74 THE VOYAGE OF H.M.S. CHALLENGER. 



familiar " Gerris " in structure and habits, and tliey skip about in the net when caught in 

 just the same manner. They are apterous, and covered with silky bluish-white down, 

 which carries down a supply of air to serve them when they dive beneath the surface, 

 which they do very readily on the approach of the net. They are only seen when the 

 sea is perfectly calm. I tried to keep two or three in a large bottle of sea-water, but 

 they very soon died." 



That all the species do not avail themselves frequently of this power of diving (if 

 indeed they possess it) seems evident from the fact that it was only on one occasion that 

 Mr. Murray had an opportunity of seeing it, and that Dr. Hay, who at my request 

 observed the habits of Halohates hayanus, and kept specimens in captivity, never saw 

 them dive. 



From these not^s it would seem that the habits of Halohates are probably much the 

 same as those of the allied genera so frequently seen on the surface of fresh water in this 

 and other countries, which, like Halohates, may often be seen congregated round any 

 small recently dead animal (such as a fly) floating on the surface. The attraction is of 

 course the juices of the animal, which they obtain by first piercing its integuments with 

 the aid of their mandibles, and then sucking the fluid by means of the maxillae.^ 



Some of the species occur close to the shore, but others are found in mid-ocean many 

 hundred miles from land. In both situations they are gregarious to a greater or less 

 extent. Their mode of progression is probably the same as that of the more closely allied 

 fresh-water genera, which by means chiefly of the long middle and hind legs run rapidly 

 (skim or scud) on the siu-face of the water, or when alarmed, progress by long jumps. 

 These fresh-water species are said to be able to dive, but I do not think that this can be 

 a common habit, for I have never seen it done by any of the thousands of specimens that 

 have come under my observation. Some of the fi-esh-water sjjecies (Gerris) are said to 

 be able to swim," but this seems open to question. On the other hand, the long fringe 

 on the middle tibia and tarsus of Halohates seems to be intended for something more 

 than merely to support the animal on the surface. This supposition is strengthened by 

 the fact that it is the middle and undermost legs that are provided with the fringe. It 

 may be that the use of the fringe is not wholly, nor even mainly, for swimming, but for 

 enabling its possessor to resist the action of the wind, by taking hold of the water, 

 which it would do by submerging these legs. The fringe is often seven or eight times 

 longer than the breadth of the tarsus, and the hairs which compose it are fine and 

 flexible. At their base is a shorter fringe of stouter haii-s, curved at the apex. It is 

 possible that the animal can exercise some control over the position of the fiinge, keeping 

 it adpressed to the limb, so as not to impede its progress, when scudding over the surface ; 

 or extending it at right angles to the limb when this is immersed. The shorter fringe 



^ Tlipy also catch and kill living insects. 

 - Kirby and Spence, Entomology, p. 479. 



