IN THE TEST. EPHEMERAE. 119 



have on their sides small fringes of hair, which, 

 when put into motion serve them as fins. It is 

 very curious to see them plying their little oars in 

 the water. These larvae scoop themselves dwel- 

 lings in the banks of rivers, and they are small 

 tubes made like syphons, the one serving for an 

 entrance, the other for an outlet. The banks of 

 rivers are often perforated with them. When the 

 waters decrease, they dig fresh holes in order to 

 enjoy the water. The season and hour when the 

 chrysalides of the different species of the Ephe- 

 merae turn into flies, maintain a kind of regularity, 

 being accelerated or postponed by the temperature, 

 or the rise and fall of the waters. 



The Ephemerae of the Rhine, appear in the air 

 two hours before sunset. These flies are hatched 

 at the same instant in such numbers as to darken 

 the air. 



The females by the help of the threads of their 

 tail, and the flapping of their wings, support them- 

 selves on the surface of the water, and drop their 

 eggs into it. 



Perhaps the most curious circumstance in the 

 history of these Ephemerae, is the fact, that after 

 they have quitted the water, and taken flight, 

 often to some distance, they have to disengage 

 themselves fiom a thin skin which covers every 

 part of them. The exuviae are generally found on 

 bushes, so that some support seems necessary to 



