576 THE TK AC HEAL 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 trachea; 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 

 trachc-al 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. 



