XXI CADDIS-FLIES 477 



different texture iiiay be seen on certain larvae (Fig. 322, B) ; 

 these may possibly be also connected witli respiration. On 

 each side of the extremity of the body there is a rather 

 large hook by which the creature attaches its dwelling to its 

 body, and the];'e are also frequently present three large bosses 

 on the anterior abdominal segment, which are supposed to 

 assist towards the same end. The hold it thus obtains is so 

 firm that it cannot be dragged out by pulling from the front ; 

 fishermen have, however, discovered a way of extracting it by a 

 strategic operation : the cases are, as a rule, partially open 

 behind, and by putting a blunt object in and annoying the 

 larva it is induced to relax the hold of its hooks and advance 

 forwards in the case, or even to leave it altogether. The firm 

 hold of the larva is maintained in spite of the fact that the body 

 does not fill the case. It is necessary that water should pass 

 freely into and out of the case, and that there should be some 

 space for the respiratory filaments to move in. The mouth of 

 the case is open, and the posterior extremity is arranged Ijy the 

 larva in such manner as to allow a passage for the water; various 

 ingenious devices are adopted by different species of larvae with 

 the object of protecting the hind end of the body, and at the 

 same time of permitting water to pass through the case. 



The mode of changing the skin, or the frequency with which 

 this occurs in the larval state of the caddis files has not been 

 recorded. The duration of hfe in this 

 stage is usually consideraljle, extending \\\ll|y/ 



over several months : indeed in our "^^^^^ 

 climate many species pass the winter in ^ ' ' 



this stage, completing the metamorphosis 

 in the following spring or summer; and 

 as one generation each year appears to 

 be the rule, it may be assumed that the 

 larval condition in such cases lasts from 

 seven to ten months. During this stage 

 the Insects are chiefly vegetable feeders, 

 some being said to feed on minute algae ; ''%!^^^'^^^^ 

 animal diet is not, however, entirely B, Mandiijies of pupa of 



avoided, and it is said by Pictet that ^^anna angustata. 



not only do some of the Phryganeidae eat other Insects, but that 

 they also sometimes devour then- companions. 



