EPHEMERA MARGINATA. 63 



mals. In land animals the function of respiration is 

 performed by lungs ; in fish by gills : in both of which, 

 however, the blood is propelled to the respiratory organ 

 to be aerated. In insects, on the contrary, the air is con- 

 veyed through the body in vessels, called the tracheae, 

 and the blood is aerated by circulating around these 

 vessels ; whence originates the term, a diffused circula- 

 tion. In place of the heart is substituted a single pro- 

 pelling organ, which, from its peculiar situation, has been 

 named the dorsal vessel. 



The circulation of the blood in this larva is shewn in 

 a magnificent manner in an achromatic microscope, with 

 a triple-cemented object-glass of an inch or half an inch 

 siderial focus. For this purpose I would recommend the 

 selecting of a young specimen about one or two-tenths 

 of an inch in length, when it has just shed its skin, at 

 which time the entire creature is exceedingly pellucid. 



Around the large air-vessel which passes down each 

 side of the body, as also along the legs, antennae, and 

 three-forked tail, the oat-shaped globules of blood may 

 be seen to pass rapidly at every pulsation of the dorsal 

 vessel. This vessel, according to Mr. Bowerbank's ob- 

 servations*, " extends nearly the whole length of the 

 body, and is of great comparative magnitude. It is fur- 

 nished at regular intervals with double valves about 

 equal in number to the sections of the body." A portion 

 of this vessel, with its valves, is represented in the an- 

 nexed figure. " Both above and below each of these sets 



* Entomological Magazine, vol. I, page 240. 



