Why Does That Blasted Blowfly Keep Flying in Circles?
Anyone who has spent much time on a farm, in a shed or simply sitting in a quiet room on a warm afternoon knows the sound. Somewhere above your head there is a blowfly going buzz, buzz, buzz, apparently flying around in circles for no conceivable reason. It does not land. It does not head for the open door that you can plainly see. It just keeps circling the middle of the room until eventually you reach for the fly swatter and wonder how an animal this apparently stupid survived millions of years of evolution.
The answer is that the blowfly is not necessarily stupid at all. It is trying to operate a sensory system designed for the outdoors inside an environment that evolution never prepared it for.
A fly does not experience a room as we do. We look across it and immediately recognise walls, windows, doors, tables and empty space. We can see that there is nothing particularly interesting beneath the ceiling and that the obvious escape route is through the open door.
A blowfly is working with very different equipment. Its compound eyes are superb at detecting movement. That is one reason flies are so annoyingly good at avoiding the swatter coming down on them. But excellent motion detection is not the same thing as seeing the world in the detailed, spatially organised way that humans do. Much of the fly's information about its environment also comes from light, smell and moving air. Put that animal inside a building and some very old navigation systems can start producing very strange results.
Artificial light is one possible culprit. Many flying insects use distant natural light sources as navigational references. The sun is so far away that its direction remains effectively constant as an insect moves across the landscape. Keeping a distant light source at a particular angle can therefore help an insect maintain its course.
A nearby artificial light is an entirely different proposition. Its apparent direction changes rapidly as the insect moves. Research on insect flight around artificial lights suggests that insects can become disoriented because they use light as a reference for which way is "up" and for maintaining normal flight attitude. What works beautifully beneath the sky can produce loops, dives and apparently nonsensical trajectories around artificial illumination. The insect has encountered the biological equivalent of giving your GPS the wrong map.
Smell adds another complication, especially for blowflies. Blowflies make their living by finding things that we generally wish they wouldn't find: carrion, manure, rotting organic material and other sources suitable for feeding or laying eggs. They are extraordinarily sensitive to airborne chemicals associated with such material.
But smell does not sit neatly in the air like a coloured road pointing towards its source. Odours travel in turbulent plumes. A fly searching for the source may repeatedly cross and recross an odour-bearing current, effectively asking: Is it stronger over here? What about over there? Have I lost it?
Outdoors, wind gives that search some structure. Indoors, where air movement may be weak, intermittent and complicated by doors, windows, heaters, computers and people, the chemical landscape can become confusing. A fly may repeatedly patrol the same volume of air because its sensory equipment keeps telling it that something worth investigating might be nearby.
Then there is the air itself. A room that feels perfectly still to us is not actually still. Warm air rises from electrical equipment, sunlight-heated surfaces and human bodies. Cooler air sinks near windows and walls. Doors create currents. Fans and air-conditioning produce still more movement. A small flying animal experiences these currents far more dramatically than we do.
What looks from our scale like a blowfly pointlessly circling the centre of a room may therefore involve the fly responding simultaneously to light, odour, landmarks and tiny air currents. The resulting flight path looks ridiculous to us because we can see the whole room. The fly cannot stand back and consult the architectural plan.
This also helps explain one of the great human-fly mysteries: why doesn't the idiot simply fly through the open door? Because "open door" is our concept, not the fly's. We recognise a rectangular gap in a wall as an exit. The fly detects brightness, contrast, airflow and other sensory cues. A brightly illuminated window may be more attractive than an open doorway, even though the window contains an invisible sheet of glass. Hence another familiar spectacle: the fly repeatedly battering itself against a window while freedom lies a metre away.
There is a broader evolutionary lesson buried in this minor domestic annoyance.
Natural selection does not produce perfect organisms. It produces organisms that function well enough in the environments in which their ancestors survived and reproduced. A navigation system that works extremely well across open country can malfunction spectacularly inside a rectangular box illuminated by electric lights and sealed with transparent glass.
Humans have created countless environments that are evolutionarily novel. Sometimes animals cope remarkably well with them. Sometimes their ancient behavioural rules encounter circumstances for which those rules were never designed.
So the next time a blowfly spends half an hour roaring around the centre of your room like a miniature aircraft whose pilot has gone mad, give it a little evolutionary sympathy. It probably isn't trying to annoy you. Its navigation system is doing roughly what it evolved to do. The problem is that nobody told the blowfly that humans had invented rooms.
