WHIRLIGIG BEETLES, S5 



In the first place, the eyes in the Dytiscidce are two in number 

 and are placed at the sides of the head, and protected from 

 injury by the anterior angles of the thorax, which for this purpose 

 are produced forward. In the Gyrinidce. the eyes are four in 

 number, two being distinctly on the upper side of the head and 

 the other two on its underside. The use of this arrangement is 

 evident, for as the Gyrinus passes most of its existence spinning 

 about on the surface of the water, these two pairs of eyes enable 

 it not only to keep a look-out for any enemy that may attack it 

 from above, but at the same time to be on the watch for prey, 

 such as small insects that may be in the water, and perhaps also to 

 make its escape from a voracious fish. 



It is worthy of note, too, that the curvature or convexity of 

 the upper and lower pairs of eyes are different ; also, that the foci 

 of the lenses of their corneas are dissimilar. By this means the 

 upper pair are suited for viewing objects in the air, whereas the 

 lower pair are adapted for seeing objects through the denser 

 medium of the water. If a portion of the cornea of the upper 

 eyes be placed by the side of a similar portion from the lower 

 ones and mounted as a slide for the microscope, it will be found, 

 on viewing any object through them, that the images of the object 

 produced by the difi^erent corneas will appear of different sizes. 



The antennae of the Dytiscidce are, as a rule, simple, although 

 there are some modifications of a different but not very striking 

 character. In the Gyrinidce, however, they are peculiar, being 

 unusually short and of a singular construction. The first joint is 

 very small and the next very large, and of an ear-like form, 

 fringed with hairs ; the remaining joints are small and inserted at 

 the side of this large joint, the arrangement somewhat resembling 

 forms we find in the antennae of the Diptera. They are inserted 

 in large circular hollows at the sides of the head between the 

 upper and lower pairs of eyes. 



The legs also difter very widely from those of the normal type 

 of water beetles. The front pair are essentially organs of prehen- 

 sion, and are employed for seizing insects, etc., and, considering 

 the size of the beetle, they are large and powerful. The posterior 

 pair are, on the other hand, organs for swimming, but not like 

 those of the Dytiscidce, which are constructed on the principle of 



