128 BRANCH ARTHROPODA 



they come forth in myriads along streams. The authors saw 

 their dead bodies piled a foot deep on an Illinois river bridge 

 just under the electric lights. So thick were they that workmen 

 came next day and shovelled myriads of them into the river. 

 They are fragile, soft skinned, and long bodied, with four gauzy 

 wings, of which the anterior pair is much the larger. The 

 abdomen ends in long thread-like anal projections. The 

 mouth parts are rudimentary. Indeed, the adults are said to 

 take no food, but to reproduce and die. The eggs are laid in 

 the water. Soon appear tiny, soft-bodied, wingless nymphs, 

 bearing leaf-like fringed gills arranged segmentally along the 

 sides, and two or three many jointed anal appendages. They 

 have strong legs and can both swim and walk. They lie on the 

 bottom of streams, and, with their powerful mandibles which are 

 adapted for biting and chewing, catch and devour other insects. 

 They eat plants also, and are themselves prized as food by many 

 kinds of fishes and other aquatic animals. 



After the ninth molt (some species have twenty-one) the wing- 

 pads begin to develop. The nymph continues to grow and to 

 molt, until finally it leaves behind its " water-nymph skin " 

 and comes forth a winged May-fly. Again it sheds its skin, 

 it may be within a few minutes or within twenty-four hours, a 

 thin layer coming off even from the wings. This is the only 

 known instance of an insect molting after acquiring its wings. 



ORDER m. PLECOP'TERA 



The stone-flies are comprised of a single family, the Per'lidce. 

 These grayish or brownish insects (Fig. 99) are | to 1| inches 

 long and have four large membranous wings, but the posterior 

 pair folds up like a fan when not in use. Unlike the dragon- 

 flies, in which the anterior and posterior wings are about equal 

 in size, the posterior wings are much wider than the anterior 

 ones. The mouth parts are adapted for biting, but poorly so 

 as compared with those of the dragon-fly. The adults probably 

 eat little. The long antennae are many jointed and the abdomen 

 is often furnished with a pair of many jointed bristles or fila- 

 ments. The 5000 or 6000 eggs are probably well scattered in 

 the swift current before dropping to the bottom. The meta- 



