MAY-FLIES. 37 



twigs amongst the rapids, where they are often found 

 in large groups. When thus congregated they have a 

 very close resemblance to a bunch of small black twigs, 

 or some other kind of submerged vegetable refuse. Their 

 ability to hold on in the swiftest currents is most remark- 

 able, but they are very feeble swimmers, and are seldom 

 seen in sufficiently calm water to admit of this method 

 of progression. They are very often found in great 

 numbers underneath large boulders in the rapids of 

 the river, where they must be comparatively safe from 

 the attacks of their numerous enemies. This nymph is, in 

 fact, solely adapted for living in extremely agitated water. 

 Its gills are not vibrated like those of most other May- 

 fly larvae, but are held erect and immovable over the 

 creature's back. In this position they are most efficiently 

 aerated by means of a rapid current of running water 

 passing immediately over the nymph. Owing to these 

 peculiarities this May-fly is extremely difficult to rear in 

 captivity, and larvae can only be kept alive for a few hours, 

 even if confined in very shallow vessels of water. In 

 deep vessels they almost immediately perish. This insect 

 can, in fact, only be reared by selecting a large number 

 of the mature nymphs, when it will sometimes chance 

 that one or two of them may be within a few hours 

 of their transformation, and these perhaps will have 

 sufficient vitality left to emerge. Unless the larvae 

 are actually mature they invariably all perish, usually 

 within less than thirty-six hours after they have been 

 removed from their native stream. 



Before emergence this larva crawls out of the water 

 on to a dry stone, and the May-fly escapes through a 

 large hole in the back of the thorax. During the latter 

 end of the spring and most of the summer, great numbers 

 of these empty nymph skins are to be found on all the 

 stones just above the level of the water. 



The sub-imago has the body dull grey in colour. The legs and tails are 

 considerably shorter than in the imago. The veins of the wings are clouded 

 with grey, and there is no brown mark on the costa near the apex. 



This period of the insect's life lasts about two and a 

 half days, and during the whole of the time it rests quietly 

 amongst the foliage on the bank of the stream. 



The perfect insect appears about the end of October ; 

 it is extremely abundant throughout the whole of Novem- 

 ber, and specimens may be taken as late as the middle or 

 end of January. It flies in groups over streams during 



