206 NATURAL HISTORY. [cH. XIV. 



Of the smaller branches, those which are nearest to 

 the common trunk are the largest, and those farthest 

 from it the smallest, consequently the general ar- 

 rangement of tnese tubes is conical. The second 

 figure represents a small portion of the branch, 

 which has been cut off and much magnified. The 

 large branch, which appeared simple in its construc- 

 tion in the first figure, is here seen to contain within 

 its cavity two tubes; each of the filaments which 

 project from the large branch is also to be seen hol- 

 low, and to contain within their respective cavities 

 two still smaller tubes. These vessels, which pre- 

 serve their shape under all circumstances, and con- 

 sequently cannot be membranous, are thus far sim- 

 ilar to the air-tubes of insects. In order to be con- 

 vinced of the fact of their being the respiratory 

 organs of the ephemera, we must examine the body 

 itself of the insect, and we shall find at the origin of 

 each fin two air-tubes, which terminate at that part 

 of the common trunk from which the two branches 

 arise. Why do these air-tubes go there, says Reau- 

 mur, if it be not to carry air to these fins, or to re- 

 ceive that which the fins bring to them ; or rather, to 

 do both 1 The constant and rapid action of these 

 fin-like appendages, seems necessary for a prompt 

 circulation of air. Probably the turning of these 

 organs in one direction may further the entrance of 

 the fluid, while the turning in the contrary may 

 further its exit. Reaumur conjectures, that a com- 

 plete knowledge of the mechanism of respiration in 

 this speciesof ephemera would afford valuable hints 

 for the improvement of our pumps. 



The larvce of other kinds of ephemera have their 

 respiratory organs planted perpendicularly along the 

 whole length of their bodies (fig. 1). At first sight 

 each fin appears to be composed of two leaves (fig. 2), 

 but on closer inspection it will be seen that this ap- 

 pearance arises simply from the peculiar manner ni 

 which each fin is folded. By spreading it out it ex- 



