282 THE BIOLOGY OF INSECTS 



no such light region surrounds the shadow of the body 

 shows that the legs keep it raised above the surface. While 

 in Hydrometra all the legs are elongate and slender, in 

 Gerris the foremost pair are relatively short and robust, 

 the feet provided with strong hook-like claws springing from 

 a cleft in the terminal foot-segment. By means of these 

 the pond-skater can anchor itself to floating objects, such 

 as the dead insects whose juices it sucks for food. The 

 middle legs of Gerris and its allies are inserted far back 

 owing to the ventral and lateral extension of the mesothorax ; 

 thus the limbs are placed close to the hindmost pair, and 

 when the insects glide over the surface all four, with long, 

 sweeping strokes, move in unison. A related form, Velia 

 currefis, common on swift streams, is more robust in build 

 than Gerris with less specialised legs. It obtains, however, 

 good support through the surface tension and occasionally 

 dives and swims under water. It makes use of the tension 

 from below, for Miall remarks that '* it can be seen to run, 

 back downwards on the surface-film, completely immersed in 

 the water ; its abdomen at such times glistens with the air- 

 bubble which overspreads it." The short hairy clothing 

 of the cuticle to which the formation of such a bubble is 

 due is especially dense in many of the aquatic Hemiptera. 



An interesting modification shown by many of these 

 insects is a partial suppression of the wings. In some 

 species of Gerris the wings may be absent in the adult, or 

 present only in a reduced condition, while in other individuals 

 of the same kind wings are fully developed. Most adults 

 of Velia ciirrens are wingless, but winged specimens are 

 occasionally found, and such must form a fairly numerous 

 minority of the whole population of the species. The 

 disappearance of wings is not surprising among insects 

 which cling through most of their lives to the surface of 

 freshwater pools and streams ; but as many of their haunts 

 are Uable to dry up, the occasional development of winged 

 members capable of flying away to fresh watery feeding- 

 places is clearly advantageous to survival. 



From consideration of some of the great multitude of 



