Flies — I mean all of them, the whole insect order Diptera — have a public relations problem. A few bad actors taint the whole group. To be sure, some of those actors are seriously bad: mosquitoes that carry malaria, house flies that transport diseases like typhoid fever, horseflies that perforate your hide. But looked at rationally, flies as a group may do more good than harm, especially in temperate climates like ours, serving as pollinators and controlling insect pests. And many flies look pretty while doing it.
For example, it’s hard to imagine a more appealing group of insects than the flies in the abundant and widespread family Syrphidae. Early fall seems to be peak season for them on the Vineyard, so now is a good time to get acquainted with these beneficial, often colorful insects.
A large family with global distribution, Syrphidae has about 6,000 species in it. Most are rather small; ours range from a few millimeters long to about the size of a hornet. No exhaustive study has been done of the Island’s Syrphids, but it’s clear the Vineyard hosts scores and perhaps hundreds of species, some of them hugely abundant.
To an entomologist, Syrphids are recognized by a so-called “spurious vein” in a particular portion of their wings: unlike normal veins that connect either to other veins or to the wing’s edge, a spurious vein leads nowhere and simply ends mid-wing. Nobody seems to know why this odd feature evolved, but there it is, if you look hard enough. Happily for most of us, it’s usually easy to recognize Syrphids by other methods.
For one thing, the flight pattern exhibited by most Syrphids has earned members of this family the common name “hover fly.” Adults spend much of their time in the air, and much of that time is spent … well, hovering, fixed in the air as if hanging from an invisible string. Also, Syrphids often feature bright colors, in particular bands of yellow and black that make them resemble bees (the resemblance is uncannily close in some cases). Syrphids can’t bite or sting, but of course bees can: by resembling armed insects, defenseless Syrphids deter would-be predators from attacking.
If they aren’t hanging motionless in the air, adult Syrphids are likely to be poised on a flower. Like most other insects that visit flowers, they’re after nectar or pollen, which they eat (research indicates that adequate pollen intake is necessary for at least some Syrphid species to successfully lay eggs). Moving from flower to flower, Syrphids, like bees, can transfer pollen from one plant to another, fertilizing the second flower visited. Syrphids tend to pick up a lot less pollen than bees do, because their bodies are much less hairy. But they make up for this deficiency through activity: Syrphids may carry less pollen, but they visit far more flowers than bees typically do.
From a gardener’s perspective, Syrphids may be even more useful before they mature. The larvae of many species of Syrphids, which resemble chubby caterpillars, are carnivorous, feeding on a range of small insects but showing, as a group, a particular fondness for aphids. Their hunting methods are primitive: apparently Syrphid larvae simply crawl randomly over plants, swinging their front end from to side until they contact an aphid, at which point they bite it and begin draining its body of liquids. But the method works: A maturing Syrphid larva may eat hundreds of aphids during its development, helping control these pest insects that feed, often in large numbers, by sucking the juices from plants. Along with ladybugs, Syrphid larvae rank among the most effective controllers of aphid populations.
Syrphids, despite their protective coloration, often wind up in the food chain themselves. No doubt birds eat them in huge numbers; robber flies seem to take them with regularity; and in particular, spiders that spin webs among vegetation prey heavily on Syphids, with the webs intersecting the flight path of the flies as they flit from flower to flower. But Syrphids are prolific and, in many cases, not too fussy in their habitat requirements. There are some Syrphid species that I bet are common in every single garden on the Vineyard, some of them producing multiple generations in the course of a single season.
In such a large, diverse family, of course there are species that don’t fit this beneficial pattern. Larvae of a few Syrphid species, for example, burrow into the roots of ornamental bulbs like daffodils, and are considered pests. Larvae of other species are aquatic, feeding on detritus and doing no particular good for humans. But overall, the flies in this family are the best friends a gardener could have. So hit the house flies, if you must, or mash the mosquitoes. But learn to recognize the Syrphids, with their bright coloration and darting, erratic flight. They’re on your side.