The pool was only wide enough for a few strokes, which she covered quickly.
The number of strokes should be increased gradually.
Comparatively only a few species are, for part of their lives, denizens of fresh water; these, as larvae, are parasitic on the eggs or larvae of other aquatic insects, the little hymenopteron, Polynema natans, one of the " fairy-flies " - swims through the water by strokes of her delicate wings in search of a dragon-fly's egg in which to lay her own egg, while the rare Agriotypus dives after the case of a caddis-worm.
This physiologist in 1867 2 showed that all natural wings, whether of the insect, bird or bat, are screws structurally, and that they act as screws when the y are made to vibrate, from the fact that they twist in opposite directions during the down and up strokes.
He further pointed out that the wings of flying creatures (contrary to received opinions, and as has been already indicated) strike downwards and forwards during the down strokes, and upwards and forwards during the up strokes.