STONEFLIES, ALDER, SMUTS, ETC. 285 



of the river, and it is said that the females do not crawl 

 down under the water to oviposit like some of the 

 Ephemeridse and Trichoptera. 



The nymphs of the genus Perla are invariably 



found in rapid parts of the 



Nymphs. stream, while of the genus 



Nemoura, according to Pictet 

 (who wrote an admirable monograph of the family), 

 some frequent fast-running, and others calm, quiet, 

 and in some cases even stagnant water. The 

 nymphs are very similar in appearance to the mature 

 insect, but at the early stages the wing-covers are 

 scarcely visible. As the nymphs grow the four wing- 

 covers gradually develop until near the next change, 

 when they are quite prominent dark ovoid pro- 

 tuberances in which the wings are beautifully folded 

 up. The mouth organs of the nymphs are fully 

 developed, mandibles, maxillae, labrum, labium, etc. 

 The legs are ciliated ; the fore and medial ones very 

 short, and the hind-legs powerful, all armed with two 

 strong curved claws. All the nymphs of the Perlidse 

 are provided with two caudal setae, and even at the 

 earliest stages this will serve to differentiate them from 

 nymphs of the Ephemeridae, of which all the genera 

 and species referred to here have three caudal setae. 



When the nymph is ready to change to the imago 



or winged and perfect insect it 



Imago. crawls up out of the water, and 



selecting a heavy stone, reed- 

 stem, or other comparatively solid body with a rough 



