Nerve-winged Insects; Scorpion-flies; Caddis-flies 241 
laid longitudinally or transversely and with projecting ends. Small snail- 
shells or bits of leaves and grass may serve for building materials. One kind 
of caddis-worm makes a small, coiled case which so much resembles a snail- 
shell that it has actually been described as a shell by conchologists. Some 
cases in California streams gleam and sparkle in the water like gold; bits 
of mica and iron pyrites were mixed with other bits of mineral picked up 
from the stream - bed to form 
these brilliant houses. An Eng¬ 
lish student removed a caddis - 
worm from its case, and pro¬ 
vided it only with small pieces 
of clear mica, hoping it would 
build a case of transparent walls. 
This it really did, and inside its 
glass house the behavior of the 
caddis - worm at home was ob¬ 
served. While most of the cases 
are free and are carried about by 
the worm in its ramblings, some 
are fastened to the boulders or Fig. 
rock banks or bed of the stream. p IG> ^32.—Two cases of caddis-worms with the 
These fixed cases are usually com- larval insects within showing head and thorax 
posed of bits of stone or smooth P r °j ectin S- ( Natural size -) 
pebbles irregularly tied together with silken threads. In all the cases silk 
spun by the caddis-worm is used to tie or cement together the foreign build¬ 
ing materials, and often a complete inner silken lining is made. 
Fig. 331. Fig. 332a. Fig. 
331. —Two cases of caddis-worms. 
Fig. 333. —Halesus indistinctus. (After Needham; enlarged.) 
The larvae within the cases are worm- or caterpillar-like, with head and 
thorax usually brown and horny-walled, while the rest of the body is soft 
and whitish. The head with the mouth-parts, and the thorax with the long 
strong legs, are the only parts of the body that project from the protecting 
case, and hence need to be specially hardened. At the posterior tip of the 
