Gnats grow from ascarids; and ascarids are engendered in the
slime of wells, or in places where there is a deposit left by the
draining off of water. This slime decays, and first turns white,
then black, and finally blood-red; and at this stage there originate
in it, as it were, little tiny bits of red weed, which at first
wriggle about all clinging together, and finally break loose and
swim in the water, and are hereupon known as ascarids. After a few
days they stand straight up on the water motionless and hard, and by
and by the husk breaks off and the gnats are seen sitting upon it,
until the sun's heat or a puff of wind sets them in motion, when
they fly away.