Speaking of Nature: The common cluster fly — Please stay outside! 

  • The brick-red eyes, “furry” golden-brown body and large black “spikes” make this common cluster fly easy to recognize. PHOTO BY BILL DANIELSON

Published: 8/7/2022 4:03:19 PM

And now for something completely different … As anyone who is familiar with my writing will tell you, I am a bird guy. I like mammals, I like reptiles and amphibians and I like trees and flowers, but I love birds. Birds occupy my thoughts like no other group of living creatures and during the summer I am free to pursue them without constraint. So, it is not unusual for me do make a daily trip down to my Thinking Chair (situated at the edge of my meadow) and sit for hours as I wait to see who flies by.

The walk down to the Thinking Chair takes me through a series of trails that I established almost 20 years ago. Some of the trails cut through a stand of white pines and others meander through a patchwork of old field and meadow. I named one of these trails after my beautiful wife Susan and I tend to take “Susie’s Trail” more often than not. Along the way I pass a variety of interesting plants, but one of my absolute favorites is a tall plant that looks something like a small sunflower.

Known as elecampane (Inulahelenium) this plant can grow to about 6 feet and it always seems to have the most interesting insects crawling over its broad, coarse leaves. In the past I have been particularly impressed with long-legged flies and different types of flower flies, candy-striped leafhoppers and even crab spiders that wait for pollinators at the edges of the flowers. But the other day I found myself looking at a large fly that I hadn’t seen before. Simultaneously beautiful and grotesque, this creature sat atop one of the aforementioned leaves in the relative cool of the morning and out of sheer luck I happened to have my closeup lens with me.

Since the theme of my columns in August is “Something Different” I have decided to feature this fly in today’s column. I am wandering well outside my area of expertise, but that is also part of the “Something Different” idea, so here we go. If there are any career entomologists in the audience who find that I have made an error, please speak up.

As far as I can tell, the fly in this photo is a common cluster fly (Polleniarudis). The only reason that I was able to come up with this identification is because I own a huge tome titled, “Flies: A Natural History and Diversity of Diptera,” by Stephen A. Marshall. This book is huge because there are over 160,000 named species of flies in the world and after almost an hour of browsing through the photos I finally stumbled upon one that matched my own. Then the story of this well-known species opened up for me.

The common cluster fly has a life cycle that is fairly standard in the world of flies. The adults are herbivores and they are attracted to flowers were they can find pollen, nectar and sap. In other cases they will search for minerals in fecal material, which is also a familiar idea to anyone with pets. To a human this seems unthinkable, but to a fly it is a reasonable way to secure certain nutrients that may not be available anywhere else.

But the flies start out their lives as meat-eating parasites. Adult females will lay their eggs in the soil and when the eggs hatch the larvae start searching for earthworms. I imagine that the earthworms blunder into the larvae more often than the larvae actually track down the worms, but once contact is made the larvae start chewing. In some cases the worms will die and the larvae have to find another host, but eventually the larvae will pupate (turning from a worm to a fly) and they will emerge from their host as adults. This is a gruesome picture to have in your head, but it is a common part of the life cycles of many insects.

The species is called a “cluster” fly because of its habit of gathering (or clustering) inside houses during the autumn. The flies are fully capable of hibernating through the winter (much like a bear would) and they will fatten up in the autumn and then slowly utilize the fat during the winter. When they emerge in the spring they appear somewhat shriveled (like a raisin) because they have used up all of their reserves.

But why hibernate in a cold underground tunnel when you can find shelter in a nice warm attic? If the temperature is a little warmer it might be easier to survive the winter with a little more energy to spare, so attics and homes are very attractive wintering spots for these flies. Unfortunately, this is not in any way appealing to the people who own the houses and cluster flies are considered a nuisance. They don’t exactly do any harm, but swarms of flies are not at all appealing and pest-removal companies are regularly called to handle large infestations.

So there you have it; the life cycle of a common insect that is probably somewhere in your yard right now. I knew that there were insects that were very “host specific,” but I never knew that one of the hosts could be an earthworm. That’s actually one of the things that I love about this column — learning. It’s a big world out there and even in the confines of your own, familiar backyard there are all sorts of things that you’ve probably never seen or thought about. Next weekend why not step outside in the cool of the morning and see what wonders you can find?

Bill Danielson has been a professional writer and nature photographer for 25 years. He has worked for the National Park Service, the U.S. Forest Service, the Nature Conservancy and the Massachusetts State Parks and he currently teaches high school biology and physics. For more in formation visit his website at www.speakingofnature.com, or head over to Speaking of Nature on Facebook.


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