466 



APPENDIX I 



chlora, two of which were originally described from this region, 

 and Leucorhina hudsonica (PL, Fig. 16). 



The May-flies, or day-flies, belong to the order Ephemenda, 

 an application which refers to the short lives of the imagoes. They 

 represent one of the more primitive groups, with mouth-parts 

 rudimentary or almost wanting in the adult, as they do not feed 

 during their few hours of existence as winged insects. The wings 

 are delicate, with a fine network of veins ; the hind wings are much 

 smaller than the fore wings, or sometimes wanting ; the abdomen 

 bears two or three long, many-jointed, bristlelike appendages, 

 while the antennae are very short. In 

 the nymph or the wingless aquatic stage 

 their life is a long one, in some species 

 often extending to two or three years. 

 The nymphs are interesting objects of 

 the streams and lakes, clinging to the 

 under sides of stones and sticks and feed- 

 ing on the smaller animal and plant life. 

 They are readily recognized by having 

 their sides fringed with tracheal gills, 

 two or three caudal appendages, and feet 

 with single claws. When the nymph 

 attains its full size, it rises to the sur- 

 face, the cuticle along the back suddenly 

 splits, and a frail-winged creature appears, 

 but this is not the true imago ; it is what 

 is known as the subimago stage. In a 

 short time another moulting takes place, 

 and we have the adult day-fly. This 

 subimago stage is unknown in any other 

 order of insects. Potamanthus marginatus, 

 the only species recorded from Labrador, 

 also occurs in northern Europe. 



Somewhat resembling the nymphs of 

 the day-flies are those of the stone-flies, belonging to the order 

 Plecoptera, or plaited-winged insects. These can, however, be 

 easily separated, the gills being in the form of tufts of short hairs 

 on the thorax and behind each leg, and not on the sides of the 

 abdomen. The feet have two claws, the legs being usually fringed 

 with hairs, and there are two caudal processes. They are found 

 in streams which are quite rapid, as they require more aerated 

 water than the nymphs of the day-flies. Reaching its full size, 

 the nymph (Fig. 17) crawls out upon the rocks or trees, the skin 

 splits along the back, and the adult appears. 

 The full-grown stone-fly (Fig. 18) is, however, very different in 



FIG. 17. 



Nymph of the Stone-fly. 



