2o6 



ANIMALS OF LAND AND SEA 



coming to the surface, some by taking it from air in the stems 

 of water plants, while in some the entrances to the tubes are 

 sealed and they are filled by air extracted from the water by 

 gills or other air extracting surfaces. Air being compressible 

 and the insect body being always full of air the insects are in- 

 capable of a deep descent in water, for under the added pres- 



FiGS. 594-611. Gastropods. 

 For explanations of the figures see pp. xx.x, xxxi. 



sure their bodies would collapse. Very few insect larvae could 

 support a pressure of two atmospheres additional. Such insects 

 as can do this counteract the effect of the compressibility of 

 air by carrying part of their supply outside, and usually under- 

 neath the body, or between the body and the wings, whereby 

 the added pressure merely decreases the volume of this external 

 surplus and slightly increases the density of the air within the 

 tubes; or they live always at a considerable depth and by gills 

 extract the air at the prevailing pressure at the depth at which 



