vii STONE FLIES 281 



It is easy to observe the escape of the fly. A full- 

 grown larva creeps out from the stream, sometimes to 

 a distance of several feet, and seeks a stone on which 

 it can securely fix its hooked feet. Then the old 

 skin begins to swell. Air is passed into the narrow 

 space between it and the new skin beneath, which 

 though still soft and flexible, is now complete in all 

 its parts. Before long the back of the thorax splits 

 lengthwise. First the new thoracic skin shows itself, 

 then the head and antennae appear, and the wings are 

 drawn out of their sheaths. The legs of the fly are 

 next freed, and when these have got a sufficient hold 

 of the ground, the abdomen and tail-filaments are 

 withdrawn. The newly emerged fly is still soft and 

 pale-coloured, but in a few hours or even, in some 

 species, in a few minutes, the organs of locomotion 

 become firm, and the Insect takes to flight. 



In spite of its large wings the imago is a poor flier, 

 turning with difficulty, and showing little power of 

 avoiding an obstacle. It is easy to capture it with a 

 net or even with the hand. Like a Grouse, it whizzes 

 along with great effort and nearly in a straight line. 

 The habits of the winged Perla do not call for great 

 agility. The males and females mate on the 

 ground, near the place of emergence. The fertilised 

 eggs, which are oval and black, project from the 

 end of the abdomen of the female, being loosely 

 fastened together by a transparent skin. These are 

 dropped into the water, and the work of the winged 

 generation is at an end. 



From these eggs small aquatic larvae issue, which 

 may be found plentifully in the same streams during 



