Ask The Entomologist

Winter Crane Flies: widespread and little-known

As I was walking over to my in-law’s place one chill and sunny afternoon, I happened to spot a fly. A gangly, long-legged fly, seeming to bounce up and down in the brisk winter air. Unlike the cluster flies lining the edges of our ceilings, this one was fairly active, despite the temperature.

Naturally, I snatched it out of the air for a better look.

It wasn’t just any fly – at first glance it appeared to be a crane fly… but parts of it weren’t quite right. It had simple eyes in the center of its forehead, something absent in true crane flies. Nor was it quite the right size – it’s perhaps 1.5 to 2 times the size of an average mosquito, whereas crane flies can be far larger, and with a broader leg-span. It was a winter crane fly.

If you look closely, you’ll find that the winter crane fly, like its true crane fly relatives,
does not have stabbing mosquito-like mouthparts.

While not true crane flies, winter crane flies are close relatives, and both develop in similar areas and eat similar foods. Wet spots on land, perhaps along a stream or seep, are perfect for a growing winter crane fly maggot. True crane fly maggots (often called “leatherjacket slugs”) prefer to swim in the stream itself, and can make excellent fishing bait. Both types of maggot like places that have plenty of moist decaying plantstuff to feed on.

Interestingly, winter crane flies have been documented gathering in large numbers underground… both here and in the Old World. I observed this in a number of western Kentucky stream caves while I was studying a group of eyeless cave beetles, but little has been written on the behavior in this organism. I wonder if it could be similar to how mosquitoes overwinter in caves…

To my surprise, Winter Crane Flies have become invasive in Antarctica in the past decade… it’s thought that they started out by colonizing the polar scientists’ sewage treatment plant, and escaped to the outside. Impressive for a wee beastie that is only active for a couple of months out of the year here. Of course, the Winter Crane Flies invading Antarctica have much less competition than those around here!

What is a pest in one place may be entirely harmless in another.

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Why are there flies in my house? There’s snow outside!

If your home is anything like ours, it has had an abundance of flies lately. You may ask yourselves “why now?” – after all, it’s snowy out, and there can’t be much food for them. Shouldn’t they die with the cold like everything else?

You’d be entirely right in thinking so. Regardless of the type of fly, most of the good food sources are rather scarce this time of year, and the cold kills many. However, insects are just as uninterested in freezing to death as we are. Most of these gathered indoor flies are trying to avoid that particular fate.

My Mother-in-Law’s elegant, clear sticky traps, thoroughly filled with Cluster Flies and Fungus Gnats.

While we see many house flies, flesh flies, and blow flies in the summer, they aren’t the primary species to invade our homes in the winter (though I’ve met a few of each overwintering inside my home).

The vast majority of our new fly guests are a different group entirely – the Cluster Flies. Until quite recently, the Cluster Flies were grouped as a special sort of blowfly, but just over a year ago, they were given their own scientific family name, Polleniidae.

Cluster Flies get their name from their habit of gathering in large groups, often in windows. Some try to stay warm outdoors, nestled deep inside rotting tree trunks. More sensible ones move inside with us to stay warm and survive the winter. Cluster Flies don’t want anything to do with your food, plants, or pets – they’re just here to avoid freezing.

Eternal optimists, Cluster Flies often come out on warmer days, and try to find their way outside.
Thus the accumulation of dead ones on your windowsills when weather turns cold again.

It’s this habit of moving inside for the winter that is thought to have introduced Cluster Flies to the New World. You see, the Cluster Flies we have now aren’t North American natives. No. They’re thought to have sailed over from the Old World with some of the first European colonists.

Back when European countries were busy sending ships to build colonies in the New World, they brought many unintentional stowaways with them. Many of these scalawags are with us today, for better or worse. In this story, we care about two such emigrants. Earthworms (especially the Green Worm) and Cluster Flies.

It’s thought that earthworms were transported across the Atlantic in ships’ ballast. Meanwhile, some Cluster Flies took refuge from bad weather inside the ships themselves, just like they’re doing in our homes this winter, and so found their way to America as well.

Earthworms flourished when ships made landfall. Many of the formerly-glaciated parts of North America no longer had earthworms, and were ripe for Lumbricid conquest. The spread of earthworms, in turn, opened up North America to the spread of Cluster Flies. Both spread like wildfire, and now can be found all over our continent.

Why are earthworms important to Cluster Flies? You see, Cluster Flies are specialist parasites. Their maggots feed almost exclusively on earthworms. Mother Cluster Flies sensibly lay their little ones near earthworm burrows, and the newly-hatched maggots then see about journeying into earthworm burrows. There they lurk in ambush, waiting for a worm to pass by.

Thinking of tiny maggots catching hold of vast earthworms with their mouth-hooks as the worm passes by very much brings scenes of Frank Herbert’s “Dune” to my mind. I like to imagine the tiny Cluster Fly maggots singing this song as they hunt their earthworm prey, and carve their way inside.

Our friendly, local Cluster Fly – Pollenia pediculata.
As you can see, they’re a bit larger and hairier than our common houseflies, as well as easier to catch.

Because of their worm-based diet, Cluster Flies are actually remarkably clean compared to other flies with carrion- or dung-based diets. While you may not appreciate Cluster Flies’ presence, at least they will not spread disease.

Additionally, both my dog and I can attest to their superb flavor.

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Louse flies

It’s autumn. Among the many little signs of this are the appearance of Western Deer Keds, or louse flies, as they’re often called. As I was walking in the woods this past week, a number of them flew about me, a couple landed on my hand and ran up and down my arms. A poor life choice for them, as they were swiftly collected.

Louse flies are members of family Hippoboscidae, and are best known for their very odd reproduction. They have their young one at a time, much like us humans. Females fertilize a single egg from stored sperm, the egg then hatches inside the mother fly’s reproductive tract. The resulting maggot nurses from a “milk” gland and molts several times inside the uterus. After about a week of this, the mother fly gives birth to a large late-stage maggot. In the case of the Western Deer Ked, the mother generally does this where the deer beds down for the night.

Liptoptena depressa, a deer ked, courtesy of my in-laws’ dog (an unsuitable host).

The late-stage maggot pupates immediately. After emerging from its pupal case as an adult in the fall, the new adult louse fly will take off in search of a suitable host. Once it finds a host, it will start feeding on blood, shed its wings, and will remain on the host until its dying day. Western Deer Keds can survive on Mule Deer, White Tailed Deer, Elk, and Moose. They may try to feed on other species – they can certainly bite. But they won’t be able to survive for long off of their proper hosts.

Keds are best known as livestock pests – sheep keds are somewhat famous for the economic damages they can inflict. Native to Europe, sheep keds immigrated with humans, and are present across almost all of North America and much of South America, as well as parts of Africa, Asia, and Australia. While sheep keds have been reported, there is not good evidence that they can survive long on Bighorn Sheep or Mountain Goats.

If you hunt turkey, and have tried for the “Grand Slam” you might have encountered turkey keds in the American southeast as well. There’s several species of turkey keds, but to the best of my knowledge, none have made it west of the Rocky Mountains yet.

Have you met louse flies before? When, and on what?

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Shiny flies

Earlier this week we visited Trego School, to see the results of the salmon snagging fieldtrip. After the fish were had been cleaned and packed away for children to take home, I saw many bluebottle and greenbottle flies, as well as a few paper wasps, flying about outside of the school, sucking up fish juices.

Bluebottle and greenbottle flies are blowflies, members of family Calliphoridae, a name which translates to “carriers of beauty”. Rather fitting, given their bright metallic hues. Additionally, adult blowflies are excellent pollinators, and a very good thing to have in your garden.

Two greenbottle flies, genus Lucilia, contemplating romance over a salmon dinner.

Blowflies have an incredible sense of smell. In most parts of the world, blowflies find bodies within minutes of death, detecting the first smells of decay long before humans are able to. When they find the right sort of decay (some blowflies prefer dung to death), blowflies quickly mate and lay eggs before flying off.

People throughout ancient Greece and Egypt believed that these shiny flies were the souls of the dead. Even today, some people in those countries consider it bad luck to swat at them, for fear that you might kill someone’s ancestor. I think this is a particularly beautiful story.

Imagine, sitting gathered with family and friends as a loved one passes on, breathes their last breath. Within minutes, you can see their soul, in the form of a beautiful fly, flying about their mouth, nose, and eyes. Almost as if the soul is sorry to leave the body, as if the person wants to stay with their family. Perhaps they say “goodbye” before flying off to the afterlife.

Today blowflies are appreciated for more than their attractive colors and spiritual significance. They are the most useful insects in forensic investigations – their young are very useful for figuring out time (and sometimes place) of death. While other insects can be used, blowflies are a forensic detective’s best friend.

Blowfly maggots have even been used in medicine, as a way to clean dead flesh out of wounds, preventing sepsis and gangrene. Medical maggot use predates antibiotics! Napoleon’s surgeons noticed that fly maggots increased soldier survival! In the American Civil War, both sides used blowfly maggots to clean deep wounds. Maggots continued to serve and save lives in World Wars I & II, and are used for some conditions even today.