THE TRACHEAL SYSTEM OF THE BLOW-FLY. 363 



spiracular trunks, but they are more easily demonstrated by 

 the behaviour of the contained air than by any other method. 

 Some sections show the valve flaps more or less perfectly. 



The small Tympanic Spiracle is situated in the tympanic 

 fissure (p. 187) ; it is connected with a complex auditory and 

 sound-producing apparatus, to be described hereafter. It is 

 controlled by a single valve, which resembles a clip, and closes 

 the tracheal tube, with which it is connected. This spiracle is 

 apparently exclusively expiratory, and I believe the issue of air 

 from it is the cause of the humming sound made by the insect, 

 it is the only spiracle from which air issues ; all the others are 

 exclusively inspiratory. 



The Abdominal Spiracles are minute circular openings ; each 

 orifice is fringed by minute setae springing from its edge. 

 These undoubtedly serve to exclude dust. Immediately 

 within the external orifice there is a short curved tube the 

 atrium the inner surface of which is thickly set with fine 

 double setae, two setae arising from a thickened base. 



The inner extremity of the highly chitinized atrium is closed 

 by the external valve ; beyond this is the spiracular sac, which 

 is separated from the true tracheal trunks by the inner valve 

 (Fig. 48). 



The External Valve consists of a half-ring of chitin, the bow 

 (Verschlussbugel) of Landois; from which a lever (Heber, Landois) 

 projects. The extremities of the bow are united by a semi- 

 lunar plate, the valve-plate (Verschlussband, Landois), by which 

 the orifice can be hermetically closed. 



The valve-plate opens inwards, and when closed its margin 

 rests against the bow (Fig. 48, 2). One extremity of the bow is 

 continuous with the lever, the other terminates in a slender 

 spiral spring, which terminates in a ring surrounding the 

 vestibule, and separates it from the trachea. This ring sup- 

 ports the inner valves, which resemble those of the veins of 

 Vertebrates, and project into the tracheal trunk. 



The lever is moved by a muscle, the contraction of which 

 separates the inner and outer valves, and draws air into the 

 vestibule or spiracular sac. The abdominal spiracular sacs 



