VI 



THE ALDER FLY 277 



in the earth. For this purpose it will often travel far 

 from its native pool, and I have lately found one 

 creeping on the surface of the ground six yards from 

 water, though the season was dry, and the soil com- 

 mon garden mould, not particularly retentive of 

 moisture. This larva had climbed up a concrete wall, 

 made its way through a thicket of Cotoneaster, and 

 reached an open flower-bed. When the Insect has 

 found a place to its mind, it enters the earth, ex- 

 cavates a little cell, casts the larval skin, and is trans- 

 formed into a pupa, which has the legs and wings free 

 from the body, though enclosed in special sheaths. 

 There are spines on the abdominal rings, as usual 

 in pupae which have to make their way to the surface 

 of the earth ready for the emergence of the imago. 

 After two or three weeks the fly emerges — a heavy 

 awkward Insect with black body, and four large and 

 coarse wings, which are clear but with conspicuous 

 black veins. The female fly is one third larger than 

 the male. At first sight the winged Insect might 

 be taken for a Caddis-fly, for the wings are rather 

 opaque, longer than the body, and form a sort of roof 

 sloping steeply away on either side. The body is 

 stouter and heavier than that of a Caddis-fly, the 

 antennae shorter in proportion, though still of good 

 length, and the mouth-parts much more complete. 

 The wings too differ considerably from those of the 

 Caddis-fly. A winged Sialis flies heavily, and is 

 easily caught. When threatened it tries to escape by 

 running rather than by taking wing. The female 

 lays patches of dark brown eggs on leaves, stones or 

 palings not far from the water. There may be several 



