AQUATIC INSECTS 



in chambers kept dry by the wings. Many aquatic nymphs, 

 stoneflies, mayflies and others have no spiracles but the 

 tracheae branch out into dehcate sac-Hke out-growths of the 

 body- wall, forming the tracheal gills (PL VIII). These 

 tracheal gills are so thin that oxygen can pass through 

 their walls into the tracheae and thence into the body. Many 

 mayfly nymphs have leaflike gills in which the tracery of the 

 trachcce is clearly visible through a hand-lens, sometimes 

 even to the naked eye (PL XV). 



Fig. 142. — Gills of mayfly nymph, Chirotenetes, 

 those of the right side turned with their under sur- 

 faces upward. 



In dragonfly larvae the lining of the hind intestine is infolded 

 in delicate frills and flutings traversed with tracheae, thus 

 forming tracheal gills. These hang into the cavity of the 

 intestine (rectal chamber) and the nymphs breathe by alter- 

 nately taking water into it and forcing it out again (PL IX). 



How insects grow. — The eggs of water insects have shells 

 which are often beautifully sculptured and provided with 

 safety devices. On the eggs of one mayfly, Tricorythus 

 (Fig. 143), are threads by which they are suspended from 

 plants out of the mud and where they secure abundant oxygen. 

 On those of another, Ephemerella rotunda, there are floats 

 which buoy it up out of the mud (Fig. 143). 



Mayfly nymphs and others go through only partial change 

 of form as they increase in size. They shed their inelastic 

 outside skeleton or skin soon after hatching and at intervals 

 through their nymphal lives. After each molt the nymph 

 grows for a period then sheds its skin again, and in mayflies 

 this may go on for twenty times or more. When it molts 

 for the last time in the water it comes forth with a shape 



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