EYES OF THE WHIRLIGIG. 63 



velocity, and always on the look-out for prey, If the eyes 

 were formed like those of the Dyticidse, the water would 

 certainly impinge against them and render the insect incapable 

 of seeing, by reason of the drops of water which would be 

 continually splashed over its eyes. In order to enable it to 

 see properly above the surface, it is needful that the eyes 

 should be placed high enough to be out of reach of the water ; 

 and to enable it to see objects in the water, it is necessary 

 that the eyes should be submerged. 



All practical entomologists are personally acquainted with 

 this latter fact ; for they know well that if they want to see 

 objects at the bottom of the water, the only way to do so is 

 to lie on the bank and submerge the eyes entirely. In some 

 parts of the world fishermen use a water-telescope for the same 

 purpose. This is nothing more than a tube, open at one end, 

 and having a plain glass closely fitted to the other. When used, 

 the closed end is pushed well below the surface, and the eye 

 applied to the open end, when it is found that objects can be 

 seen nearly as well below as above the water. 



The eyes of the Whirligig Beetles are in fact water-telescopes. 

 Instead of being placed in two masses, one on each side of the 

 head, each is divided by the portion of the head which carries 

 the antennae ; so that half of the eye-cluster is well out of the 

 water, and can see objects above the surface, while the other 

 half is submerged, and can see objects beneath it. 



The Common Whirligig, Gyrinus natator^ which is shown 

 on Plate III. Fig. 3, is very gregarious in its habits, and may 

 generally be seen in small companies, whirling about on the 

 surface of the water in very still and sheltered places. As 

 they dart about, they often strike against each other ; but the 

 shock does no harm to their hard and polished bodies, and 

 they go on with their unceasing round as if nothing had 

 happened. Their chief object in thus continually darting over 

 the surface is to obtain food, which consists principally of 

 small flies, Beetles, and other insects which fall into the water. 

 They use their long fore legs in the capture of prey. They 

 are watchful little Beetles, and if they fear danger they dive 

 to the bottom, and there remain until they think that they 

 can return in safety to the surface. 



As in the case with the Dyticidoe, these insects -are furnished 



