A GUIDE TO THE STUDY OF BRITISH WATERBUGS. 153 



habitually moving in the water,* but are eminently adapted to 

 insects skimming lightly over the surface. The anterior legs do 

 not seem to be, as states De Geer, usually employed for the 

 capture of prey, though they hold in position the dead insects 

 upon which Gerris apparently generally feeds. Meinert states 

 that, although the anterior legs are structurally similar in both 

 sexes, their principal use, in the male sex, is to hold the female 

 firmly during cofmlation. I have not, however, been able to 

 find, among the males, any specially modified clasping organ, 

 such as obtains in certain Coleoptera. 



The normal position of Gerris on the water is as follows : 

 the posterior legs slope somewhat backwards, the middle legs 

 sloping strongly forward, so that the tarsi come to rest nearly as 

 far forward as the head, but directed outwards to the side. The 

 anterior legs are nearly perpendicular, the tarsi almost touching 

 at their bases, thence diverging. Gerris rests on all six legs, 

 elevated upon them to a considerable height from the surface, 

 all the species, especially the larger kinds in their adult state, 

 depressing the surface of the water, and forming a moderately 

 deep oval "bowl" or "dimple" for each leg. The length, or 

 larger diameter, of each dimple is proportionate to the length of 

 that portion of the leg wherewith they rest on the water, viz. to 

 the length of the tarsi in the case of the anterior and inter- 

 mediate legs, and to the length of the tibiae and tarsi in the 

 posterior legs. The width, or smaller diameter, is half that of 

 the larger diameter. 



Movements are of three kinds — (1) drifting or floating, (2) 

 skating or rowing, and (3) leaping. 



In the drifting or passive movement the bug is carried forward 

 on flowing water, and even on stagnant ponds, by the undulating 

 ripples caused by puffs of wind. 



The commonest of the active movements is that by which the 

 bugs do not lift themselves from the surface of the water by 

 means of their legs, but make a few strong synchronous strokes 

 with the intermediate pair of legs ; while the anterior and pos- 

 terior pairs are quiescent, or move so feebly that the motion can 

 be observed only with difficulty. A proof of the fact that the 

 legs are not upraised from the water is that the dimples wherein 

 they stand retain their form and size, as may be seen clearly 

 both in nature and in captivity. When the bug stands on the 

 surface of a shallow sandy pool (or a glass vessel with a sandy 

 bottom) in the sunshine, the dimples throw shadows on to the 

 bottom, each surrounded by a bright ring, caused by tbe refrac- 

 tion of those rays which pass through the curved surface. The 

 central shadow thrown by the body is not surrounded, however, 

 by a bright ring, and so, when the bug moves, no bright ripples 



* Cf. the short stout posterior legs of Notonecta, Corixa, and Dytis- 

 cus, &c. 



