16 NATURAL HISTORY OF AQUATIC INSECTS 



with few exceptions, are lighter than water. 1 Hence, 

 if by loss of respirable air, or from any other cause, 

 they are disabled, they have only to cease to struggle 

 in order to come to the surface, and it will in all cases 

 be found that they float upwards in such a position 

 that the part of the body by which air is taken in 

 comes to the top. 



Locomotion in any medium by an animal of 

 complex structure involves some power of orientation, 

 and especially a rapid and instinctive judgment as to 

 the horizontal plane and the upward and downward 

 directions. The pull of gravity may furnish the 

 necessary indication, but an animal which swims in 

 water, or flies in air, may be unable to determine the 

 horizontal plane with facility, especially if its body is 

 nearly of the same density as the surrounding 

 medium, since the pressure will be approximately 

 equal on all sides. The semi-circular canals of 

 Vertebrates and the so-called auditory organs or 

 otoliths of many Invertebrates are probably organs 

 for the perception of the horizontal position. Some 

 animals, however, such as Insects, possess no organs 

 of this kind, although they are excellent fliers or 

 swimmers. How are they enabled to maintain their 

 course at a fixed distance above the surface of the 

 earth or below the surface of the water ? Sight may 

 help, but that sight does not furnish the sole indica- 

 tion may be inferred from the behaviour of sightless 

 animals, such as the inhabitants of dark caverns, or 

 animals temporarily blinded for the purpose of 



1 Among the exceptions are Corixa, and the larvae of the, 

 Gnat, Agabus, and Hydrobius. 



