24 INSECT PHYSIOLOGY 



means of the hydrofuge surfaces and hairs already discussed (p. 6). 

 These external air stores have several functions: (1) They have an 

 important hydrostatic function in enabling the insect to reach the 

 water surface in the correct position to take in oxygen. (2) They pro- 

 vide a store of oxygen which is gradually used up by the submerged 

 insect ; for example, the oxygen in the sub-elytral air of Dytiscus may 

 fall from 19-5 per cent, at the moment of diving, to 1 per cent, or less 

 in three or four minutes. (3) They provide a mechanism for obtaining 

 dissolved oxygen from the water, and so function as a kind of physi- 

 cal gill. For the 'invasion coefficient' or rate of diffusion of oxygen 

 between water and air is more than three times as great as that of 

 nitrogen. Consequently, as the partial pressure of oxygen in the air 

 store becomes reduced, equilibrium will be restored by the diffusion 

 of oxygen inwards, rather than by the diffusion of nitrogen outwards. 

 Of course, some nitrogen will diffuse out, but as long as any re- 

 mains undissolved the process can go on, and the insect can extract 

 dissolved oxygen from the water. This mechanism is of more or less 

 value to all aquatic insects that carry air stores ; and small forms like 

 Corixa can obtain enough oxygen in this way even at summer tem- 

 perature so long as they do not swim actively. The striking effect of 

 the process may be shown by quoting a single experiment : Notonecta 

 lived five minutes submerged in water saturated with nitrogen, thirty- 

 five minutes in water saturated with oxygen, but six hours in water 

 saturated with atmospheric air - the air store, in each instance, being 

 first charged with the dissolved gas. 



In a few insects, such as the beetles Haemonia and Elmis and the 

 Naucorid bug Aphelocheirus, the hydrofuge hairs which carry the 

 film of air are bent over at the tip so as to provide a hydrophile 

 surface enclosing a layer of air which cannot be replaced by water. 

 This firmly held layer of gas is termed a 'plastron' ; it enables such 

 insects to become independent of the atmospheric air and to obtain 

 all their oxygen from the water. 



A much more common mechanism, by which the insect has be- 

 come completely adapted for the respiration of dissolved oxygen, is 

 the obliteration of all the spiracles and the development of tracheal 

 gills. In many cases, the general surface of the larva, richly supplied 

 with fine tracheae, provides the respiratory surface (in Chironomus, 



