Gibb says shad-flies swarmed each year along both banks of the St. Lawrence, as far upstream as Montreal.
The swarms usually appeared in May, sometimes June, as shad moved upriver to spawn.
In the air, the flies looked like heavy snowfall; in one locality, the harmless swarm lasted only two or three days.
Gibb’s explanation was mass hatching: millions of flies emerged from pupae and aquatic larvae in waterways along the riverbanks, making them seem to fall from clouds.
Gibb reports that shad chiefly fed on the flies and that catches and market supplies were generally abundant when the swarms first appeared; he describes a seasonal coincidence, not spawning as the cause of hatching.