LEGS AND WINGS OF DYTISCUS 83 



remarkable inasmuch as in either its larval 

 or its mature condition it can exist in water. 

 There are reasons, however, for supposing that 

 these creatures are modified terrestrial in- 

 sects. A peculiar feature of their history is 

 that they thrive better in the cooler waters 

 of the earth. Lapland is one of the parts of 

 Europe richest in Dytiscidae. About 1,800 

 species are known. Although both larvae and 

 imagos are perfectly at home in the water, 

 they must come up to the surface to get air. 



The hind pair of legs is the chief means 

 of locomotion. These swimming legs are de- 

 serving of admiration on account of their 

 mechanical perfection. 



The wing-cases so fit the body that the 

 air carried down by them under the water is 

 held, as it were, in an air-tight compartment, 

 and is distributed through the spiracles to the 

 tracheal system. When v the dytiscus feels the 

 necessity for fresh air, it exposes the tip of its 

 body exactly at the level of the water. Eespi- 

 ration is effected by this means as well as by the 

 store of air carried down under the wing-cases. 



