228 



APPLIED ENTOMOLOGY 



silk, and between these spin a silken tube in whicli to live. Close to this 

 they spin more or less funnel-shaped webs, the mouth up-stream and so 

 arranged that tiny animals swept down by the current within the outer 

 limits of the funnel come within reach of the larva lying in its tube. 

 While the food of these larvae is carnivorous, in most of the species plant 

 materials are consumed. 



The larvae, in most cases, breathe by tracheal gills which are slender 

 filaments, frequently grouped in clusters, and attached to the abdominal 

 segments. Other structures present in some species are also suspected of 

 being concerned with respiration. 



Fig. 225. P'lG. 226. 



Fig. 225. — Caddice-fly larvae: larva with head and thorax extended out of its case, 

 above; larva removed from its case, below, showing tracheal gills. About twice natural 

 size. {Modified from Lenckart's Wandtafeln.) 



Fig. 226. — Examples of different types of cases formed by Caddice-fly larvae. (From 

 Sanderson and Jackson, Elementary Entomology: after Furneaux.) 



When full-grown the Caddice-worm forms a sort of lid or door grating 

 across the front opening of its case, though not complete enough to pre- 

 vent water from entering and supplying the in.sect with the oxygen it 

 needs. After pupation in its case the adult swims to the surface and 

 grasps some object, from which it takes its flight. In some species it is 

 apparently the pupa which when ready to become the adult, comes 

 to the surface and passes its final molt there. 



The Trichoptera is quite a large group of insects and representatives 

 of it are found in almost all parts of the world. Probably not many more 

 than a thousand species have been described, as they do not appear to be 

 of any economic importance unless their consumption of decaying vege- 

 table matter in pools can be considered as desirable, but it is very likely 

 that there are from five to ten thousand kinds in existence. Their cases 

 have been found as fossils and adults have also been preserved in this way. 



