380 Annals Entomological Society of America [Vol. VI ^ 



May-flies must swarm in like manner at night is testified by 

 the great numbers often caught in webs in the early morning. 

 The important functions of this stage are the fertilization 

 and laying of the eggs. 



IV. MODIFICATIONS OF STRUCTURES OF THE NYMPH. 



Nymphs of Ephemerinae and Heptagenin^e (Needham) 

 live fairly within the limits of two ecological situations. The 

 Ephemerinee inhabit mud or muddy water exclusively. Most 

 of the Heptagenince live in riffles of streams or upon the wave 

 washed shores of lakes. The B^etinse inhabit gentle currents 

 or open waters and intermingle with the mud and cascade 

 dwellers as well. They have become adjusted to very dif- 

 ferent situations and they show a wide range of specialization. 



All of the Ephemerinae which have been found here live in 

 the same situation and are very similar in their habit of life. 

 Ephemera and Hexagenia are true burrowers in the mud;, 

 Polymitarcys occasionally adopts the digging habit and Pot- 

 amanthus crawls upon silt covered stones and muddy bottoms- 

 in the same locality. 



The members of the Heptageninas are also very homo- 

 geneous habit. They live in running water, clinging or moving 

 about upon the under sides of stones. Iron and Epeorus 

 dwell in the swiftest water of the current, in riffles and falls;. 

 Ecdyurus and Heptagenia live in the gentle currents along 

 the borders of the stream and sometimes beneath the stones 

 in quiet pools. 



The Baetinae dwell in a variety of situations. Siphlurus 

 and Callibaetis clamber upon the aquatic plants or dart about 

 on the alga covered bottoms of still pools and inlets, while 

 Ameletus more often frequents moving waters and nymphs 

 of Blasturus hide among decayed leaves in ponds and brooks. 

 Leptophlebia and Habrophlebia cling closely to the surface 

 of stones, usually upon the under side and often in fairly 

 rapid water. Most members of the genus Ephemerella have 

 a similar habit, but there is a wide divergence among the species 

 of this genus. Two genera, Baetis and Chirotonetes, are 

 dwellers in water falls, and the latter has become remarkably 

 well adjusted to its habitat. Tricorythus and Caenis are 

 adjusted to life in mud and sand and show structures especially 

 well fitted to their surroundings. These two extremes of. 



