Food of the Ycllozvslonc Trout 477 



streams, especially where there are trout, are hke other mountain 

 streams. Indeed one may say that the Hfe of trout streams as far. 

 as the fish are concerned is absolutely dependent on stone-flies.. They 

 are indispensible in the natural propagation of trout. While in the. 

 water, they are more or less flattened, six-legged creatures (fig. 114),- 

 varied in. color and size, and generally may be found under stones 

 or clinging to them, busily searching for food. Hence they are 

 known to zoologists as " stone-flies." Most of them are carnivores 

 in the aquatic stage; they eat other insects, flatworms and other 

 aquatic animals, and even turn cannibal, eating their own kind or 

 their young. Curious to say, the largest and most conspicuous 

 species of .the stone-flies {Ptcronarcys calif arnica) found in the 

 rivers of Yellowstone Park is almost entirely herbivorous, eating the 

 various algae and diatoms ofi^ the rocks, and also bits of wood and 

 bark that come down the stream. 



Beginning with July, the stone-flies "emerge," that is, they crawl 

 out of the water onto the shore or on exposed rocks, split the larval 

 skin, draw out the thorax, head, legs, and finally the abdomen, and 

 expand their two pairs of. wings (see figs. 115-116). They generally 

 come out in enormous numbers from early morning till early after- 

 noon. After the wings are dried, which takes from ten to fifteen 

 minutes, they fly toward some upright object. Naturally, any per- 

 son standing or moving along the stream will be taken for a con- 

 venient resting place by the stone-flies, much to the delight of the 

 trout fishermen who encounter flights of these beautiful creatures. 



According to local habit, these stone-flies are known by various 

 names in different sections of the country. Thus, they are called 

 willow flies, trout flies, mountain flies, salmon flies, yellow flies, river 

 flies, red flies, black flies and so on, according to color, place of oc- 

 currence and the fish found in the particular neighborhood. , 

 . Stone-flies are almost wholly confined to rapid streams (figs. 

 117-119), as their distribution is controlled by the degree of aeration 

 of the water. Since wave action along the shores of lakes provides 

 fair aeration, stone-flies are found also along lake margins, although 

 not in such profuse numbers as in the mountain streams. They are 

 but seldom found in quiet streams or ponds and then only in rela- 

 tively small numbers. 



After becoming winged they generally spend the day in short 

 flights and in copulation (figs. 120, 121). In the dusk of the even- 

 ing they fly upstream for considerable distances, and then oviposit. 

 This is not a nuptial flight, for mating appears to precede the flight, 



