ii2 THE BOOK OF NATURE STUDY 



These are as a general rule those inhabiting running as opposed 

 to still water, and the teacher should not fail to point out the 

 predominance of caddis cases among the rubbish found at the 

 sides of streams after floods, and to emphasise the dangers to 

 which the forms found in running water are subject. A very 

 interesting case is that constructed by the stream-dwelling Hal- 

 esus auricollis, which is made of vegetable matter, but is weighted 

 at the lower end by a tiny pebble, or even fastened down if the 

 current is very strong. 



Very little observation will show that the particles of which 

 the case is composed are woven together by a silky substance 

 which is secreted by a gland which opens on the under surface 

 of the head, and is quite similar to that which secretes the silk 

 in a caterpillar. As the object of the case is to protect the lank 

 soft body from possible enemies, such as fish, the caddis, what- 

 ever their natural tastes, will avail themselves of any building 

 substance supplied to them. It is thus possible to make them 

 build nests of beads, little fragments of brightly coloured minerals, 

 or even transparent glass. In the last case the resultant tube 

 allows the movements of the larva to be discerned through it, 

 which is justifiable if a detailed study of habits is to be made ; 

 but otherwise the teacher will be well advised to limit herself 

 to substances which occur naturally in the ponds and streams 

 in which the caddis live. 



The case is added to as the contained larva grows, and is 

 always considerably larger than its inhabitant, so that not only 

 can the caddis completely withdraw into its tube, but it can even 

 turn in it and cut off the posterior end as this becomes too narrow 

 for the abdomen. Larvae ejected from their tubes, or which have 

 been supplied with fragments of glass out of which a transparent 

 tube can be manufactured, will show the interesting breathing 

 organs. These consist of delicate filaments borne on the 

 abdomen, which in the water have a feathery appearance. The 

 filaments are abundantly supplied with branches of the tracheal 

 system, which are so modified as to permit the larvae to take 

 advantage of the air dissolved in water. As the respiratory 

 filaments lie permanently within the case they must be liable to 

 suffer from the stagnation of the water, especially in the case 



