AND THE LOWER ANIMALS. 77 



level were pierced by numerous little round holes 

 from three to four millimetres wide, and usually 

 grouped in pairs. These are the entrances of as 

 many arciform galleries extending for about six to 

 eight centimetres into the earth, and inhabited by 

 ephemera larvae. Each larva is about two centi- 

 metres long ; its head bears two very large compound 

 eyes, a pair of mandibles which are used in burrowing, 

 and jaws adapted to grind the slimy mud upon which 

 the creature seems to subsist. The thorax is quite 

 distinct, and has six legs already attached to it ; the 

 abdomen ends in three long hairy filaments, and is 

 covered with large fringed lamina3, which the creature 

 moves with great rapidity. These laminae are actual 

 gills, that is to say, they are organs of aquatic 

 respiration. Large tracheal trunks ramify through 

 them, and extract from the surrounding water the air 

 which is necessary to the preservation of life. 



The ephemeron remains in this larval condition for 

 about two years, gradually acquires its proper size, 

 and then becomes a pupa. But this, quite the reverse 

 of all the insects we have been describing, does not 

 alter its larval mode of life. It lives in the same 

 gallery, is just as active as before, and only differs in 

 the possession of rudimentary wings attached to the 

 upper part of the thorax. We see then, that this first 

 metamorphosis is in certain particulars an incomplete 

 one. It is not so with the second; this takes place 

 every year at the same period, and occurs daily, nay 

 hourly, without being influenced to any decided 

 extent by variations of temperature. The nymphs or 

 pupse may be seen emitting the water for dry land, 

 between eight and half-past eight every evening, from 



