AQUATIC INSECTS 887 



minutes, as in Caenis, or it may be delayed twenty-four hours or 

 more, as in most of our larger species. Caenis probably lives but a 

 few hours after leaving the water ;J}ut the larger forms live through 

 two days, their transformation from the nymph occurs in one night, 



FIG. 1358. A newly-emerged Mayfly, Hexagenia bilineata. 



their final moult the next night, and their period of adult activity 

 and egg-laying and their death the next evening. 



The adults are peculiar in the venation of their wings (Fig. 1387) 

 and in the extent of the longitudinal furrowing of the same, in 

 the lack of functional mouth parts and in the buoyant function 

 assumed by the alimentary canal, which, being no longer used for 

 food, is filled with air. While highly specialized in most respects, 

 one very generalized character has been retained in the group: 

 the openings of the oviducts of the female are paired and separate. 



The males of most species indulge in graceful ante-nuptial 

 flights, that to the observer appear most delightful and exhilarating. 

 They assemble in little companies and dance up and down, alter- 

 nately rising and falling, flying upward and falling down again on 

 outspread wings in long vertical lines. The crepuscular species 

 such as Ephemera and Hexagenia, that compose the well-known 

 swarms, fly out over the surface of the water, where the females 

 meet the males, and afterwards settle down upon the surface of the 

 water to liberate their eggs. Caenis swarms over the edge of the 



