312 NATURAL HISTORY OF AQUATIC INSECTS en. 



before sunrise, should be drawn so powerfully towards 

 a lantern. The person who held the light had a bad 

 time of it ; in a few moments he was covered with the 

 flies, which came in all directions as if to overwhelm 

 him. The luminous sphere about the light was 

 crossed at all angles by the orbits of the circling 

 Insects, which after performing one or two revolutions, 

 fell to the earth. 



" After half an hour or less the swarms were less 

 dense, and by ten o'clock only a few scattered Ephe- 

 merae could be seen on the river, while no more came 

 round the light. 



" Next day the Ephemerae appeared in undiminished 

 numbers, but on August 2ist they hardly amounted 

 to one-third of the previous flights. Each day they 

 appeared between a quarter and half past eight ; 

 from nine to half past nine they filled the air, while 

 by ten o'clock they had almost ceased to fly. 



" In the afternoon of the 2i.st the air was cold for 

 the time of year, the thermometer standing at I7°R. 

 (70°F.).^ One might suppose that heat would hasten 

 the transformation of these larvae, as it hastens the 

 development of the chrysalis or the egg of other 

 Insects. However, in spite of a relatively low 

 temperature, the flics emerged at the same hour as 

 before. 



" On the 22nd the thermometer fell to I5°R. (66°F.); 



^ I have given in brackets the accepted equivalents on Fahren- 

 heit's scale of the readings quoted by Reaumur from his own 

 thermometer. They are obviously too high. Reaumur's ther- 

 mometer was somewhat differently graduated from any modern 

 ones. 



