THE TRACHEAL SYSTEM OF THE BLOW-FLY. 361 
in a membranous fold of the air-vessel (Pl. XXII., Figs. 1 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, 
