THE ANIMALS OF THE FRESHWATER AQUARIUM 113 



of caddis found in still water. To remedy this the larva undul- 

 ates its abdomen, and so causes a current of water to flow through 

 the case, much as a crayfish causes a current of water to flow 

 through its gill-chamber by movements of certain of its appen- 

 dages. The movement in the case of the caddis, even when 

 the animal is enveloped in an opaque tube, may be demonstrated 

 by putting some fine particles into the water, when they will 

 be stirred by the intermittent respiratory current. It is the 

 existence of this current which explains why the tube is per- 

 manently open at its posterior end. 



Caddis live nearly a year in the water, so that if your specimens 

 are taken from the ponds in early spring, and are then of con- 

 siderable size, they may be expected to pass into the pupa stage 

 in early summer. The approach of this condition is indicated 

 by the little creature ceasing to feed (it has hitherto fed freely 

 on vegetable matter) and ceasing to be active. It no longer 

 protrudes the anterior part of the body from the case, and the 

 ends of this may be observed to be closed by a delicate web, so 

 arranged as to permit of the continued entrance and exit of 

 water for respiratory purposes. Sometimes, to make security 

 doubly sure, the ends are partially blocked by minute pebbles 

 or morsels of stick. The caddis in their case should at this stage 

 be removed to a separate dish, if they have hitherto been in the 

 general aquarium. Within the closed case extraordinary re- 

 constructive processes go on. These changes go on within the 

 case and also within the larval skin. They result in the forma- 

 tion of what is called the nymph, which has in general terms 

 the structure of the adult caddis-fly. As the nymph is formed 

 the larval skin is shed piecemeal within the case, and the nymph 

 skin, at first soft, gradually hardens. When the changes are com- 

 pleted, after a period of some weeks, the nymph eats its way 

 out of the case by a pair of strong jaws which it bears for the 

 purpose, and swims through the water in search of some weed, 

 stick or stone by means of which it may crawl to the surface 

 of the water. Once raised above its level, the skin of the nymph 

 splits and allows the perfect insect to emerge. It dries its wings 

 in the sun, and flies away to begin the free aerial life. The perfect 

 insect is moth-like in appearance, and does not travel far from 



VOL. II. 8 



