362 THE TRACHEAL SYSTEM OF THE IMAGO. 
with a portion of the tracheal sacs and the surrounding integu- 
ment, by an incision made about one millimetre from the edge 
of the spiracle, and surrounding it. If this is done, the 
tracheal sacs will be found full of air. If before removing the 
spiracle the insect is immersed for a few seconds in alcohol to 
remove the wax, the vestibule will be found full of fluid. The 
efficiency of the internal valve is then seen to be very great, as 
no air will escape from the tracheal sacs through the spiracle. 
Of course, the dissection must be made under water or dilute 
Miiller’s fluid. I use one part Miiller to ten of water. 
Fic. 48.—The Abdominal Spiracular Apparatus of the Imago: 7, a semi-diagram- 
matic representation of the spiracular apparatus of the third abdominal spiracle ; 
2, another view of the same, with a portion of the wall of the atrium removed to 
show the external valve ; 3, a section of the vestibule and trachea, showing the 
flaps of the internal valve ; a, atrium ; /, lever; s, spiracle ; ¢”, trachea; v, con- 
tractile vestibule. 
I have frequently observed that the tracheal sacs expand 
when relieved from the pressure of surrounding parts, showing 
that they contain compressed air. 
There are several valves opening inwards in the course of the 
