134 NATURAL HISTORY OF WASPS. 
structures in which it is placed sufficiently trans- 
parent for our purpose. 
Under the microscope the crescentic outline of the 
stigma almost disappears, and the opening assumes 
a long oval form. On one side, the posterior, it 
presents a thick, round overhanging lip, which is 
slightly striated transversely on the surface, and 
Fig. 9.—Diagram of the right metathoracic spiracle 
of the hornet, seen from the outside. 
On the left appears the thick margin transversely 
striated, and the horny slip into which the wall on 
this side is continued. On the right is the thin 
membrane, with the hook by which its tension is 
regulated, The membranous edge of the vocal 
larynx is not displayed fully in this figure. 
‘QQ 
hollowed out beneath. Farther in, this edge gives 
attachment to a membrane, like that which forms 
the pouch of the abdominal spiracle, and having the 
line of its connection with the wall of the trachea 
marked, in the same way, by a horny slip. The 
other margin is thinner and, again like the abdominal 
spiracle, gives attachment to a delicate membrane, 
the edge of which has space to play in the hollow 
just noticed behind the opposite overhanging lip. 
This is the vocal larynx, similar to that already de- 
scribed in the abdominal spiracles in its general 
structure, but more powerful as an organ of sound, 
from its larger size and the nature of the frame in 
which it is set, as well as from the freer current 
of air which can be sent through it. The instrument 
actually producing the sound seems to be the mem- 
branous tongue which vibrates in the hollow behind 
the thickened margin of the stigma. I have never 
