WHIRLIGIG BEETLES. 57 



that of the Gyrinidae, or Whirligig Beetles, so called 

 on account of the manner in which they whirl them- 

 selves about on the surface of the water. 



There is not the least difficulty in determining 

 whether or not a Water Beetle belongs to the 

 Gyrinidse. Besides the distinctions before mentioned, 

 all the Gyrinidae appear to differ not only from the 

 Dyticidae, but from Beetles generally, in having, or 

 rather in appearing to have, four compound eyes 

 instead of two. The real fact, however, is that, in 

 order to suit the peculiar habits of the insects, the 

 structure of the eye is modified. 



These Beetles pass the greater part of their time 

 on the surface of the water, rowing themselves about 

 with wonderful velocity, and always on the look-out 

 for prey. If the eyes were formed like those of the 

 Dyticidae, the water would certainly impinge against 

 them and render the insect incapable of seeing by 

 reason of the drops of water which would be continu- 

 ally 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 sub- 

 merged. 



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. 



