Maple and basswood line the banks of the Mississippi River.

We hike along the Mississippi River as a cloudless sky, calm wind and a mild 52°F settle in for the afternoon. (The average high for Minneapolis-Saint Paul is 28°F today.) Blue jays, black-capped chickadees, and red-bellied woodpeckers call; gray squirrels rattle through the crisp fallen leaves. The river’s surface is unmarked by ripple or wave, and two Canada geese float a few feet from shore, honking softly to each other. 

Winter crane flies (Trichocera species) swarm and mate on mild winter days.

Wherever rays of winter sunlight stream through bare branches of basswood and maple, the air shimmers with small swarms of winter crane flies. The low December sun ignites their wispy forms into sparks of gold. They zip around as energetically as though it were July.

Most types of crane flies are abundant in summer. Just a few species mature and mate in the winter. The swarms shimmering in the afternoon sun consist mainly of males. Females join them briefly to mate, and then lay their eggs in rotting leaves and soil. When not in flight on mild days, the adults rest in protected areas such as hollow trees and caves. (What could they possibly find to eat during a Minnesota winter? Not much. In fact, adults typically don’t eat at all! The larvae eat rotting vegetation, fungi and animal scat.)

They sparkle like tiny gold jewels in patches of winter sunlight.

Winter crane flies are most common in late autumn, early spring and sometimes on mild winter days. Today, they never stray from the sun’s mild warmth — nor do we! A rare Minnesota December day, indeed, and one to remember when winter weather returns.

Further reading:

Winter Crane Fly (Family Trichoceridae)

Winter Crane Flies