RESPIRATION IN INSECTS. 283 



wholly in the air.* Those of the Scolia hortorum 

 (Fab.) are delineated in Fig. 373, considerably 

 magnified. 



If an insect be immersed in water, air will be 

 seen escaping in minute bubbles at each spiracle ; 

 and in proportion as the water enters into the tubes, 

 sensibility is destroyed. If all the spiracles be 

 closed by oil, or any other unctuous substance, the 

 insect immediately dies of suffocation ; but if some 

 of them be left open, respiration is kept up to a 

 considerable extent, from the numerous communi- 

 cations which exist among the air vessels. Insects 

 soon perish when placed in the receiver of an air- 

 pump, and the air exhausted ; but they are gene- 

 rally more tenacious of life under these circum- 

 stances than the larger animals, and often, after 

 being apparently dead, revive on the readmission 

 of air. 



Aquatic insects have tracheae, and sometimes also 

 branchiae ; and are frequently provided with tubes, 

 which are of sufficient length to reach the surface 

 of the water, where they absorb air for respiration. 

 In a few tribes a complicated mode of respiration 

 is practised ; aerated water is taken into the body, 

 and introduced into cavities, where the air is ex- 

 tracted from it, and transmitted by the ordinary 

 tracheae to the different parts of the system.! 



Such, then, is the extensive apparatus for aera- 

 tion in animals, which have either no circulation of 



* Leon Dufour, Annales des Sciences Naturelles; viii. 26. 



f Dutrochet conceives that the principle on which this operation 

 is conducted is the same with that by which gases are reciprocally 

 transmitted through moistened membranes ; as in the experiments 

 of Humboldt and Gay Lussac, who, on enclosing mixtures of oxy- 



