WATEK-BEBTLES. 221 



their prey they frequently throw their heads so far back as 

 to rest them upon their backs. These larvae must be handled 

 with great gentleness or they will die. 



These beetles do not obtain air in the same way as the 

 DytigcidcB do, by protruding the apex of their elytra above 

 the surface of the water, but by means of their antennae, 

 which they raise above the water, and which conduct the au- 

 to the hair-like surface of the lower part of the thorax, and 

 thence to the spiracles. This method of respiration gives 

 to the breasts of these beetles a silvery appearance while 

 they are under the water. 



The male {H. pieeus) is distinguished from the female by 

 having the last joint but one of the tarsi of its fore-feet 

 dilated into a kind of triangular plate. The colour of the 

 elytra of these beetles is a beautiful olive-black. The 

 abdomen beneath is also black, having quite distinct yellow 

 spots upon the margin. The antennae, which are reddish, 

 are nine-jointed, the four terminal joints forming a club. 

 There is upon the under-part of the body a prominent 

 ridge ending in a sharp spine. The posterior tarsi are well 

 adapted for swimming, being m.uch compressed and cUiated. 



These beetles may be found in the ponds, ditches, and 

 canals of England, chiefly the southern part, but they seem 

 to be rarer than formerly. They may be generally bought of 

 those who deal in aquarium requisites, however, at from 

 Is. to 2s. 6d. a pan-, according to the dealer and the season. 

 The aquarium in which these insects are confined should be 

 covered, or they are likely to escape, especially just about 

 the time they lay their eggs ; but anyone, I think, who 

 keeps them will come to the conclusion that they are the 

 most interesting and the least trouble of any of the aquatic 

 coleoptera. 



The only species belonging to the genus Hydrous is 3. 

 caraboides, an insect somewhat similar in appearance to 

 Rydrophilus pieeus, but smaller in size, being about fin. long, 

 difEerent in puncturation, and darker in colour; its keel is also 

 much shorter in proportion to the length of its body. The 

 shining blackness of its elytra occasionally has a greenish or 



