"Hey Jim," a recent email read, "How about a Source article describing the nature and intent of 'winter flies?' They come into our houses—fly around slow and stupidly—do not feed—and within a day or two, they die. When they do, they seem almost completely dehydrated and they actually crumble to dust and body parts when you try to pick them up." Then the writer asked, "Why don't they stay outside and just die there? Best wishes, Philip."
So, what about those "winter flies?"
To begin with, it's probably impossible for a common housefly to die outside in winter, even at 30 below zero. They are similar to many of our-wintering arthropods and have "blood" (not like ours) which becomes antifreeze in winter — the colder it gets, the better the antifreeze.
Many flies, however, become trapped in our houses at the end of summer, and as Philip and many of you have noticed, most of them end up dying in the comfort and "safety" of our warm home. That's because their physiological time clock (set by the length of daylight — and perhaps by the angle of the sun) has put them into an "outside" hibernation mode and they use up all their stored energy.
At the end of summer, instead of using the food they ingest for mating and laying eggs, they have put on layers of fat that will keep them fueled up through the long winter hibernation. But being in a warm environment near the woodstove, or in the kitchen, keeps their "blood" flowing, which prevents them from going to sleep, so they use their winter fat reserves for flying about.
Bad news... Because we do not leave much fly-food lying about in winter, and further, the house environment is a lot cooler than summer, the flies' digestive system will not operate efficiently — and well — there you go... Sadly, they starve, dry out and fall apart.
It would be the same way for bats and butterflies if they got stuck in the house over winter and were forced to use their fat reserves; they wouldn't make it to spring, either. Mourning cloak and tortoiseshell butterflies are sleeping under woodpiles, logs and otherwise out of direct cold and snow where they'll be OK if they're left alone, while bats are sleeping in our lava caves that are a stable 4 degrees.
On warm days in February it's not uncommon to see a mourning cloak or tortoiseshell out flying about, building up their oxygen supply—but it's a bit dangerous, as there is a fine balance between fat use and oxygen demands.
Unlike insects, bats have blood similar to ours, which can't turn to antifreeze. The thin-skinned species have gone south, while "our" bats have settled into dark areas with stable temperatures above freezing where they will use their fat reserves to safely sleep through the winter. That's why some of the lava tubes near Bend are closed to the public, so bats can survive the winter.
Another problem for flies trapped in our houses for winter is predators. Yes, those lovely little spiders you discover in the sink or bathtub, running out from under the bed and getting blamed for welts you otherwise can't explain, eat flies. Sheet web spiders are elegant fly-catchers and probably dispose of many flies (and clothing fabric eaters) long before they are seen flitting about, dying and falling apart, as Philip has noted.
"Outdoor" flies, such as mosquitoes, robber flies and crane flies, die in winter, and leave their eggs to start the next generation the following summer. However, gnats (which are also flies), have an astounding behavior in winter.
Some species of gnats seem to wait for winter to prepare for mass breeding orgies. From October to March, on any day when the temperature rises to above 60 degrees, gnats will take advantage of the situation and form a mating swarm to get a head start on the rest of the insects. It's probably good strategy, as there are very few birds out looking for insects, bats are sleeping, and (most) predatory insects are not about, so they have the world all to themselves.
Which all goes to prove, most times, it is difficult to fool Mother Nature...