Fly Day Friday: Snow Flies

As I write this, the outdoor temperature here in Colorado Springs is a whopping 5° F. There are going to be few, if any insects out and about in those frigid conditions. A sunny winter day, even higher up in the mountains, could provide a different story. There are wingless crane flies that can be seen crawling across the snow in the dead of winter.

Last October 27, 2012, I was fortunate enough to take a trip with other members of the Aiken Audubon Society to Guanella Pass (Clear Creek County) in search of ptarmigans. We got skunked on the birds, but one person in our party spotted a tiny insect creeping across the snow. I instantly recognized it as a wingless crane fly, though I had only seen images of them until then.

There are sixteen species of “snow flies” in the genus Chionea in North America. They are in the crane fly family Limoniidae, formerly a subfamily of the Tipulidae. While western species are mostly confined to mountain ranges, eastern species can be found in most forested areas, especially in regions that experienced glaciations during the most recent ice age. They appear most commonly in October and November, and again in February and March.

While these dark flies are most conspicuous on the surface of the snow, most of the time they are concealed in the tunnels of small mammals, beneath leaf litter, or fairly deep in caves. These situations help to insulate them from truly severe weather, and an adult snow fly may live up to two months.

Drawing by Dr. George Byers

The life cycle of Chionea remains mostly a mystery. Every attempt to rear them in the laboratory has failed because no one knows what the larval stage feeds upon. We do know that an adult female fly can produce up to 194 eggs, each one laid singly. Lacking wings, the female fly can store eggs even in the thoracic cavity that is normally packed with wing muscles.

Eggs take eight days to three weeks to hatch under laboratory conditions. Larvae have fairly hardened mouthparts, suggesting they may be able to take in solid food. Just what that nourishment could be is up for conjecture. Maggots of distantly-related flies feed on fungi that grow on bat guano in caves, but that information may or may not be relevant to snow fly larvae. We do know the larvae feed and grow during the summer months, with pupation taking place in the fall.

Adult snow flies have few enemies, but we know that mice are among their predators. Believe it or not, this conclusion is reached from knowledge of the cycle of parasitic tapeworms. Rodent tapeworm eggs expelled in the feces of a mammal must be consumed by an insect, usually a larva. Once inside the insect, the tapeworm metamorphoses into a “cysticeroid” stage. There it remains until the insect is in turn consumed by a rodent. Tapeworm eggs have been found in the gut of adult snow flies.

Rock-crawlers, primitive insects related to mantids and cockroaches, are also predators of snow flies. Grylloblattids, as rock-crawlers are known in the scientific community, are found in isolated populations at high elevations in western North America.

How do snow flies keep from freezing to death? First, snow flies occupy the “subnivean” environment: a microclimate that exists in cavities beneath the snow created by arching grassblades, leaf litter, and rodent tunnels. These nooks and crannies offer protected niches with temperatures that are milder than the surface and air temperatures above.

Snow flies also have a body chemistry to cope with the cold. Many insects have glycerol in their body fluids, which acts as an antifreeze, preventing the formation of sharp, life-threatening crystals of ice inside the insect’s tissues. Differing enzyme systems also allow snow flies to function at lower temperatures than most insects. In fact, they are susceptible to overheating!

The temperature in the room where I write this is not that warm, either. I am not believing the 66° registered on the digital thermometer and, lacking my own internal antifreeze, will go open a can of soup. Stay cozy, friends.

Source: Schrock, John R. 1992. “Snow Flies,” Kansas School Naturalist 38(2): 1-16.

Housemates

Besides the human and canine residents at 24 Sugarloaf Street, our house is also occupied by a variety of arthropods, mostly spiders. Crystalyn says she is not fond of spiders, so I’m hoping she doesn’t read my blog, at least for the sake of the health of these arachnids living with us.

The first spider to meet my acquaintance was this very gravid (egg-laden) female “prowling spider” of the family Miturgidae, on the ceiling of the bathroom. She is most likely a member of the genus Cheiracanthium, known also as “longlegged sac spiders.” The genus used to be in the family Clubionidae, but was moved to the Miturgidae in 1997, a decision still being debated in arachnological circles.

These spiders are mostly nocturnal, prowling during the night then spinning silken retreats in which they spend the day. This girl spun a bivouac as I watched.

Later I would find her mate in the kitchen. Well, it was an adult male at least, evidenced by the modified palps. They appear as the dark “boxing gloves” near his mouth. The palps (or pedipalps) are leg-like mouthparts that double in the male as intromittent sex organs. He deposits sperm on a mat of webbing, then draws it into each palp. These are complex appendages, designed to fit like a key in a lock, the lock in this case being the female’s sex organ called the epigynium. Like the female, the male also “sacks out” during the day.

Cheiracanthium had long been considered a genus of spiders at least mildly venomous to humans, but research has proven that this is a myth. So, I let them roam and carry out pest control activities. We definitely have too many mosquitoes here!

Crane flies of the family Tipulidae are very abundant now, some species being attracted to lights at night, and inevitably one strays indoors. This one has patterned wings. It is on the bathroom ceiling, too, but I don’t know whether one of the spiders got it. Crane flies are often mistaken for giant mosquitoes, or are referred to as “mosquito hawks,” but the adult insects probably do not feed, let alone take prey. Their size is deceptive as they are fragile insects. Look at one cockeyed and a leg will fall off. Their lifespan as winged insects is brief, just long enough to find a mate and reproduce. Crane fly larvae live in an incredibly variety of habitats, with diets that vary with the genus to which they belong.

A more permanent guest is yet another spider. This is what I imagine is a juvenile long-bodied cellar spider, Pholcus phalangioides, because adults are much larger. This one is well on its way, though, especially if it continues catching prey in the bathroom doorjamb like this. Cellar spiders are totally harmless to people and pets. While they do build tangled webs, they will also stray from those webs into the webs of other spiders and kill and devour them. Pholcids are frequently referred to as “daddy longlegs,” but that colloquial name is also given to harvestmen, arachnids of the order Opiliones (not spiders), and, ironically, to crane flies, especially in England. A crane fly would be more than this little one could handle.

Crystalyn is so busy that she sometimes forgets to turn the oven off, let alone observe the room for roaming arachnids. Her dog Ruby did see one, once, in the dining room, and drew our collective attention to it as it crossed the ceiling out of reach.

On the Radio

I awoke this morning to my usual radio station, but the DJs were bantering about “giant mosquitoes” that one of them was seeing in his house recently. I knew immediately that what he was referring to were harmless “crane flies” in the family Tipulidae. I also knew they were emerging in fair numbers because I photographed this pair (female above, male below) as they rested on a pillar at the University of Arizona campus during the recent Tucson Festival of Books.

I got up, went to the phonebook, and found the number for the station. As luck would have it, I got right through to Blake and Jennie, and told them what I knew.

”Are they good dog food?” asked Blake, “because my dog sure loves to eat ‘em.” We all got a collective chuckle out of that, and I replied that “They won’t do your dog any harm, let’s put it that way.” We hung up, and I went back to bed for a minute to hear what would happen next. Well, they played our conversation on the air, mystery solved.

I’d really enjoy doing a regular “spot” on a drive time radio show, answering questions about insects and arachnids from the listening public. Meanwhile, you can meet my over-the-phone friends Jennie and Blake at the website for 92.9 the Mountain, KWMT FM in Tucson.