THE DRAGON FLIES AND THE MAY FLIES 29 



Within a very short time the skin is again cast, even to a 

 thin covering from the wings, and the true imago comes 

 forth. A molt in the winged state is known nowhere else 

 among insects. Though the reduced mouth parts make it 

 impossible for the adult May fly to take any food, the ali- 

 mentary canal is not useless. Air is taken in at the mouth, 

 and the capacious stomach acts as a balloon, being provided 

 with valves so that the air cannot escape. 



Definition of Ephemerida (Gr. ephemeros, " lasting but a 

 day"). The May flies make up the order Ephemerida. 

 They may be distinguished from other insects by the two 

 pairs of lace-like wings, of which the hind pair is much the 

 smaller. Three or two fine, hair-like appendages extend from 

 the posterior end of the abdomen. The aquatic nymphs 

 have chewing mouth parts, but in the adults the mouth 

 parts are not functional. From the earliest nymph stages 

 to the imago there is a very clearly marked, though grad- 

 ual, change or incomplete metamorphosis. 



