If you’ve ever found a large, bristly, blueish-black fly banging against a windowpane in late autumn or early spring, chances are you were looking at a fly in the genus Calliphora. Also known as bluebottle flies, these insects seek out sheltered sites to overwinter, and resume activity as the weather warms, often ending up inside human dwellings.
Or perhaps you’ve noticed smaller, iridescent green flies, possibly large numbers of them, flitting about your yard in the summer. There are likely members of the genus Lucilia, or greenbottle flies. Both these insects represent the family Calliphoridae, or blowflies. Precise identification is tricky, depending on characteristics — the coloration of the face or the number and arrangement of spines on the body — that are hard to capture in photographs. But Calliphora vicina appears to be the Vineyard’s usual bluebottle fly, and among my photos of Lucilia, the only one I’ve been able identify with any certainty appears to be Lucilia sericata.
Perhaps you’ve left a chicken carcass in your trash barrel for a bit too long, and found a mass of whitish maggots wiggling around it. Those, too, are probably blowflies, in larval form. You’d need to know more than I do to be able to identify blowfly maggots, but if you’re truly curious, you can put a few in a jar with a bit of whatever they’re feeding on, and wait a week or two for the larvae to pupate and then emerge as adults.
Blowflies often survive the winter as adults, and with quickly maturing larvae, they can run through multiple generations in a year. So you can encounter blowflies at any season, as long as local conditions are warm enough (the low 40s seems to do it if the sun is out) for these flies to be active. I find blowflies in modest numbers, even outdoors, throughout the winter, and in the warmer months, they are often the most numerous, or at least the most prominent, insects in our yard.
Blowflies rank among the most widely distributed of insects. Nearly anywhere offers something an adult blowfly is willing to swab up with its sponge-like mouthparts. And almost as widespread are the food sources — decaying matter of all kinds, but especially rotting flesh — that blowfly larvae depend on.
Adult blowflies are simply geniuses at finding dead things, zeroing in unerringly, sometimes within minutes of the demise of the food-source-to-be, on chemicals that flesh produces as soon as it begins to decay. Eggs, laid on or near the increasingly aromatic food source, hatch in days or even just hours. Larvae feed on the decaying flesh, pupate, and finally emerge as adults. The duration of the process depends on temperature, but under ideal, warm, moist conditions, it seems like two weeks or so suffices to produce the next generation of adults.
The development of blowflies, including the effect of temperature on the process, has been much studied, and the habits and ubiquity of these insects make them important in the field of medical forensics. Blowflies almost invariably find and lay eggs on an exposed dead body, and through identification of the fly species, reconstruction of the ambient temperature, and examination of the stage of development of the maggots, a forensic entomologist can help establish the time of death of the victim, sometimes to a window of just a couple of hours. Blowfly maggots also have a limited, but perhaps growing, medical role in keeping wounds clean, since sterile maggots, judiciously applied, cheerfully devour any dead or decaying tissue around a wound.
More generally, blowflies can be thought of as recycling agents, converting the nutrients and energy stored in dead animal back into living, moving form. A wide range of birds and insects will feed on blowfly maggots, and the adults are likewise fairly easy prey for predators.
Moreover, adult blowflies (especially Lucilia, among the kinds found on the Vineyard) function quite well as pollinators. Pollen and flower nectar are among the many substances adult blowflies will eat, and with spine-covered bodies and legs, these insects readily carry pollen from one flower to another.
The association of blowflies with death and decay makes it hard to love these insects. And even I will admit that there is something unnerving about the thought of a fly visiting a dead animal, or a dog turd, and then settling in on your hamburger (a broad range of nasty germs are known to be transported by blowflies).
But there is also something amazing about the efficiency of these insects, and if you can overcome your aversion, there is something admirable about the roles they play in moving nutrients through the food chain, in pollinating plants, and in simply doing what they’ve evolved to do. So please think twice before you swat.