THE ANIMALS OF THE FRESHWATER AQUARIUM 121 



its carnivorous ally. One reason, no doubt, is that it seeks its 

 food on water-weed, over which it clambers, and does not attack 

 moving prey like Dyticus. Like that species, it breathes at the 

 surface. The mechanism is somewhat similar, in that there is 

 again an air-space beneath the elytra, but the down on the under 

 surface of the thorax, as well as that on the antennae, plays an 

 important part in the respiratory process. 



The female spins a cocoon for her eggs a somewhat curious 

 fact. In the rounded bag which she spins many eggs are en- 

 closed, and with them a bubble of air. The cocoon soon hardens, 

 and after the lapse of some weeks the larvae hatch and swim 



FIG. 55. The silver beetle and its larva. 



away in the water. They are soft, fat creatures, reaching a 

 length of 2 inches, and furnished with strong jaws by means 

 of which they obtain their prey. They seem to feed largely on 

 molluscs. Like the adult, the larva is obliged to come to the 

 surface to breathe. As in Dyticus, pupation takes place at the 

 margin of the pond. 



In searching the ponds for water-beetles one may often come 

 across numbers of the whirligig beetles, so called because of the 

 curious rotatory movements they perform at the surface. A 

 common form is Gyrinus natator, which whirls about at the sur- 

 face, its small black body shining in the sun like burnished metal. 

 If an attempt is made to capture these beetles they will be 

 observed to sink down at once, for the two posterior pairs of 





