Friday, June 14, 2024

Adult fruit flies are tiny, about an eighth of an inch long. (Photo: Katja Schulz)

Fruit flies! They seem to appear from out of nowhere to pester you and your ripening bananas. They are especially common this time of year because the optimal temperature for their growth and reproduction is around 85 degrees.

A few of these tiny flies could have entered your house unnoticed when you opened a door. Or the fruit flies may have hitchhiked into your home on your bananas or other fruit – their eggs are too small to see with the naked eye. The larvae that hatch from the eggs are not easy to see either. It’s not until dozens or hundreds of flying adults emerge that you take notice.

A fruit fly in Rock Creek Park, Washington, D.C., on July 4, 2022. (Photo: Katja Schulz)

A fruit fly matures from egg to adult in about a week. A female can lay up to 100 eggs in a day, so in a week you can go from almost zero to hundreds of fruit flies in your kitchen. No wonder they seem to spontaneously generate!

The fruit flies we see in our house are often called “small fruit flies” or vinegar flies because they are attracted to fermenting or overripe fruit or vinegar. Often the term “fruit fly” refers to a family of small fruit flies called Drosophila, which are used in genetics research. A related family of flies, sometimes called “true fruit flies” are considered agricultural pests. 

Small fruit flies usually have red eyes. (Photo: Katja Schulz)

Small fruit flies have an excellent sense of smell and are strongly attracted to ripening bananas, potatoes, onions and other produce that is not refrigerated. The fruit flies we find in our homes do not actually eat fruit. They are interested in the yeasts and fungi that cause fruits to overripen. The point at which food doesn’t taste good to us anymore is the point at which fruit flies most like it. Fruit flies are attracted to the vinegarlike smell of fermenting fruit that indicates to the female that the fruit is a good place to lay eggs. So you might find fruit flies in bowls of (over)ripening fruit, compost bins, drains or garbage pails. Banana peels are one of the most common carriers of fruit fly larvae. If you put the peels in outside compost, you can reduce the occurrence of indoor flies. 

A true fruit fly (Eurosta comma) in Dunstable on Sept. 23, 2022. (Photo: Tom Murray)

Some parasitic wasps lay their eggs in the bodies of fruit fly larvae. The wasp grubs that hatch from the eggs eat the fruit flies from the inside out. But one species of fruit fly (Drosophila melanogaster) has a secret weapon: If the female spots a parasitic wasp, she lays her eggs on fermenting food with a high alcohol content. The fly larvae consume the alcohol at such high levels that the wasps inside their bodies are killed, while the flies suffer few side effects. Under normal circumstances, 90 percent of fruit fly larvae reach adulthood. When parasitic wasps are around, that number falls to just 10 percent. But if the larvae were born on alcoholic food, 50 percent reach adulthood.

Male fruit flies of the genus Drosophila have the longest sperm cells of any known organism. The fruit fly that we encounter in our kitchens has sperm 35 times longer than human sperm – perhaps why some of these flies mate by traumatic insemination: The male pierces the female’s abdomen and injects his sperm directly into the abdominal cavity.

A true fruit fly (Eutreta noveboracensis) in Bolton on Aug. 18, 2022. (Photo: Tom Murray)

Even though fruit flies are household pests, beginning in the early 1900s scientist began using them for genetics research. Because fruit flies are easy to raise and reproduce quickly, scientists can study multiple generations in a short time. And believe it or not, fruit flies have many similarities to humans! Almost 75 percent of the genes that cause diseases in humans have equivalent genes in fruit flies. This has helped scientists who study fruit flies earn six Nobel prizes for groundbreaking research by using this diminutive fly.

How in the world can scientists’ study something so tiny? Often, with a microscope. First, they dose the fly with carbon dioxide to keep it asleep on a pad under a microscope. Then they use a soft tool such as a feather or paintbrush to manipulate the unconscious fly. Under a microscope, scientists can view the stages of fruit fly development or examine genetic markers such as eye color or wing length.

The next time fruit flies take over your kitchen and seem like nothing more than a pesky nuisance, remember that they have made enormous contributions to our understanding of genetics, human development and conditions such as Alzheimer’s disease, Parkinson’s disease and autism.


Reader photo

Caroline Royer spotted these garter snakes in Cambridge’s Alewife Reservation on June 13, 2022.


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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.