558 



AN INTRODUCTION TO ENTOMOLOGY 



end of the body and the legs from the case when they traveh The 

 cases of different species differ greatly in form and in materials used 

 in their construction; 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 ; but in most cases 

 it is not spun into a thread, but is poured forth in a glue-like sheet 

 upon the objects to be cemented together; some species, however, 

 build nets of silken strands. 



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. Ex- 

 amples of different types of cases are figured later. 



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

 water to transform, as do nearly all other aquatic larvae, 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 silt; 

 but usually provision is made for the ingress of water for respiration. 

 Some species merely cement a stone or grains of sand over each open- 

 ing of the case; others build a sill-cen lid with a slit in it; and still 

 others build a silken grating in each end of the case. Frequently 

 caddice-worms leave the open water in which the larval life has been 

 spent and seek some more secluded place in which to transform, such 

 as crevices in bark or among roots, or they may 

 burrow into wood or into the soil. 



The pupae are of the exarate type, that is, the 

 wings and legs are free (Fig. 688). Some pupae 

 have tracheal gills, others do not; this, however, 

 is not correlated with the presence or absence of 

 tracheal gills in the larva; tracheal gills may be 

 present in either of these stages and absent in the 

 other. 



In the case of those caddice-flies that emerge 

 from rapidly flowing water, as the net-building 

 species, the wings expand instantly when the in- 

 sect 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 

 nimibers of them chat are attracted to lights. 



Fig. 688.— A, pupa 

 of Phryganea pi- 

 losa. '(After Pic- 

 tet.) B, mandi- 

 bles of pupa of 

 Molamia angusta- 

 ta. (From Sharp.) 



