324 INTRODUCTION TO ZOOLOGY chap. 



Within the closed case, in a few days' time, the larva casts 



its skin piecemeal, and the soft white pupa is exposed ; on it 



can be seen the antennae, quite free from 



the body, and tlie legs and folded wings of 



the imago (Fig. 243). This pupa is peculiar 



in possessing several organs not found in 



the imago. There are respiratory hairs on 



the abdominal segments, and the head bears 



great toothed mandibles which cross over 



the face (Fig. 244). The presence of the 



mandibles is specially interesting, since the 



imago possesses as a rule no 



trace of them. They are of 



distinct use to the pupa, for it 



is with them that it breaks 



down the covering at the end Fi°- 244.— 



of the case, when it is ready V'^^^^ff 



ofa Caddis (After *° emerge ; these mandibles are pupa, seen 



Pictet.) shed with the pupal skin. from in 



The pupa retains the power ^ ' 



of movement in its abdomen, and thus is able to keep up the 



current of water which is necessary for respiration. 



Emergence After two or three weeks, the pupa breaks down 



of the the end of the case, and — still enveloped in the 



Imago, pupal skin — it swims, back downwards, to the 



surface of the water, and climbs out into the air by means 



of some projecting weed, or other object (Plate III., P). 



When at a height of a few inches above the water it stops and 



rests, then its skin swells and splits, and the new Caddis-fly 



quickly emerges and flies away — the complete empty pupal 



skin remaining like a pupal ghost behind. 



The imago is at first rather pallid and greenish, but it 

 darkens in colour in a few days ; the antennae are thin and 

 tapering, and longer than the body ; the legs also are long, 

 but the body is nevertheless usually held rather low down 

 against any support, the drooping wings often hiding the 

 legs. 



This is the history of the life lived by most caddises, 

 though a few species differ in some details of their habits. 

 There are certain small caddises which do not actually leave 

 the water before the imago emerges, but merely float to the 



