304 THE TRACHEAL SYSTEM OF THE IMAGO. 
open and close like bellows, on a hinge formed between the 
bow and the spiral prolongation of the bow which supports the 
inner valve. 
There is a layer of muscle fibres extending between the rings 
which support the inner and outer valves, and these, by their 
contraction, effectually empty the spiracular sac, forcing the air 
which it contains through the inner valve into the trachee. 
The outer valve is opened by the contraction of the muscle 
of the lever which depresses the bow; as soon as this muscle 
relaxes the outer valve closes. This is apparently effected by 
the elasticity of the curved atrial tube and of the bow; the 
muscle of the spiracular sac then contracts and draws the inner 
and outer rings together, at the same time opening the inner 
valve and driving the air into the tracheze. The spiracular sac 
is opened again by the spring-like action of the spiral which 
supports the inner valves, and the depression of the lever 
which opens the outer valve. In short, the vestibule or 
spiracular sac is a small bellows which pumps air into the 
tracheal tubes. 
The trachez, which arise from the first and second pairs of 
abdominal spiracular sacs, communicate directly with the 
aérostats (p. 217) or abdominal pulmonary sacs; those of the 
remaining abdominal spiracles form arborescent tracheze which 
are not directly connected with the thoracic trachez and with 
each other, except by small or capillary branches. These 
trachez do not dilate into air-sacs in the Blow-fly, although in 
many insects the abdominal trachez, like those of the head 
and thorax, dilate and form thin-walled air-sacs. 
b. Structure of the Trachee and Air-sacs. 
The Trachez and Air-sacs, in dissections made under water or 
glycerine, exhibit, as is well-known, a silvery lustre, and appear 
with reflected light like tubes or vesicles filled with mercury. 
This appearance is due to reflection from the surface of the 
contained air. It is very beautiful, and may be preserved in 
dissections mounted in glycerine; for if the cells are properly 
sealed, the air is not dissolved out of the larger or even moderate- 
