XIX MAY-FLIES 437 



pletely shed, and the creature is thus transformed from a water- 

 breather to an Insect breathing air as usual. In addition to this 

 change there are others of great importance, such as the develop- 

 ment of the great eyes and the complete atrophy of the mouth- 

 parts. The precise manner of these changes is not known ; they 

 occur, however, within the nymph skin. The sudden emergence 

 of the winged Insect from the nymph is one of the most 

 remarkable facts in the life-history of the may-fly ; it has been 

 observed by Sir John Lubbock, 1 who describes it as almost in- 

 stantaneous. The nymph floats on the water, the skin of the back 

 opens, and the winged Insect flies out, upwards and away ; 

 " from the moment when the skin first cracks not ten seconds are 

 over before the Insect has flown away." The creature that thus 

 escapes has not, however, quite completed its transformation. It 

 is still enveloped in a skin that compresses and embarrasses it ; 

 this it therefore rapidly gets rid of, and thus becomes the 

 imago, or final instar of the life-cycle. The instar in which the 

 creature exists winged and active, though covered with a skin, is 

 called the sub-imago. The parts of the body in the sub-imago are 

 as a whole smaller than they are in the imago, and the colour is 

 more dingy ; the appendages wings, legs, and caudal setae are 

 generally considerably shorter than they are in the imago, but 

 attain their full length during the process of extraction. The 

 creatures being, according to Riley, very impatient and eager to 

 take to the wing, the completion of the shedding of the skin of 

 the sub-imago is sometimes performed while the Insect is flying 

 in the air. 



The food of young Ephemeridae is apparently of a varied and 

 mixed nature. Eaton says 2 

 that though sometimes the 

 stronger larvae devour the 

 weaker, yet the diet is even 

 in these cases partly vege- 

 table. The alimentary canal 

 frequently contains much 



mud ; very small organisms, Fia 2 83.-Lingua of Heptagenia longicauda, 

 SUCh as diatoms and con- * 1 6. ?, Central; I, lateral pieces. (After 



fervae, are thought to form 



u large part of the bill of fare of Ephemerid nymphs. Although 

 1 Tr. Linn. Soc. xxv. 1866, p. 483. 2 Ann. Nat. Hist. (3) xviii. 1866, p. 145. 



