THE TRACHEAL SYSTEM OF THE I! LOU '-FLY. 361 



in a membranous fold of the air-vessel (PI. XXII., Figs, i and 3). 

 This valve is controlled by a small fan-shaped muscle. The 

 internal valves were figured and described by Landois [150] 

 and Krancher [154]. 



The Vestibule. I have applied this term to the membranous 

 sac between the external and internal valves. The vestibule is 

 covered externally by a layer of muscles, which arise from the 

 integument at the edges of the spiracle. The muscle may be 

 termed the compressor vestibuli. 



The Mechanism of the Spiracle and its Valves. The external 

 valve plates are of unequal size ; the larger one is apparently 

 fixed. The smaller valve is only attached by its base, and 

 opens inwards. Landois describes it as opening outwards ; 

 but it is easily dislocated, so that it can then be turned out- 

 wards. Both valve-plates may be removed with a little care, 

 even in the living insect, when the vestibule is exposed, and 

 the movements of the internal valve can be watched. 



The upper edge of the spiracular opening exhibits a kind of 

 flange, against which the outer surface of the margin of the 

 smaller valve-plate rests. The smaller valve-plate is opened by 

 a few muscle fibres attached to its fixed border, and it is 

 probably closed by the pressure of the air in the vestibule when 

 the compressor vestibuli contracts. 



The internal valve opens and closes rhythmically, like a pair 

 of side curtains. Its movements were first seen by Landois. 

 The removal of the external plates is not difficult, but it is most 

 easily done from the anterior spiracle, as it is necessary to cut 

 off the abdomen to see the valves of the posterior spiracle. 

 Even after this operation the internal valves continue to open 

 and close rhythmically for some time. The rate of movement 

 after such an operation cannot, however, be regarded as 

 normal ; and is probably far less frequent than when the insect 

 is uninjured. 



It appears to me certain that the function of the vestibule is 

 that of a pump, which drives air into the tracheal vessels. 



The Anterior Spiracle only differs from the posterior in its 

 form. It is possible to remove the whole spiracular apparatus, 



