CHAPTER III 



THE MAY-FLIES (PLECTOPTERA) AND THE DRAGON- 

 FLIES (ODONATA) 



The sun comes forth and many reptiles spawn ; 



He sets and each ephemeral insect then 



Is gathered into death without a dawn, 



And the immortal stars awake again. SHELLEY. 



May-Flies. The May-flies (Fig. 14), which stand in litera- 

 ture as the type of brief and purposeless existence, are deli- 

 cately constructed, pale insects, with usually four finely 

 veined wings and two or three long white filaments project- 

 ing from the end of the abdomen. The eyes are compara- 

 tively large, but the mouth-parts are so reduced that no food 

 can be taken during adult life, which in most species lasts 

 only a few hours. May-flies appear in countless numbers in 

 late spring or early summer, dance about in the air at dusk 

 in swarms so dense that the atmosphere seems one mass of 

 moving forms, and, after laying their eggs, perish with the 

 day, forming a great food-supply for fishes and birds. 



The eggs are laid in the water and hatch into nymphs 

 (Ephem'era, Fig. 14), which do not at all resemble the adult, 

 and are adapted to an aquatic existence by the presence, along 

 the sides of the abdomen, of outgrowths of the body-wall pene- 

 trated by tracheae. These outgrowths are called tracheal gills. 

 The delicate skin of which the gills are formed permits the 

 passage of oxygen from the surrounding water inward, and 

 allows the escape of carbon dioxide gas. The posterior divi- 

 sion of the heart (or an accessory chamber) is so arranged, 

 that it propels blood backward into the abdominal filaments, 

 so that they, too, act as an organ of respiration. The young 



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