“I have traveled extensively in Concord,” said Thoreau, famously if apocryphally. And truly, exploration is as much a matter of mindset as it is of location. In recent years, more and more of my nature study has taken place close to home, in our scant quarter-acre yard on the fringe of the Oak Bluffs metropolis. I’ve converted much of the yard to meadow and established a long list of native wildflowers to attract insects. It looks a bit unconventional, I suppose, but it produces enough variety to keep me constantly surprised.
You might think that the snow and cold weather Veterans Day weekend would have put an end to the year’s bug-watching. But some insects stay active late into the autumn. A few actually flourish, evidently adapted to life at a point in the year when predators and competitors are few. And my gardening choices ensure that, even in mid-November, pollinators find a food source. Some of these insects turn out to be pretty odd.
A brief period of midday sun last weekend brought temperatures up near 60, impelling me to get out the camera and poke around. Still going gangbusters in the backyard was a stand of cowpen daisy — a fast-growing annual, resembling a small-blossomed sunflower. It’s not a Vineyard native but occurs widely across North America; I value its vigor, its very long bloom season, and its appeal to pollinators.
Camera in hand, I simply staked out the cowpen daisies and waited for something to visit. The first to appear was an interesting fly, Eristalis tenax, sometimes called a drone fly. It’s a member of very large fly family, Syrphidae or the so-called “hover flies” (appropriately named for their erratic flight pattern). While there are native Eristalis species, drone flies are Eurasian, introduced to North America sometime around the mid-19th century.
They’ve spread to inhabit much of North America, and unlike many such introduced species, they don’t seem to do much harm. There’s no evidence they’ve displaced any native insects, and it appears they’ve simply taken their place among our continent’s thousands of native fly species. Adult drone flies are beneficial insects, typical hover-flies that visit flowers avidly and serve as effective pollinators. But the species turns out to have some hidden talents.
Like many hover flies, Eristalis resembles a bee, with a hairy body and black-and-yellow stripes. The similarity serves as defense mechanism: the adult drone fly can’t bite or sting, but bees certainly can, and by resembling a bee, the fly deters potential predators. Adult drone flies eat pollen and are said to favor yellow flowers, especially those in the composite family; cowpen daisy fits the bill, with its wealth of pollen-rich, golden blossoms, and I was not surprised to find a half-dozen drone flies taking advantage of the offering.
Researching this fly, I learned that the adult is actually much less interesting than the larval stage. So-called rat-tailed maggots (a name that represents a public relations nightmare if ever there was one!), Eristalis larvae are plump little blobs, perhaps half an inch long, with a thin tail often longer than the body extending from their sterns (hence “rat-tailed”). The tail is in fact a tube, and it serves as a snorkel for the immature fly. Living under water, the larvae literally breathe through their butts, and their access to oxygen allows them to survive in water of even the most appalling quality. If fact, they like it that way, feeding on the bacteria and other organic matter found in puddles near dung hills and similar unappealing locations. The ability to reproduce in a niche that few competitors can occupy surely helps account for the successful spread of this species in North America.
It gets better. To survive in such cruddy conditions, Eristalis larvae have apparently evolved a tough exterior that resists attack by bacteria or erosion from chemicals. They can, in fact, sometimes survive the juices of the human digestive tract, and there are cases of drone fly larvae, after being swallowed accidentally, managing to survive inside humans, presumably eating the contents of the digestive tract — a condition called intestinal myiasis. The condition is rare and, while sometimes unpleasant, evidently self-limiting and not lethal. And fortunately, you’re safe from it if you avoid drinking filthy water with Eristalis larvae in it.
But it gets even better than that. There are documented cases of larvae entering other bodily orifices — the nose, the anus, or the urinary tract — and subsisting on whatever organic matter they find, while breathing through those handy tubes emerging from their back ends. This is what you call a resourceful fly.
Don’t worry. Instances of Eristalis larvae living inside humans are rare (though myiasis by other insect species is said to be a significant medical problem in the tropics) and require unusual circumstance to occur. But you never know what kinds of critters you’re sharing the landscape with. For the alert naturalist, there is always something surprising out there, ready to be discovered, and that’s what Thoreau meant about traveling extensively in your home town.