Sickening science

A school holiday special: science for the strong-stomached (6 minute read)

Jul 11, 2025

It’s school holidays in New Zealand and Australia at the moment, and I’ve been spending some time with my 7-year-old nephew. He’s rapidly turning into a keen biologist, with a little encouragement in that direction from me. So, I decided that this week I would bring you some science that you can share with younger friends and family members. Some discretion is advised, though, since some of this article may not be suitable for those of a squeamish or sensitive nature, or for younger children.

Would the world be better without flies?

Buzz, buzz. The smell of your delicious dinner has wafted through the air and attracted a fly. It flies though the open window and is heading straight for your plate. Better eat up!

But why is it so bad for a fly to land on your dinner?

Do you know what that fly was doing before it flew in the window? It might have been sitting on a flower, lapping up some nectar. Maybe, if you’re lucky. Or it might have been making a meal of some dog poo. Or perhaps it was snacking on a dead rat. Now, it wants to crawl all over your food with its grubby little feet. If you don’t shoo it away, it will slobber saliva and vomit out some of its stomach contents, which will soften up your dinner so the fly can slurp it up. Not only is this disgusting, but it can spread microbes which cause food poisoning. The fly won’t be the only one vomiting if you’re not careful.

It's tempting to think that the world would be a better place without animals as revolting as flies, but is that really true? Is there anything good about flies?

To know that, we have to understand what flies are. The common housefly belongs to a large group of insects known as the Diptera. It’s easy to know if an insect belongs to the Diptera or not, because Diptera have only two wings, while the other insects you see flying around have four1. As well as house flies, the Diptera group also includes blowflies, mosquitoes, fruit flies and New Zealand’s blood-sucking sandflies.

Today, I’m not going to talk about any of the bloodsuckers – that’s for another time. I’m mostly going to talk about the kind of flies like houseflies and blowflies, which feed on putrid things like rotten meat, poo and other stinky stuff, although they are also happy to dine on your delicious dinner. These flies have an important job – they are part of nature’s cleanup crew. Poo and death are both a part of life, and nature’s cleanup crew break down poo and dead things and return them to the soil, where they can be used to help plants grow. Since everything we eat depends on plants, we also depend on the cleanup crew – including flies.

There’s something else important that many flies do as well. You might have heard that bees are important because they help plants to produce seed by pollinating their flowers. Flies are important pollinators too. They pollinate many types of flower and are important pollinators of some crops, such as avocado, mango and carrot. And without tiny flies known as midges to pollinate cocoa plants, there would be no chocolate!

Some flowers work extra hard to make sure they attract flies to pollinate them. How do they do this? They produce the same kinds of chemicals which are found in things that flies love, such as poo and rotten meat. To carry these foetid fragrances further, some plants even produce heat inside their flowers. This causes the stinky chemicals to evaporate and waft on the breeze, enticing passing flies. As the flies get closer, they might think that the flowers look like rotten meat as well, because many of them are coloured in shades of reddish-brown which look like a piece of meat left out of the fridge for too long. Both the world’s largest flower and the world’s largest flower spike are sometimes known as corpse flowers because of their foul odours for attracting flies.

There’s something else amazing (and also disgusting) that flies do for us. To understand it, I need to explain a bit about the fly’s life cycle. This begins when an egg is laid on something suitable to feed a growing fly, such as a dead animal. The egg hatches and the young fly, known as a larva or maggot, emerges. As it eats and grows, the maggot sheds its skin, going through different stages. A housefly maggot goes through three stages but other types of fly can have more. An expert entomologist can look at a maggot and recognise the stage it’s at. When a maggot is fully grown, it turns into a pupa, which doesn’t feed, and then eventually emerges as an adult fly.

How long it takes for a fly to grow and move from one stage of its life cycle to the next depends on how warm or cold it is where the fly is living. If the weather is cold, it grows more slowly. As the weather gets warmer, the fly grows faster, until it gets too warm. Then, the fly’s growth slows right down and it can die, or the maggot will wriggle off to find somewhere cooler. Scientists have grown different species of fly at different temperatures in their labs so they know exactly how long these flies take to grow at different temperatures.

Why does this matter?

Sometimes dead bodies are found in out-of-the-way places, and it’s important for the police to know when the person died. If there are maggots on the body – and there will be if it’s been lying around for long – experts will collect the maggots and examine them. They will determine the growth stage of the maggots and combine that with information on what the temperature has been to find out how long the body has been there. Maggots have helped to catch murderers, and in one case they proved a woman who was in prison for murder was actually innocent.

There’s one more important thing that maggots do for us. We are used to thinking of flies as dirty and disgusting, because they feed on dirty and disgusting things. But if we grow them on sterile food, they will be perfectly clean too. Then, we can put them to work in the most surprising way – cleaning up infected wounds. Maggots have been doing this dirty job for us since ancient times. The maggots eat infected and dead parts of the wound, but leave healthy flesh alone. They also make their own medicines which fight the microbes causing the infection.

Forty years ago, doctors thought that using maggots in medicine was a particularly revolting part of history. However, the microbes which cause infections can now resist many of the drugs we have been using. Sometimes, people have wounds which just won’t heal – that is, until some miraculous maggot medics are applied!

I’m not saying you shouldn’t swat flies, because you certainly don’t want a fly sharing your dinner. But flies are important insects which help us in many ways. So, the next time you shoo a fly away, you might think to say thank you as well.