XVII PERLIDAE 405 



The Perlidae l^eing of aquatic habits in their early stages, 

 and, notwithstanding their ample wings, very poor adepts in the 

 art of flying, are rarely found at any considerable distance from 

 their native element. They are specially fond of running water, 

 and delight in the neighbourhood of waterfalls, or other spots 

 where the current is broken l^y obstacles so that a foaming water 

 results. It is probable that the larvae which breathe by means 

 of gills find an advantage in living in strongly-aerated water. 

 Mountain streams and torrents are therefore specially affected by 

 them ; but Pictet informs us that they do not like the waters 

 descending from glaciers. The food of the larvae is believed to 

 be chiefly young may-flies, or other small, soft creatures, and it 

 may possibly be owing to the absence of these that the Perlidae 

 do not affect the glacier streams. Although Perlidae are remark- 

 able for their capacity for enduring cold, it is possible that they 

 may require warmth of the water at some period of their 

 development, and this the glacier-streams cannot offer to them. 

 They are among the earliest Insects to appear in the spring in 

 Europe. Mr. Barnston says that on the Albany river in Canada 

 the nymph of Capnia vernalis comes up frequently in the cracks 

 of the ice and casts its skin there ; " it frequently comes up 

 when the thermometer stands at freezing." Of Nemoura glacialis, 

 which inhabits similar localities, he says that " it appears in the 

 spring (end of March or beginning of April) when the ice 

 becomes honeycombed, and even before then, at the same time 

 as Capnia vernalis. It pairs in the crevices of decaying ice. 

 The male has long antennae, and his wings are generally rumpled 

 as if glued together." Xewport entertained the idea that those 

 Perlidae that live at low temperatures are of lower organisation 

 than the other forms of the family. 



It is a remarkable fact that several Perlidae frequently 

 have like Nemoura glacialis the wings of the male much 

 reduced in size ; this being the contrary of the rule that 

 usually prevails among Insects to the effect that, when there is a 

 difference in the powers of flight, or even in the size of the wings, 

 it is the male that is superior. Mr. J. J. Lister met with a very 

 interesting Perlid at Loch Tanna in Arran at the beginning of 

 April 1892. In this Insect, which is, according to Mr. 

 M'Lachlan, a form of Isogenus nubecula, the wings of the female 

 (Fig. 258, B) are reduced to a size much less than those of ordinary 



