CALIFORNIA FISH AND GAME 



123 



The great majority of beetles eaten were aquatic, most of them 

 being either larvae or adults of riffle beetles (Fig. 30) that live in the 

 gravel beds in swift water. A few, 35 of the 293 beetles eaten, were 

 typical land forms and nonaquatic. This is a surprising find and is 

 the only instance in the hundreds of stomachs I have examined where 

 riffle beetles formed a major portion of the food. A few other aquatic 

 beetles were eaten, such as diving and whirligig beetles, and are included 

 in the figures given here. 



True-flies found in these rainbow trout formed only 13.02 per cent 

 of the total foods, while it will be recalled that in the young steelheads 



Fig. 32. Adult stonefly (Sal- 



FiG. 31. Nymph of stonefly mon-fly), Pteronarcys. Com- 



( Salmon-fly), Pteronarcys. One mon in streams of northern 



and a half times natural size. California from early June to 



late August. Drawing by Miss 

 Helen Thorsen. One and a half 

 times natural size. 



they formed approximately 36 per cent. Of the 108 flies, exactly 100 

 were typically aquatic larvae and pupae, only eight being secured as 

 adults after they had left the water. 



Caddis-flies which ranked first in numbers in the young steelhead 

 trout, here rate but fourth place (7.35 per cent) and all of them eaten 

 were immature larvae and pupae. 



About equal numbers of mayfly and stonefly nymphs (Figs. 33 

 and 31) had been eaten and are surprisingly low, for members of both 

 groups are abundant in the Merced River as shown by our bottom 

 collections. Most anglers believe mayflies and stoneflies to be major 

 foods of trout and large numbers of artificial flies are patterned after 

 these insects. Mayflies often are eaten in large numbers but stoneflies 

 or "salmon flies," as they are usually called, are extremely "spotty" 



