ADAPTATIONS TO HAUNTS AND SEASONS 281 



wise along the dorsal axis of the thorax, and through the 

 slit of the body, head, feelers, jaws, legs of the developed 

 insect are all withdrawn upwards. The empty pupal 

 cuticle may float at the surface, serving as a raft whereon the 

 gnat may rest and let her wings spread and harden. 



The surface tension of a sheet of water not only enables 

 such submerged larvae and pupae to breathe atmospheric 

 air ; it also allows a large majority of insects to live and move 

 freely over the surface without getting wet. A quiet pool 

 or the calmer reaches of a stream often bear on a summer 

 day numbers of insects, gliding, skating, or even leaping 

 on the surface-film. Among these the slender, active, long- 

 legged flies of the family Dolichopodidae are often con- 

 spicuous ; they stand in readiness on the water and dart 

 across the surface to catch suitable prey. Springtails of 

 several distinct groups which live in large assemblages on 

 the surface of water — Podura aquatica, for example — often 

 fall victims to these flies. But the surface- living habit is 

 especially well developed among the Hydrometridae, a 

 family of Hemiptera, insects with sucking and piercing 

 mouth-parts which pass through no marked transformation 

 in their life-history. Most of these aquatic bugs are slender 

 and elongate in form. Hydrometra stagnorwn, with its 

 long, narrow head and body and stiff thread-like legs, is 

 wonderfully adapted for life on the surface-film which 

 supports it without showing any appreciable depression. 

 The true pond-skaters belonging to the genus Gerris, of 

 which there are several common British species, are less 

 slender in body than Hydrometra, but their middle and 

 hind-legs, which spread far out on the surface, are straight, 

 thin, and rigid except at the joints. The short, two- 

 segmented feet bear on the surface-film, which they depress 

 to such an extent that they make, as Miall remarks, " little 

 dimples on the water," which are apparent when the sun 

 is shining on one of these insects standing in a shallow pool, 

 as then '' the dimples cast shadows on the bottom, each 

 surrounded by a bright ring due to refraction of the rays 

 which pass through the curved surface." The fact that 



