TRICHOPTERA 181 



struction ; but silk is used in building all of them. This silk, like that of 

 caterpillars, is secreted by modified salivary glands and is emitted 

 through an opening in the labium. 



Some caddice-worms build their cases entirely of silk; but most of 

 the case-building species use other materials also; these may be grains 

 of sand, small stones, bits of wood, moss, or pieces of leaves; and some 

 species fasten shells of small mollusks to their cases. The materials used 

 are glued together with silk; and the case is lined with silk, so as to 

 form a suitable protection for the soft abdomen. 



When the caddice-worms are full-grown they do not leave the water 

 to transform, the pupae being as truly aquatic as the larvae. Some of the 

 case-building species change the form and material of their cases at this 

 time; and nearly all of them partly close their cases so as to keep out 

 intruders and sift; but usually provision is made for the ingress of water 

 for respiration. Some species merely cement a stone or grains of sand 

 over each opening of the case; others build a silken lid with a slit in it; 

 and still others build a silken grating in each end of the case. Within 

 these modified cases the larvae transform to pupae. 



In the case of those caddice-flies that emerge from rapidly flowing 

 water, as the net-building species, the wings expand instantly when the 

 insect reaches the surface of the water and are then fitted for flight; it is 

 evident that if much time were required for the wings to become fit for 

 use, as is the case with most other insects, the wave succeeding that 

 which swept the insect from the water would sweep it back again and 

 destroy it. 



The Trichoptera can be regarded as beneficial insects, as the larvae 

 form an important element in the food of fishes, and especially of the 

 brook trout. Sometimes in cities near rivers, the adults are annoying on 

 account of the great numbers of them that are attracted to lights. 



This is a large order and includes several families. Each species of 

 those which make cases builds a par- 

 ticular kind of tube. Some caddice- 

 worms are carpenters, 

 building their houses of _ _ „ , , ,. 



o - Fig. 318. — Case of caddice- worm. 



straws or sticks placed 



lengthwise of the body (Fig. 318); but certain species that 

 make their houses chiefly of straws fasten the straws cross- 

 wise like the logs of a log-house (Fig. 319). These log-house 

 fig. 319— Log- builders often have the curious habit of decorating their 

 . a m type o case. k ouses j-^ f as tening snail-shells to the outside. 



There are caddice-worm houses closely resembling in plan those just 

 described but differing in appearance, being-made of bits of moss. Some- 

 times the houses are built of leaves ; these may be fastened so as to form 

 a flat case; or are arranged in three planes, so as to form a tube, a cross- 

 section of which is a triangle. 



Other caddice-worms are masons, building their houses of grains of 

 sand or of small stones. Sometimes these houses are tubes very regular 

 in outline, being composed only of grains of sand fastened together with 

 silk; but certain species of mason caddice-worms fasten larger stones on 

 each side of this tube of sand (Fig. 320). Some of the species that build 

 tubes of sand make spiral houses which very closely resemble in form 

 snail-shells (Fig. 321). 



