OF RESPIRATION. 385 



which thus stands in constant connexion with the air, whereas, when 

 this continuation of the epidermis forms an internal cavity, the oxidised 

 respiratory medium is received in it. These cavities, which are every- 

 where distributed throughout the bodies of insects, we have described 

 above, according to their most general forms, as air tubes or tracheae ; 

 they constitute the respiratory organ, which is consequently neither 

 external nor partial, but is distributed throughout the entire compass 

 of the cavity of the body in uniform perfection. The structure of the 

 respiratory organ will, therefore, be fully known when we shall have 

 proved that these air tubes and no other portion of the body actually 

 constitute it. Commencing with this proof, the subsequent divisions of 

 this chapter will be occupied with the mechanism of respiration, and 

 its effects upon the corporeal functions. 



227. 



With respect to the proofs that the tracheae are the actual respi- 

 ratory organs of insects, the most superficial anatomical inspection of an 

 insect shows us that air is found in these tubes, and that we nowhere 

 find internal apertures to these tracheae, but constantly external ones. 

 Besides, air is seen to pass through the external orifices, or spiracles, 

 when living insects are cast into water, as air bladders rise from them 

 to the surface of the water. But Treviranus's * experiment is the 

 strongest proof; he placed the large green locust (Locusta viridissima) 

 beneath a turned up glass filled with water, and then saw an air bubble 

 rise from the spiracle between the meso- and meta-thorax, which regu- 

 larly decreased with the respiratory motion of the creature, and again 

 increased with its distension. Hausmann also observed an ascent and 

 descent of the water in a glass tube closed above, the superior space of 

 which contained air and a green locust, and this took place syn- 

 chronally with the inspiration and expiration of the insect t- Other 

 facts which prove the function of the air tubes as respiratory organs 

 are, for instance, the speedy death of all insects whose spiracles are 

 closed with oil or gum, so that no fresh air can enter the tracheae, 

 besides the ascending to the surface of all such water insects which 

 have no branchiae, and lastly, the projection to the surface of the air- 

 tubes whilst the remainder of the creature is immersed in the water. 

 In addition to these direct observations upon the respiratory function of 



* Biologie, vol. iv. p. 158. t De Animal, exsang. respirat., p. 8. 



c c 



