120 THE BOOK OF NATURE STUDY 



may lay eggs in captivity. The eggs are laid in early spring, 

 and are inserted into the stems of aquatic plants by means of 

 the ovipositor. From them the larvae hatch in a few weeks. 

 In any case, however, the larvae are to be found abundantly in 

 the ponds in which the adults occur. When full-grown they 

 reach a length of about 2 inches. Growth is very rapid, and 

 is completed in less than two months, and in consequence the 

 young are as voracious as their parents. They are much less 

 active, and hide their lank brownish bodies in the mud. Breath- 

 ing is effected at the surface, the tail being thrust through the 

 surface film much as it is in the gnat larva. Three pairs of slender, 

 non-swimming legs are present, and also a pair of long but slender 

 jaws. The mouth is minute, the little creature living on the 

 juices of its prey somewhat after the fashion of a spider. 



As the larva is a true air-breather no less than the adult, 

 and as it must rise periodically to the surface to breathe, it is 

 obvious that the pupa cannot remain within the water. In 

 point of fact, the full fed larva quits the water and becomes a 

 motionless pupa after burying itself in the ground near the pond 

 in which it has hitherto lived. The winged adult returns to 

 the water, but, as already indicated, uses its wings to make its 

 escape if the conditions are unfavourable. It thus does not 

 require any special adaptations to fit it to resist drought, for 

 example. 



A more useful inhabitant of the general aquarium than 

 Dyticus marginalia is the Silver Beetle (Hydrophilus piceus), 

 which is largely vegetarian and therefore does not cause such 

 widespread havoc. It is unfortunately rarer, but can be readily 

 obtained from dealers, and should be liberally supplied with 

 water-weed. This beetle reaches more than i \ inch in length, with 

 a width of f inch. It is of a dark green colour above and black 

 below, but the under surface is largely covered with closely set 

 yellow hairs, in which the air becomes entangled, and which 

 thus shine like silver under water. The antennae are short, 

 instead of being long as in Dyticus, and the short fore-legs end 

 in two sharp claws. The last pair of legs are turned into 

 swimming paddles as in Dyticus, but they move alternately 

 instead of together, and the animal is a less swift swimmer than 



