INTRODUCTION 15 



with the behaviour of Insects living at the surface of 

 water. The particles close to the surface are in a 

 peculiar condition of aggregation, and temporarily 

 cohere to form a film. The film, as we have seen, 

 offers resistance to the passage of solid bodies, and 

 can therefore support a weight or keep a buoyant 

 object from rising through it. It also exerts a pull, 

 and we shall find that advantage is taken of this pull 

 by certain aquatic Insects. The extreme tenuity of 

 the film, which is thinner than our imagination can 

 realise, helps us to understand how it is that only 

 small objects are affected by it. Ships, boats, swim- 

 ming quadrupeds, and all objects whose weight is 

 large in comparison with their contour, are practically 

 uninfluenced, but objects whose dimensions are given 

 in fractions of an inch may be largely controlled by the 

 peculiar properties of the surface-film. The larva of the 

 Gnat turns these properties to account in a peculiarly 

 interesting way, as do many other aquatic Insects, 

 Crustacea and floating plants. On the other hand, 

 minute aquatic animals, approaching the surface of 

 water without precaution, may find it a death-trap, as 

 we learn by observation of certain small Crustacea.^ 



Equilibrium of Aquatic Insects. 



The density of water is so much greater than that 

 of air as to require peculiar adjustments in the 

 equilibrium of aquatic animals. A simple case is pre- 

 sented by the air-breathing aquatic Insects. These, 



1 D. J. Scourfield, Linn. Joiirn., Zool., Vol. XXV., pp. i — 19, 

 pi. I, 2, 1894. 



