FRESH-WATER BIOLOGY 



559- The nymph of 

 'olymitarcys alba. 



water just as darkness falls. Some of the less nocturnal species, 

 as Leptophlebia and Choroterpes, swarm out in the sunlight in 

 sheltered places of late afternoons, or dance up and down among 

 the mixed shadows and sunlight beneath the 

 canopied crowns of tall stream-side forest trees. 

 The females of Baetis creep beneath stones at 

 the surface of the water and deposit their eggs in 

 single-layered patches just beneath the surface. 



The adult life of Mayflies is truly ephemeral 

 and is concerned wholly with reproduction; and 

 the struggle for existence is transferred largely to 

 the immature stages. The nymphs are highly and 

 independently specialized. They are adapted 

 to all sorts of aquatic situations. A few, like 

 Hexagenia, Ephemera, and Polymitarcys (Fig. 

 1359), are burrowers beneath the bottom silt. 

 A few, like Caenis and Ephemerella, are of seden- 

 tary habits and live rather inactively on the FlG - 

 bottom, and on silt-covered stems. Many are 

 active climbers among green vegetation; such are Callibaetis and 

 Blasturus; and some of these can swim and dart about by means of 

 synchronous strokes of tail and gills with the swiftness of a minnow. 

 The species of Leptophlebia love the beds of slow-flowing streams, 

 and all the flattened nymphs of the Heptageninae live in swiftly mov- 

 ing water, and manifest various degrees of adaptation to withstand- 

 ing the wash of strong currents. The form is depressed, and margins 

 of the head and body are thin and flaring, and can be appressed 

 closely to the stones to deflect the current. So diverse are the 

 nymphs in form that the genera may be distinguished among them 

 by a beginner more easily than among the adult Mayflies. 



Mayfly nymphs feed largely on dead vegetable substances the 

 decaying stems and leaves of aquatic plants. They are of first 

 importance in the food of fishes. But we are as yet largely in 

 ignorance of the conditions that make for their abundance. 



The study of this group has been greatly neglected by entomol- 

 ogists and our .Mayfly fauna is very insufficiently known. The 

 ecology of the immature stages is especially in need of investigation. 



