They seem to appear out of nowhere to infest your kitchen, each a tiny speck of winged annoyance. They hover around your five day old bunch of bananas and take great interest in your unwashed wine glasses, but they truly revel in the aroma of your garbage can. And with no greater sustenance than the tiny yeasts which grow on your food waste, these prolific pests quickly multiply into a swarm that may seriously threaten your sanity, if not your food supply.
Of course I speak of the fruit fly, particularly members of the genus Drosophila. But how does one wage war against such a diminutive foe? One possible approach is to jump around the kitchen while clapping your hands together, hoping to squish the life out of each tiny insect. I’ve tried this method with some success, but it has several drawbacks. Fruit flies are surprisingly swift in their aerial maneuvers, so hitting them is a challenge. They’re also very tough: if your hands are on target, you may have the experience of watching the crushed insect on your finger unfold its wings and fly away. And even if your violent rampage is successful, frequent handwashings become obligatory.
What then is to be done about the fruit fly menace? Before you place a call to a fumigator or a real estate agent, let me tell you about a much simpler solution. I, your friendly essayist, have come to your aid with the blueprints for an effective fruit fly trap. The materials you need for this trap are quite common: a juice jar, a sheet of scrap paper, scissors, and some masking tape. You also need bait for the trap, which I’ll discuss later. When used properly along with regular garbage removal habits, this trap might just help reclaim your kitchen!
Before I go any further I must divulge that this fly trap is not entirely my own creation, but closely resembles the traps used by laboratory researchers for decades. Clearly, the fly trap is an example of science making life better for all of us, but it’s certainly not the only contribution to human endeavor made by Drosophila research. After all, the fruit fly has been an important study organism in the field of genetics since the early 1900’s, when scientists named Morgan, Sturtevant, Muller, and Dobzhansky puzzled over the process of inheritance in various Drosophila species. Why were they so preoccupied with fruit flies? Well, if you can figure out how genetics works in the fruit fly, much of it will be applicable to other species as well. And Drosophila was a convenient model: fruit flies breed just as well in the laboratory as they do amongst your orange peels and coffee grounds.
Speaking of the fruit flies in your garbage can, you might wonder how they came to be there. Of course they can’t really “appear out of nowhere”, no matter how much it seems like week-old garbage breeds them. After all, the idea that living things arise by “spontaneous generation” was put on shaky ground in 1668 when Francesco Redi proved that rotting meat doesn’t produce maggots: when he kept the flies away, none appeared. The notion was finished off by Louis Pasteur, who in 1859 showed that airborn bacteria are necessary to make chicken broth spoil. In light of those findings, it’s likely that your fruit flies slipped in through an open door or window, or hitched a ride in with the groceries.
But what about the larger question of why an insect with such odd habits should be around at all? If you live in America, then a few hundred years ago there were probably no garbage cans to raid in your neighborhood. Where were the ancestors of our “domestic” Drosophila before human settlements became their homes as well? The history of one common trash dwelling species, Drosophila melanogaster, has its roots in Africa, where I presume they knew of food sources other than garbage cans. Perhaps ten thousand years ago, this species expanded into Europe and Asia. Curiously, this roughly mirrors the history of our own species, Homo sapiens, and although it might not have been human migration that helped melanogaster spread, I can’t help imagining a little swarm following the trail of garbage left by the first of our kind to leave their African homeland. But how in the world did fruit flies reach the Americas? Probably the same way Europeans did, stowing away in the ships of explorers and traders.
Navigating back to the subject of fruit fly removal, the construction of the trap is quite simple. Fold the paper into an ice cream cone shape, leaving a small opening at the bottom of the cone about the size of a pencil eraser. Tape the paper to keeps its shape: you’ve just made a paper funnel. Set it on top of the juice jar, narrow end down, and use scissors to trim your funnel down to a couple inches above the jar opening. Next, set the funnel aside and drop in your bait: for your insect ambrosia I suggest either a piece of banana peel or a squished grape. Finally, tape the funnel into place, all the way around the jar opening, to leave no escape route for the flies. How does the trap work? Fruit flies are drawn to the scent of the bait, landing on the paper funnel and gravitating toward the strongest smells in the jar. Although they have little trouble finding their way into the jar, they almost never find their way out. It’s a very small opening they’d need to fly through, and their instinct is to fly up toward the sealed mouth of the jar instead.
Once your trap begins seducing its captives, you’ll have a great chance to observe them up close. Take a close look at the eyes: each compound eye has hundreds of facets. Do they all have red eyes? They should, unless they’re mutants! The early geneticists found many unusual eye colors and named genes after these mutations: white, brown, sepia, burnt orange, rosy, scarlet, vermilion, and others. If your own eyes are particularly keen, maybe you can learn to tell apart the females and males. The females are larger, with elongated abdomens; the males are smaller, with a more pronounced dark spot at the tip of the abdomen. If you see that both sexes are present, you may get the chance to observe fruit fly courtship behavior. Believe it or not, a male fruit fly will serenade a female by vibrating his wing while he follows her or dances around her.
Have I mentioned that I’m a Drosophila researcher? My job is to take fruit fly DNA and discover what it has to say about evolution. Perhaps that colors my perspective a bit, because even though fruit flies are unwelcome guests at my dinner table, when I see them my irritation is tempered with contemplation. I think about the lineage from which these insects descend. Their cousins helped us understand the principles of genetics, and by looking at fruit fly genes we have expanded our understanding about everything from how species evolve to how the molecular basis of development produces a complex organism. Recently Drosophila melanogaster had the honor of being one of the first animals to have every ‘A’, ‘C’, ‘G’, and ‘T’ of its genome decoded, about 180 million of these DNA bases in all. This number is actually rather small when compared to the content of your 46 chromosomes, which add up to about three billion bases! The compactness of the fruit fly genome may make it easier for us to study, and the knowledge we gain from it will help us to understand the human genome, which will in turn spur new advances in medicine and our understanding of our own biology.
My research leads me to ask what these genes can tell me about the history of a population. Was it started by a small group of founders, having bravely endured a generations-long voyage across the Atlantic? Did the flies need to adapt to harsh winters, dry buildings, or a new diet? These trials are written in their DNA, if we can learn to read the record. I study the genetic differences between individuals, between populations, and between species. For example, one fruit fly may have a ‘C’ where another has a ‘T’. If I find that genetic diversity is very low at a particular gene, it tells me natural selection may have recently acted on that gene, because the gene favored by selection has replaced all other copies of that gene in the population. But if I find that pattern of low diversity all across the genome, I conclude that population history is the cause, and specifically that the population being studied had gone through a “bottleneck” (a period of reduced population size). If only ten fruit flies got on the boat to America, the New World population is only going to have genes from those ten individuals until mutation creates more variation or additional immigrants arrive.
DNA is more than a blueprint for a fruit fly, a barnacle, a virus, an olive tree, or a human being. Its double helix forms a thread of life that can be traced back through evolutionary time. Darwin realized that following any two lineages back in time leads to a common ancestor, even if one of those lineages is yours and the other belongs to the small insect hovering next to that orange peel. The family resemblance has faded somewhat, I’ll grant you that.
Once your trap
has yielded a bumper crop of fruit flies (and you’ve spent time
studying
them, of course), you might start wondering what to do with them.
Well, if you let them sit around for a while, you may get the chance to
witness other stages of the Drosophila life cycle: the egg,
larva,
and pupa. I realize that some readers may not be interested in
raising
additional fruit flies, so if after reading this essay you still just
want
to get rid of them, you have a couple of choices. The “cold
efficiency”
option is to put the trap in the freezer for an hour or more every few
days. Or, if you’ve started to feel a kinship with
your fruit
flies, there’s the “humane alternative”. Take
your trap outside and
remove the funnel, then watch as your captives rise to greet their
newfound
freedom, each one straining its antennae to scent out the nearest
aromatic
rot, and each one striving to perpetuate its lineage and extend its own
tiny thread through life’s rich tapestry.