552 HEMIPTERA-HETEROPTERA CHAP. 



walks on the surface-film of water, above which its body is held 

 well separated. It is easily drowned, and if submerged it has 

 great difficulty in escaping from the water. This genus repre- 

 sents the sub -family Hydrometrides, and is apparently almost 

 cosmopolitan. Velia currens is another common British Insect ; 

 it loves the eddies and currents of backwaters on burns and 

 streams, and is very abundant in Scotland. An American ally, 

 Rhagovelia plumbea, appears to be not uncommon on the surface 

 of the ocean in the Gulf of Mexico, near the shores. The great 

 majority of the family belong to the division Gerrides, of which 

 the curious, long Insects that float so lazily and skim so easily 

 on the surface of quiet streams are typical. The species of the 

 genus now called Gerris, but formerly known as Hydrometra are 

 apparently distributed all over the world ; we have ten in 

 Britain. They have very long legs, and on being alarmed move 

 away with the greatest ease. 



The genus Hcdobates includes at present fifteen species. They 

 are found on the ocean, where the surface-water is warm, in 

 various parts of the world. They are destitute of any trace of 

 alar organs, the meso- and meta-thorax are closely united and 

 large, while the abdomen is very small, so that the body is of 

 oval form ; the middle legs are thrown so far back that they are 

 placed immediately over the posterior pair. When the sea is 

 calm these Insects skim over the surface with rapidity, but 

 disappear as soon as it becomes agitated. They are believed to 

 feed on small animals recently deceased ; Witlaczil says on 

 the juices of jelly-fish. The young are frequently met with, 

 and there can be no doubt that the whole life-cycle may be 

 passed through by the Insect far away from land. The Italian 

 ship Vettor Pisani met with a bird's feather floating on the 

 ocean off the Galapagos Islands, covered with eggs which proved 

 to be those of Halobates in an advanced stage of development. 

 It was formerly believed that the female carries the eggs for 

 some time after their exclusion, and although this has since 

 been denied, it is nevertheless an undoubted fact, for it was 

 observed by Mr. J. J. Walker, 1 to whom we are indebted for 

 a specimen having the eggs still attached to the body, as shown 

 in Fig 265. Mr. Walker believes the bugs shelter themselves 

 when the sea is at all rough by keeping at a sufficient distance 

 1 Ent. Mag. xxix. 1893, p. 227. 



