METAMORPHOSIS— CADDIS-FLIES 235 



broods are hurried on to enjoy the summer, to spread 

 and increase its abounding life. In this way the life- 

 history of the modern insects become divided into 

 periods characterised by adaptive structural features. 

 The earlier period is one of growth, the last is one of 

 active colonisation, and as these functions become 

 mutually incompatible, a gradual change from the 

 earlier nymph to the later fly has been abandoned. 

 An intermediate period is introduced during which 

 the transition is effected. Aided by this, the earlier 

 or nymph stage can now more freely adapt itself to 

 circumstances that favour growth, the fly can acquire 

 new combinations of structure and instinct that will 

 aid the spread and welfare of its race, whilst the 

 transition state itself becomes adapted to that pro- 

 tection which it increasingly needs as it becomes the 

 seat of that complex remoulding of the old traditional 

 life to meet fully the new activity. To the insect 

 in this transitional period the term pupa is applied. 



Caddis-flies. — The caddis (or hairy-winged) flies 

 offer a convenient example of this more complex life- 

 history. These insects abound near rivers and ponds, 

 and are not widely different from certain small moths, 

 though their four wings are provided with hairs 

 instead of with scales. The eggs are laid in or near 

 water, though occasionally far from any stream or 

 pond. The larvae, well known as caddis-worms, have 

 ' something of that versatility which characterises 

 the whole class of insects.' ] The great majority of 



1 Miall, Natural History of Aquatic Insects, p. 255. 



