240 INDIRECT BENEFITS DERIVED FROM INSECTS. 



known to the farmer, than that in which the Ephemerae of a 

 particular river are to emerge is to the fishermen. Yet a 

 greater degree of heat or cold, the rise or fall of the water, 

 and other circumstances we are not aware of, may accelerate 

 or retard their appearance. Between the 10th and 15th of 

 August is the time when those of the Seine and Marne, 

 which Reaumur described, are expected by the fishermen, 

 who call them manna : and when their season is come, they 

 say, " The manna begins to appear, the manna fell abundantly 

 such a night ; " — alluding, by this expression, either to the 

 astonishing quantity of food which the Ephemerae afford the 

 fish, or to the large quantity of fish which they then take. 



Reaumur first observed these insects in the year 1738, 

 when they did not begin to show themselves in numbers till 

 the 18 th of August. On the 19th, having received notice 

 from his fisherman that the flies had appeared, he got into 

 his boat about three hours before sunset, and detached from 

 the banks of the river several masses of earth filled with 

 pupas, which he put into a large tub full of water. This tub, 

 after staying in the boat till about eight o'clock, without 

 seeing any remarkable number of the flies, and being threat- 

 ened with a storm, he caused to be landed and placed in his 

 garden, at the foot of which ran the Marne. Before the 

 people had landed it, an astonishing number of Ephemerae 

 emerged from it. Every piece of earth that was above the 

 surface of the water was covered by them, some beginning to 

 quit their slough, others prepared to fly, and others already 

 on the wing ; and every where under the water they were to 

 be seen in a greater or less degree of forwardness. The 

 storm coming on, he was obliged to quit the amusing scene ; 

 but when the rain ceased to fall he returned to it. As soon 

 as the cloth with which he had ordered the tub to be covered 

 was removed, the number of flies appeared to be greatly 

 augmented, and kept continually increasing : many flew 

 away, but more were drowned. Those already transformed, 

 and continually transforming, would have been sufficient of 

 themselves to have made the tub seem full ; but their number 

 was soon very much enlarged by others attracted by the light. 

 To prevent their being drowned, he caused the tub to be 



