6 INSECT PHYSIOLOGY 



The effect of surface forces becomes still more evident when hairs 

 or bristles are present on the body. When a fine pile of these hydro- 

 phobe hairs is set vertically on the body surface, they hold the water 

 at a distance, so that the insect, when it goes below the water surface, 

 can carry with it a film of air (Fig. 2, E, F). We shall see that this 

 film has some remarkable properties in relation to the respiration of 

 aquatic forms (p. 24). If a single bristle is inclined at an angle - for 

 simplicity we will suppose an angle of 45°, and we will suppose that 

 the angle of contact is 135° - and brought up to the water surface 

 from below, as can be seen from Fig. 2, C, the surface tension of the 

 water will draw it over so as to increase the angle of inclination. It 

 will behave as though the one surface were hydrophile and the other 

 hydrophobe. Many insects hold themselves at the water surface for 

 purposes of respiration by means of rows or circles of such bristles 

 (Fig. 2, D). 



Terrestrial insects avail themselves of surface forces in quite a 

 different way: namely, in order to cling to surfaces too smooth to 

 provide a purchase for their claws. The physical mechanism of the 

 organs (pulvilli and such-like) that are used for this purpose is by no 



fig. 2. — Diagrams illustrating the properties of the insect cuticle in 



contact with water 



A, hydrophobe object supported by the surface tension of water; B, respiratory siphon 

 with hydrophobe rim similarly supported ; C, surface forces acting on a hydrophobe hair 

 brought obliquely to the water surface; D, a crown of such hairs supporting a respiratory 

 siphon; E, hydrophobe hairs brought vertically against the water surface; F, insect body 

 with a ventral covering of such hairs carrying a layer of air 



