POLYMORPHA WHIRLIGIG-BEETLES 



215 



are five ventral segments visible along the middle, six at the 

 sides, as in the families of Aclephaga generally. There is said 

 to be a great difference in the nervous systems of Carabidae and 

 Paussidae, but so little is known on this point that we cannot 

 judge whether it is really of importance. 



Fam. 11. Gyrinidae (Whirligig beetles'). Antennae very 

 short; four eyes; middle and hind legs forming short broad 

 ^addles ; abdomen with six segments visible along the middle, seven 

 <i/<>n// each side. These Insects are known to. all from their habit 

 of floating lightly on the surface of water, and performing graceful 

 complex curves round 

 one another without 

 colliding ; sometimes 

 they may be met with 

 in great congregations. 

 They are admirably con- 

 structed for this mode 

 of life, which is com- 

 paratively rare in the 

 Insect world ; the 

 Hydrometridae amongst 

 the bugs, and a small 

 number of different 

 kinds of Diptera, being 

 the only other Insects 

 that are devoted to a 

 life on the surface of the 

 waters. Of all these, 



FIG. 99. A, Larva of Gyrinus (after Schiodte) ; B, 

 under side of Gyrinus sp. (after Ganglbauer). 1, 

 Prosternum ; 2, anterior coxal cavity ; 3. meso- 

 thoracic episternum ; 4, mesoepimeron ; 5, meso- 

 sternum ; 6, metathoracic episternum ; 7, middle 

 coxal cavity ; 8, metasternum ; 9, hind coxa ; 10, 

 ventral segments. [N.B. The first ventral segment 

 really consists, at each side, of two segments united ; 

 this may be distinctly seen in many Gyrinidae.] 



Gyrinidae are in their 

 construction the most 

 adapted for such a 

 career. They are able 

 to dive to escape danger, 

 and they then carry with them a small supply of air, but do not 

 stay long beneath the surface. Their two hind pairs of legs are 

 beautifully constructed as paddles, expanding mechanically when 

 moved in the backward direction, and collapsing into an extremely 

 small space directly the resistance they meet with is in the other 

 direction. The front legs of these Insects are articulated to the 

 thorax in a peculiar direction so that their soles do not look 



