THE STONE-FLIES 



(Order Plecoptera.) 



This order is not a large one and contains only the single 

 family Perlidae, of which thirteen genera are represented in North 

 America and less than a hundred species. The stone-flies have 

 mouth-parts formed for biting; the body is long and soft and 

 flat; the wings are four in number and are membranous, the 

 hind wings being much larger than the fore wings, folded in 

 plates, and lie upon the abdomen when at rest. The antennae 

 are long and thread-like. The larvae are aquatic and are usually 

 found under stones in running water. The flies are commonly 

 seen about water courses in the first warm days of spring, and 

 the cast skins of the nymphs, or pupae, sticking to stones and 

 logs on the banks of streams, are very common objects. The 

 eggs are produced in enormous numbers. They are small and 

 are probably dropped on the surface of the water, as with the 

 May-flies, but some of them, even after they issue from the ab- 

 domen enclosed in a kind of capsule, are carried about by the 

 female. One female may deposit 5,000 or 6,000 eggs. The 

 larvae when hatched are very active and are carnivorous in habits, 

 feeding upon the young of the May-flies and other soft-bodied 

 aquatic animals. They are as a rule very flat in form, which 

 enables them to crawl under heavy stones at the bottom of swift 

 running streams. The head is large and flat and the eyes are 

 large and compound. They resemble the adults in general 

 structure except that they lack the wings. There are as a rule 

 two long protruding filaments at the anal end of the body, and 

 they breathe by means of tracheal gills. The legs are flattened 

 and are fringed with hairs, fitting the insect admirably for swim- 

 ming. The long antennae are present in the larvae also and when 

 the larva is hidden beneath a stone frequently the antennae curve 

 around upwards, apparently to give it warning of the approach 

 of prey. The jaws are strong and toothed but are hidden by the 



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