104 FIRST BOOK OF ZOOLOGY. 



worms, and are inclosed in cases of cylindrical and other 

 shapes. These are variously made of grains of sand, bits 

 of bark and sticks, and other fragments of convenient size 

 cemented together. Some of these cases, built of small 



FIG. 100. CADDIS- WORM, WITH ITS CASE. 



grains of sand, look like coiled snail-shells. Other larvse 

 shelter themselves in bits of straw, or the fragments of 

 hollow stems of plants. 



Fig. 100 represents the larva of one of these insects in 

 its case, which is made of bits of sticks arranged in a spiral 

 course. The larva drags about this case, and as it grows col- 

 lects material for the enlargement of its tube. 



In almost any quiet pool or running stream these curious 

 cases may be found, containing the larvse within. 



FIG. 101. GALL-FLY. 



96. The curious round balls called gall-nuts, which are 

 found on the leaves of the oak and of other trees, are pro- 

 duced by an insect called the gall-fly. 



The eggs are deposited in the substance of the leaf, and 



