i8 NATURAL HISTORY OF AQUATIC INSECTS 



sequently rises when they are energetically moved, 

 but the attitude is very easily reversed. 



If such an animal ceases to use its limbs, and is 

 moved through the water by a current or by its own 

 weight, it will tend to move back foremost. Hence 

 a dead Lobster or Crayfish thrown into water will 

 commonly come to rest lying on its back. Swimmers 

 in water, such as some Ephemera-larvae, find it easy 

 to invert their bodies. Some Crustacea swim in all 

 sorts of attitudes, back upwards, back downwards, or 

 sideways, and change incessantly from one position 

 to another. This is effected by the bending of the 

 body. If the body is much curved, the centre of 

 gravity will lie outside it, in the concavity and well 

 below the line about which the moments on both sides 

 form a couple. The convex side will tend to sink. 

 If the ventral side is made convex it will tend to 

 turn downwards, and so with any other side.^ 



The Wintering of Aquatic Insects. 



Winter, of course, brings many hardships upon 

 aquatic Insects, as the great reduction in their num- 

 bers proves. The enormous number of eggs laid by 

 so many of them is doubtless connected with the 

 heavy risks to which they are exposed during half the 

 year. I find comparatively little information recorded 

 even upon so fundamental a point as the stage in 

 which particular Insects pass the winter. It is much 



I have derived useful notions on this subject from a short 

 paper by Bethe (" Erhaltung des Gleichgewichts," Biol. Central- 

 blatf, 1894), which contains, however, some serious mistakes. 



