72 ON THE ANATOMY OP THE FLY. 



The anterior thoracic spiracle (plate V., fig. 17) is oval and 

 narrowest above. Its margin consists of a thickened ring 

 chiefly visible internally, and formed by the extremities of 

 the process already described as extending from the posterior 

 extremity of the central portion of the pro-sternum to the 

 Bpiracle. 



The anterior spiracle is situated between the pro-thorax and 

 meso-thorax, but the meso-thoracic tergum reaches over its 

 superior margin, and joins the pro-thorax in front of the 

 spiracle ; beneath it is entirely bounded by the pro-thorax. 

 The edge of the spiracle is continued as a thin semi-lunar 

 fold, which reduces the opening to a mere slit. This is 

 strengthened by chitinous arborescent nervures, which project 

 beyond its free edge, so as to fringe it with a delicate fringe of 

 sharp spicules. I shall call it the peritreme, although in 

 the posterior thoracic spiracle the same structure forms a pair 

 of leaf -like valves. 



Close behind the peritreme is the true valve by which the 

 aperture is closed This resembles a pair of callipers jointed 

 below, but the two sides shut against each other like the leaves 

 of a book. I shall call this structure the epitreme (plate Y., 

 fig. 16). It is closely united to the wall of the tracheal vessel, 

 and is probably developed in its inner layer or that which 

 corresponds to the protoderm. By breaking away a small 

 portion of the peritreme of the anterior spiracle of the living 

 fly, the epitreme may be observed opening and closing regularly 

 with each act of respiration. As has been already observed, 

 when a fly is held by the wings it makes regular efforts with 

 its legs ; during these the peritreme remains closed, but may 

 be observed to open as soon as the effort ceases. 



The peritreme of the anterior spiracle is opened and closed 



