206 



Aquatic Organisms 



duns. Then this outer skin is shed, and they come 

 forth with smooth and shining surfaces and brighter 

 colors, as imagos, fully adult, and ready for their 

 mating flight. Lacking mouth parts and feeding not 

 at all, they then live but a few hours. 



There are few phenomena 

 of the insect world more strik- 

 ing than the mating flight of 

 mayflies. The adult males fly 

 in companies, each species 

 maneuvering according to its 

 habit, and the females come 

 out to meet them in the air. 

 Certain large species that are 

 concerted in their season of 

 appearance gather in vast" 

 swarms about the shores of all 

 our larger bodies of fresh water 

 at their appointed time. By 

 day we see them sitting 

 motionless on every solid sup- 

 port, of ten bending the stream- 

 side willows with their weight ; 

 and when twilight falls we see 

 all that have passed their final 

 molt swarming in untold num- 

 bers over the surface of the 

 water along shore. 



The nymphs of mayflies are all recognizable by the 

 gills upon the back of the abdomen. These are 

 arranged in pairs at the sides of some or all of the first 

 seven segments. The body terminates occasionally in 

 two but usually in three long tails. The mouth parts 

 are furnished with many specialties for raking diatoms 

 and for rasping decayed stems. Mayfly nymphs are 

 among the most important herbivores in all fresh waters. 



FIG. 113. The nymph of the 

 mayfly, Siphlonurus alternatus. 

 (Photo by Anna Haven Morgan.) 



