Midge eggs develop into larva. Larvae are wormlike aquatic versions of the insect.
Prior to emergence, larvae seal themselves inside of an immobile pupal chamber and transform to a pupa.(Dr. Ernest Schwiebert told me the most important thing the presence of midge adults tells you is that there are pupa in the film.)
Prior to emergence the pupa congregate in the slow water, or “cushion” at the bottom of the stream. (Wild Trout also concentrate there because fish can't spend their llives fighting the current). As the concentration of pupae increase, the trout notice them and begin to feed on them.
When it's time for the pupa to "hatch" they ascend from the cushion, to the surface. The trout follow them to the surface and suspend.
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