AQUATIC INSECTS 



883 



where these are deposited, and when and how, are matters not 

 yet estabhshed. One species of Capnia, an undetermined, late 

 appearing species that occurs in Lake Forest, 111. in May, is 

 viviparous. 



The nymphs of stoneflies are much easier to find and to collect 

 than are the adults. By lifting stones or other obstructions out 

 of the bed of rapid permanent streams, and quickly turning them 

 over to look on the under side, the nymphs may usually be seen 

 lying flat, outspread, with widely extended legs clutching the sur- 

 face. They are always associated 

 with Mayfly nymphs of similar ap- 

 pearance, but are easUy distinguished 

 by the presence of two claws on the 

 tip of each foot, where the Mayfly 

 nymphs have but one, and by the 

 lack of gills upon the dorsal side 

 of the abdomen. The nymphs of 

 larger species, as Perla (Fig. 1354), 

 are not easily managed in ordinary 

 aquaria. They cannot Hve long in 

 still water, and soon after being 

 placed in it, they manifest their 

 discomfort, by a vigorous swaying 

 of the body up and down. This 

 motion brings their tufted gills into 

 water. Running water aquaria are 



Fig. 1354. The nymph of a stonefly, Perla. 



better contact with the 

 essential for their maintenance. 



Their transformation may often be easily observed where it 

 occurs naturally out of doors. It always takes place near to the 

 edge of the water. Often rocks that project but a few inches above 

 the surface are favorite places of emergence, and the exposed sur- 

 faces of these may sometimes be found covered several layers deep 

 with the skins of the nymphs that have come from the bed of the 

 adjacent parts of the stream. Transformation usually occurs at 

 night, but early and late stragglers may often be found by morn- 

 ing or evening light. The change from nymph to adult is, for insects, 

 comparatively sKght: wings and accessory reproductive organs are 



