MAY-FLIES AND THEIR NEIGHBOURS 217 



a two-foot scabbard, you would say that there 

 was some conjuring trick or sleight-of-hand ; but 

 the May-fly does what is practically the same 

 thing, as a natural detail of its development. 



Fig. 142 shows the brilliant and fully-developed 

 May-fly just emerged and prior to its flight, leaving 

 its subimago skin on the grass blade behind. It is 

 almost saddening to think how often these miracles 

 are performed in vain, for incalculable myriads of 

 these flies perish so soon as emergence takes 

 place, being devoured by fishes, swallows and 

 other birds. 



During the five or six hours which is probably 

 the average time of their winged life, the female 

 May-fly finds a partner, and the eggs are scattered 

 upon the waters. And at the end of this time 

 the prodigious swarms of insects, in which the 

 males largely predominate, fall away as the in- 

 dividuals gradually become weaker and weaker, 

 and the rish enjoy a feast which is probably 

 equivalent to our "strawberry season." Some 

 species develop large swarms for several succes- 

 sive evenings, and then that generation of the 

 species disappears beneath the water for some 

 two or three years, although, of course, alternate 

 generations make an unbroken annual emergence ; 

 while other species, notably the Common May-fly, 

 may emerge in moderate numbers during the 



