The grannom hatches are massive and in late March or April thousend of insects can be seen in flight or resting on rocks and trees.

The common name in many languages refers to the female, which has a characteristic green butt.

Despite their massive presence I cannot recall of having ever seen any fish surface activity during the hatches I observed.

It might be that they emerge fast and without drifting, so that the fish prefer feed on the submerged pupa

The characteristic green butt is the egg sack.