CHAPTER VII 



PERLID.E 



STONE-FLIES (PERLA) 



IN streams, and especially in clear streams which 

 flow down from the hills along stony beds, the larvae 

 of Perla are plentiful during early summer. Lift any 

 good-sized stone, as large as the hand or larger, and 

 it is likely that two or three dusky objects, resembling 

 small Shrimps, will run along it to seek the side which 

 is turned away from the light. In the stream which 

 I have most diligently frequented two aquatic larvae 

 occur side by side on such stones. One is that 

 of Baetis (Ecdyurus) fluminum, an Ephemerid ; the 

 other that of a Perla. The Baetis is smaller than the 

 Perla, though a good inch in length when of full size, 

 and has three tails. The Perla is somewhat larger, 

 and has only two tails. 



In June the larvae of Perla leave the water, and 

 shortly after the winged flies emerge. The stones 

 which border the brook are now strewn with larva- 

 skins, curiously like living larvae in shape, but dry 

 and empty. Along the back of the thorax is the 



