viii MAY-FLIES 309 



" The head of the larva is furnished with slender, 

 many-jointed antennae, a pair of long, curved mandi- 

 bles, well suited to the excavating of earth, and 

 behind these with two pairs of maxillae, the hind pair 

 being united to form the labium. The fore legs are 

 stout, as if for digging ; they are directed forwards, 

 as also are the legs of the second pair, while those of 

 the third pair turn backwards. I have seen larvae, 

 when taken from their holes and placed in a bucket 

 of earth and water, use their fore legs to make a fresh 

 hiding-place. 



" The larvae of Polymitarcys are very hard to please 

 in the matter of fresh water ; they perish in four or 

 five days if placed in large buckets full of water ; 

 those of some other species, such as Chloeon dipterum, 

 on the contrary, will live for months in shallow 

 vessels without renewal of the water, and will there 

 undergo their transformation. 



" The Ephemera described by Swammerdam 

 (Palingenia longicauda) emerges about St. John's 

 Day ; the fly of Polymitarcys much later, viz., in mid- 

 August. Palingenia appears about six o'clock in the 

 evening, Polymitarcys at sun-down or later. Differ- 

 ences of temperature and other less obvious causes 

 render the appearance of the flies earlier in some 

 seasons than in others. 



" In 1738 I resolved to attend to the emergence of 

 the fly, and engaged an angler of Charenton to warn 

 me when the first signs appeared, which were expected 

 between St. Laurence's Day and " Notre Dame 

 d'Aout," that is, between the loth and I5th of August. 

 This year the flies appeared on the i8th. On the 



