270 THE LIFE OF AN INSECT. 



fishermen expect them with the greatest confidence 

 during these few days, nor are they ever dis- 

 appointed. Millions upon millions suddenly rise 

 into the air between eight and ten o'clock in the 

 evening, and this generally for three successive 

 nights."* Whatever be the temperature of the 

 atmosphere, whether it be cold or hot, these flies 

 invariably appear at the same hour in the evening, 

 that is, between a quarter and half-past eight; 

 towards nine they begin to fill the air; in the 

 following half hour they are in the greatest num- 

 bers ; and at ten there are scarcely any to be seen. 

 So that in less than two hours and these always 

 the same this infinite host of insects leave their 

 pupa state, become perfect insects, perform their 

 appointed work, and vanish. The same phenome- 

 non of regularity of limit to the pupa state occurs 

 also in other insects, though, perhaps, less strik- 

 ingly. Some insects constantly leave the pupa at 

 break of day ; others in the full tide of noon- 

 day, and others when the shadows of declining 

 day come over the landscape. These, however, 

 are certainly exceptions to the general rule, 

 which appears to lay down no precise period 

 * This scene is depicted in the Frontispiece to the last Part. 



