Tom Turpin
Professor of
Purdue University







Seeing Cluster Flies? It Must be Spring!

Ah, the signs of spring.  Robins hopping on the lawn, crocuses emerging from the ground, and spring peepers singing in water-filled ditches along the roadside.  An even better harbinger of spring, though, is flies bouncing off windowpanes.

In the poem “Spring,” Oliver Wendell Holmes penned these lines:

“The housefly, stealing from his narrow grave,
Drugged with the opiate that November gave,
Beats with faint wing against the sunny pane,
Or crawls, tenacious, o'ver its lucid plain.”  

The fly that Holmes describes is not the house fly, but a similar-looking fly known as the cluster fly.  Cluster flies are parasites.  In their immature stages, cluster flies feed on earthworms.  In the adult stage, they hibernate to wile away the long, winter hours in temperate regions of North America.

Cluster flies prefer a nice, cozy place in which to hibernate.  So each fall, millions of little buzzers invade buildings, barns, garages and houses.  Now most people don't mind sharing their barns or garages with a few flies, but it's a different story when it comes to houses.

The cluster flies normally find their way under eaves and into chimney chases and attics, where they settle down for a little serious hibernation.  However, the increasing minutes of sunlight associated with late winter days frequently warms attics.

The increased temperatures do not go unnoticed by our slumbering parasitic fly.  It assumes that spring has sprung and begins to search for some earthworm on which to lay eggs.  However, the return journey to the great out-of-doors frequently results in a slight detour of the house.

Once indoors, the creatures try to fly out through windows.  While windowpanes do a good job of keeping insects out of our houses, the glass also does a fine job of keeping flies in.  The hapless creatures just crawl around or, as Holmes states, “Beat with faint wings” against the pane.

Of course, the presence of flies makes most homeowners a bit fidgety.  Some run for the good ol' insecticide spray can.  Others just fire up the vacuum cleaner.  A few engage in hand-to-tarsal combat with the unwelcome invaders.  Regardless of the elimination approach, there always seem to be more.  In fact, hibernating flies can several inches deep in corners of attics.

Let's hope that most of the cluster flies can find their way out as easily as they found their way in!


Writer: Tom Turpin
Editor: Carol McGrew