INTRODUCTION 19 



to be desired that observing persons would attend to 

 this and like subjects instead of piling up lists of 

 species, which tell us so little that signifies. 



The common rule is, I think, that aquatic Insects 

 winter as larvae. Nearly all aquatic Diptera, Dragon- 

 flies, May-flies, Stone-flies and Caddis-flies do this. 

 Occasionally, however, the winged individuals hiber- 

 nate. Examples are furnished by the Gnat, and one 

 of the Dragon-flies (Lestes). 



Fully armoured aquatic Insects, such as Beetles and 

 Bugs, commonly pass the winter in the winged state, 

 burying themselves in the mud or in the earth during 

 unusually severe weather. 



Aquatic Insects which have wintered as larvae 

 usually undergo transformation and lay their eggs in 

 the following spring. From these eggs a summer 

 generation proceeds, which becomes ready for egg- 

 laying in September, and so the cycle comes round. 

 This is the usual course with aquatic Diptera of small 

 size. There may be more than one summer-brood, 

 and sometimes the new generations appear so fre- 

 quently that larvae, pupae and flies can always be 

 found during the summer half-year. This is the case 

 with Chironomus, for instance. Large Insects, and 

 the great majority of the aquatic Beetles, produce 

 at most one brood in the year. 1 Egg-laying takes 

 place in spring or early summer, and the transforma- 

 tion takes place in the height of summer, though not 

 always in the summer which immediately follows the 

 hatching-out of the eggs. Upwards of a year may be 

 spent in the larval stage. 



1 Hydrobius produces a second brood in autumn. 



C 2 



