904 FRESH-WATER BIOLOGY 



Beetles {Order Coleoptera) 



Of this great group of insects only a few families are wholly 

 aquatic, and a few others are partially so. The order as a whole is 

 predominantly terrestrial, and the aquatic families show xmmistak- 

 able signs of having been developed from terrestrial ancestors. 

 All the adults and pupae are strictly terrestrial in their mode of 

 respiration, and nearly all the larvae likewise get their air supply 

 from above the surface of the water. The pupae of all are formed 

 either on land, or in direct commimication with the air. The 

 families that are strictly aquatic are the Dytiscidae, HaHplidae, 

 Parnidae and Amphizoidae; those that show complete intergrada- 

 tion in habits are the Hydrophihdae and DascyUidae. The Chrys- 

 omelidae are scarcely to be called aquatic at all in any proper 

 sense, although two of the subfamilies live on water plants. 



There is such great diversity of habits and structure in water 

 beetles that the families may be best considered separately. We 

 begin with those that are least aquatic in habit. 



Two small groups of leaf beetles of the great family Chrysomehdae 

 feed upon water plants; the GalerucelHnae, upon the floating leaves 

 of members of the water-Hly family. These dingy little beetles 

 lay their yellow eggs in small clusters on the upper surfaces of the 

 leaves, and the black-banded larvae, hatching therefrom, feed upon 

 tlie tissues, quite as their more familiar relatives feed upon land 

 plants. The other subfamily, the Donaciinae, or long-horned leaf- 

 beetles, is much more interesting. The larvae feed upon the roots 

 of aquatic plants, far beneath the surface of the water. They are 

 provided with a pair of spiracles near the end of the body and 

 these spiracles are armed with sharp corneous processes, capable 

 of being thrust into plant stems, of reaching the air spaces on the 

 inside, and of obtaining the air, rich in oxygen, contained therein. 

 Thus the larvae, while destitute of gills, and strictly air breathers, 

 get their air supply through the medium of the plants, while Uving 

 always beneath the water. The pupal stage likewise is passed 

 in the place where the larva lived on the roots, but the pupa is 

 inclosed in a water-tight cocoon, attached to the plant tissue and 

 containing air in free communication with that in the air spaces of 



