THE PHR YGANIDAl. 
367 
them, and only project the front part of their bodies. Should 
any danger threaten them, or should they be alarmed, they 
retract their bodies within the case immediately. Each species 
makes its case in a different manner, and employs different 
materials. Some of the larvae of caddis flies invariably use 
gravel or very small stones for their cases, and others the 
small fresh-water snail and other shells, which they glue together. 
Many use bits of stick or morsels of plants, so that each species 
the nymph and larva of species of the genus Phryganea . (Magnified.) 
may be known by its tube. Some live in fixed habitations, 
which are made in the same manner as the others, but which 
are not to be moved. 
So numerous were the caddis worms in the olden time that 
a very thick limestone in the Auvergne is made up of the tubes 
or indusiae of them. 
The larvae always live in water, and they have respiratory 
filaments fitted for aquatic breathing upon the sides of the 
abdomen. They have a scaly head, and the thoracic segments 
are clothed with leathery plates. The legs are long, and the 
