ORDER EPIIEMERIDA 73 



toward evening and fly for an hour or two, deposit their 

 eggs, and then disappear. Other species Kve for several 

 days, some perhaps for a week or two. Their first flight 

 is in what is cafled the pseudimago stage. When coming 

 to rest they attach themselves to some convenient object, 

 and there the skin of this stage splits and the insect escapes, 

 leaving the skin still clinging to the object. After this 

 moult they are mature adults with full-sized wings, but with 

 a difference in texture, and in genital organs, which are not 

 fully developed in the pseudimago stage. For many of the 

 species it is probably safe to say that they mate the same 

 day that they come out of the pseudimago stage. The 

 normal method of mating is on the wing. Eggs are probably 

 deposited within a day or two, being laid either on the 

 surface of the water or else underneath the surface. This is 

 done by the adult folding the wings and descending beneath 

 the surface. In one species the eggs are extruded in long 

 packets and then the insect descends close to the water and 

 deposits them on the surface. 



The eggs after deposition in one way or another — in or 

 on the water — hatch in a short time and the larvse grow and 

 probably migrate more or less in the water. They do not 

 have to come to the top of the water for respiration, as they 

 are fitted with organs which provide for aquatic respiration, 

 and probably live two or three years in the water. The 

 adults appear every year, and if the larvse require two or 

 three years to grow, there must be different broods, and if 

 there are two or three different generations in the lake, there 

 must be an enormous shifting of its population. They con- 

 stitute an enormous body of animal life, furnishing the basis 

 of food supply for other forms of animal life, fishes, etc., 

 which in a few days shifts its position and disappears, thereby 

 lessening the food supply materially. The larva; of these 

 nymphs come to the surface and the skin splits and the 

 pseudimago form issues and flies. The exuvise or skins which 

 are left on the surface drift in on shore and pile up in wind- 

 rows on the beach. 



