into rods, which articulate with the edge of the discal 

 sclerite. In Musca domestica (Plate IX, fig. 5) there are three such 

 rows of teeth, the lateral borders of those of the two distal rows separat- 

 ing from one another and gaining attachment to the membrane between 

 the terminations of the pseudotracheae, wiiile those of the proximal row 

 articulate with the sclerite. There are usually four to six teeth in each 

 row, at each side. 



The articulation between the labium and the labella is divided into 

 two joints, one anterior and the other posterior. Their importance from 

 our present point of view is that they are the homo- 



logU6S f the J intS n which the biting a PP aratus of 

 the blood-sucking forms acts. 



The anterior joint is formed by the articulation of the discal sclerite 

 with the rods which form the anterior boundary of the labial gutter. 

 The discal sclerite of Musca is a strong loop of chitin, roughly resembling 

 a racket in shape. The rounded distal portion forms the boundary 

 of the prestomum, and has attached to it the inner walls of the labella, 

 or the pseudotracheal membrane, and the prestomal teeth. The portion 

 corresponding to the handle of the racket consists of two stout rods, 

 of about the same length as the looped portion ; these lie in almost 

 the same plane as the loop, and diverge from one another as they 

 pass backwards. The sclerite is attached to the lateral rods of the 

 labial gutter by short tendons which pass between the distal ends of the 

 latter and the posterior surface of the sclerite, at the junction of the 

 looped portion and the parallel rods (Plate IX, fig. 8). 



The posterior joint lies between the distal end of the mentum 

 and a horse-shoe shaped arch of chitin termed the furca (Plate IX, fig. 4). 

 The distal end of the mentum is somewhat contracted, and has in 



