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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 sete 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 setz, two sete 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 
(Verschlussbiigel) 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 
