376 THE TRACHEAL SYSTEM OF THE IMAGO. 
of atmospheric air, and the efflux of air, vitiated by carbon 
dioxide, are produced by respiratory movements of the thoracic 
and abdominal walls. 
This view originated with Treviranus; but both Reaumur [5] 
and Lyonet held that the air only passes into the trachez by 
the spiracles, and that expiration is effected by the transfusion 
of the carbon dioxide formed in the tissues, and of the inert 
nitrogen of the air through the whole surface of the integument 
and the walls of the alimentary canal. 
Not only is the view of Treviranus negatived by my researches 
on the structure of the respiratory organs, but also by physical 
difficulties which are insuperable, and these, as will be seen, 
entirely disappear when we return to the theory of Reaumur 
and Lyonet. This theory is supported not only by anatomical 
evidence, but by numerous physical facts, which I shall now 
proceed to investigate. 
The Respiratory Movements of the Abdomen can only affect 
the compression of the air within the tracheal vessels, and 
could therefore only be expiratory. A very little considera- 
tion will show that the contraction of the muscles, which 
form a layer beneath the abdominal integument, must 
diminish the extent of its cavity. This is effected by the 
invagination of the segments, and the approximation of the 
lower edges of the dorsal arches and the sternal plates. The 
loose syndesmotic integument is infolded by muscular con- 
traction, and although the elastic recoil of the dorsal arches 
may to some slight extent increase the vertical and transverse 
diameters of the abdomen, its elasticity is powerless to pro- 
trude the invaginated segments. 
The expansion of the abdomen is effected by the elasticity of 
the contained air, and the abdomen of a dead insect collapses. 
When the imago fly first emerges from the pupa, its abdominal 
integuments are soft and thick, and the whole cavity is small; 
it is only when respiration has been fully established, and the 
tracheal vessels are distended, that the abdomen is enlarged to 
its full size; the expansion and thinning of the integument is 
produced by the pressure of the inspired air, and is not the 
cause of inspiration. 
