440 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 Ephemerida, 
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 antennze 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 1s 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, 
Fic. 17. the only species recorded from Labrador, 
Nymph of the Stone-fly. 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 aérated 
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 
