CADDIS-FLIES. 59 



of the Phryganeidae are of very great and general interest. 

 The larvae are aquatic and are well known to all anglers 

 under the name of Caddis-worms. They construct for 

 themselves cases of a great variety of materials, some 

 species selecting sand, others stones, and others again 

 twigs or various vegetable fragments floating in the water. 

 These cases are, in some of the groups (Sericostomatides, 

 Leptocerides and Hydroptilides) tubular and movable ; 

 whilst in others (Hydropsychides and Rhyacophilides) 

 they are permanently fixed to the stones in the bed of the 

 stream. When removed from their cases, caddis-worms 

 bear a considerable resemblance to the caterpillars 

 of moths : in order to move about they must put their 

 head, and the three pairs of legs at the front of the body, 

 out of their tube or case, and they then look very like 

 case-bearing caterpillars. The part of the body that usually 

 remains under cover is different in texture and colour, and 

 frequently bears outstanding processes, or filaments, con- 

 taining tracheae for the purpose of extracting air from the 

 water. Some peculiar spaces of a different texture may 

 be seen on certain larvae, and these may possibly be also 

 connected with 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 there are 

 also frequently present on the anterior abdominal segment 

 three large bosses, 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 ; fisher- 

 men 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 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 fila- 

 ments to move in. The mouth of the case is open, and 

 the posterior extremity is arranged by the larva in such a 

 maimer 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-flies, 



