X POND-SKATERS 349 



the water below, so among the aquatic Rhynchota 

 some run on the surface without being wetted, while 

 others are almost constantly submerged. The surface- 

 life is that practised by Hydrometra and the Pond- 

 skaters. 



Hydrometra, sometimes called the Water-gnat, is 

 distinguished by its peculiar shape and habitat. The 

 whole Insect is very slender, and the head is drawn 

 out to a length of one-third that of the body. 

 Mounted on long slender legs, the Insect walks 

 sluggishly on the top of the water, or creeps about 

 the grassy banks. 



Closely related to this are the much more lively 

 Pond-skaters (Gerris). Like Hydrometra they have 

 a narrow, elongate body, but the head is not so pro- 

 longed. The fore-legs are prehensile, and said to be 

 used for capturing and holding living prey.^ As often 

 happens in such cases, they are separated from the 

 next pair by the prolongation of the first segment of 

 the thorax on which they are borne. The second 

 and third pairs of legs are long and slender, and 

 support Ihe weight of the body. When the Pond- 

 skater darts about the surface of the water, the inter- 

 mediate legs are most active, while the hind pair 

 steer the course. Not only the legs but the whole 

 body is covered with a velvety pile, which renders it 

 incapable of being wetted. Though the legs do not 

 break the surface-film, they depress it, making little 

 dimples in the water. When the Insect stands on the 

 surface of a shallow sandy pool in the sunshine the 



^ This is De Geer's account. I believe, however, that Gerris 

 is timid, and prefers a victim aheady dead or nearly so. 



