CLASSES OF CONDITIONS 135 



or pupa to the winged subimago or imago is, of 

 course, a misnomer, as the word hatch means the 

 emergence of the young larva or nymph from the egg. 

 The late T. P. Hawksley when hatching Ephemeridse 

 and Trichoptera from eggs in captivity found those 

 in a strong light invariably hatch sooner than others 

 of the same brood placed in warmer positions but in 

 comparative darkness. It is also suggested that warmth 

 encourages the growth of weed which thus provides 

 more food for the nymphs and thereby advances the 

 date of the appearance of the flies in winged form. 



The temperature of the air has very little effect on 

 the temperature of the water in a chalk-stream, the 

 more so as the volume of water is continually being 

 increased by springs filtering up at many places 

 throughout the course of the stream. The difference 

 between the summer and winter temperature of the 

 water in a chalk-stream is far less than one would 

 imagine, and it is, therefore, not surprising to find 

 that to the hands the water of these streams always 

 seems cold in the summer and warm in the winter. 

 This, too, is one of the reasons why our Hampshire 

 rivers are seldom frozen, excepting in eddies or very 

 still and nearly stagnant parts. 



The object of this dissertation on the temperature 

 of the water in the chalk-streams is to impress upon 

 the student's mind the fact that his readings of the 

 thermometer in the air are unsafe criteria of the 

 probable hatch of fly. It may, too, serve to illustrate 

 the absurdity of what we read at times in the sporting 



