STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 405 



The discal sclerites (fig. 1, ds.) are united afc the posterior 

 end to form, when the oral lobes are expanded, a U-shaped 

 structure, with the limbs constricted in the middle where the 

 ends of the thickened margins of the labium-hypopharynx 

 articulate. They are sunk in deeply between the two oral 

 lobes at the base of the oral pit with the free ends of the U 

 anterior, these being spatulate and curved anteriorly. 



The two oral lobes are normally connected by a bead and 

 groove attachment along their anterior edges, but under 

 pressure the connection is severed, and the oral disc presents 

 a heart-shaped instead of the normal oval appeai^ance. The 

 oral lobes ai'e covered on their upper aboral surfaces by 

 sensory setae, the large marginal setae being* different in 

 structure from the rest. On the lower or oral surface a large 

 number of channels, the pseudotrachea3 (fig*. 1, ^^5.) run from 

 the internal margins of the oral lobes to the external borders. 

 The channels of the pseudotracheae are kept open when the 

 lobes are extended by means of small incomplete chitinous 

 rings, which give the channels a tracheal appearance, hence 

 their name. Each of these incomplete rings has one end 

 bifid, and as the bifid ends alternate the opening into the 

 channel has a zigzag appearance. The number of pseudo- 

 trachete on each lobe is generally thirty-six, and they are 

 grouped in three sets. The anterior set of twelve all run 

 into a single large pseudotracheal channel running along the 

 anterior inner margin of the lobe, and a posterior set of 

 twenty-one all run into a channel running along the posterior 

 inner margin ; between these two sets three pseudotracheae 

 run direct into the oral aperture. The oral aperture lies at 

 the base of the small oral pit, which is a space kept open 

 between the oral lobes by means of the discal sclerites. The 

 pseudotracheee do not extend as far as the discal sclerites, 

 but on entering the oral pit the rings cease and the sides of 

 the channels are covered by overlapping teeth, which extend 

 back to the discal sclerites. Between the pseudotrache^ the 

 membranous surface of each oral lobe is thrown into two 

 longitudinal sinuous ridges, and projecting up from the 



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