?6 



AN INTRODUCTION TO ENTOMOLOGY. 



Fig. 73. — Nymph of Stone-fly, 

 Acroneura. 



It is easy to obtain the nymphs of these insects; by lifting 

 stones from the water of swiftly flowing streams the young stone-flies 



may be found closely adhering to their lower 

 surface. They present a wonderfully flat- 

 tened appearance (Fig. 73) ; the body is de- 

 pressed, and closely applied to the stone ; 

 while the legs, antennae, and caudal setae ra- 

 diate from it on the surface of the stone. In 

 our common forms there is a tuft of hair- 

 like tracheal gills just behind the base of 

 each leg. And the more mature individuals 

 present conspicuous wing-pads. 



The nymphs of stone-flies constitute an 

 important element in the food of fishes. I 

 have found them more often than any other 

 insects in the stomachs of brook trout. 



When about to transform to the adult 

 state the insect crawls from the water upon a 

 stone or some other object. Their exuviae 

 are common in these situations. The females of certain exotic spe- 

 cies have been observed to carry their eggs about with them, 

 attached in a globular mass to the end of the abdomen, for some 

 time before they are laid in the water. Probably our species pre- 

 sent a similar habit. 



The greater number of our species belong to the genus Pcrla. 

 In this genus the wings have but few transverse veins ; the anal 

 space of the posterior wings is large and folded ; the palpi are bris- 

 tle-like ; and there are two caudal setae. The genus Ptcrondrcys 

 (Fig. 68) is of great scientific interest, owing to the presence of 

 tracheal gills throughout the entire life of the insects. The gills of 

 Pteronarcys consist of little tufts of short slender filaments, of which 

 there is a pair on the ventral aspect of each thoracic, and the first 

 and second abdominal segments. The wings in this genus are 

 densely net-veined. 



There are several species of stone-flies that appear in the adult 

 state upon the snow on warm days in the latter half of winter. 

 They become more numerous in early spring, and often find their 

 way into our houses. The most common one in Central New York 

 is the small snow-fly, Cdpnia pygma>a. It is black, with gray hairs. 

 The female measures 9 mm. in length, and has an expanse of wings 

 of 16 mm. The male measures only 4^ mm. (0.18 inch), and has 



