The flies can't all be bad

For each person on Earth, there are 17 million flies. They pollinate plants, consume decomposing bodies, damage crops, spread disease, kill spiders, hunt dragonflies. Some have even lost their wings so as to live exclusively on bat blood, spending their lives scuttling about the fur of their hosts, leaving only to give birth to a single larva - usually. "That's why I love them. They do everything. They get everywhere. They're noisy. And they love having sex," said Erica McAlister, a curator of Diptera - flies, to the rest of us - at the Museum of Natural History in London.

Erica has captured her affection for the Diptera in The Secret Life of Flies, a short, rich book by turns informative and humorous, both a hymn of praise to her favourite creatures and a gleeful attempt to give readers the willies. Her book is also the source of the 17 million number, which, she pointed out, is just an estimate. Like other fly writers before her, Erica has more than fun in mind. She wants to remind the world at large of the importance of flies to humanity, and to the planet. They are not just something to swat.

Without them, to take just one example, there would be no chocolate. Erica herself hates chocolate, but she is fond of the kind of flies that pollinate the cacao plant - a variety of biting midge. The midges are tiny, mostly blood-feeding insects, but the chocolate midges like nectar and carry pollen from one plant to another. Biting midges are, in fact, part of Erica's speciality.

She is fond of all flies, but focuses on those that are included in the lower Diptera, which include mosquitoes, black flies and, as she puts it, "everything that's bitey, stabby, nasty." Her life among flies involves both museum work and field research. For her, this is a dream job. She recalled the first time she went behind the scenes at the museum, as a student, before she actually worked there. "I'd been let into a building that had 34 million insects. I said, 'Oh hello, I quite like you.'"

Many flies do an enormous service for us and the planet by cleaning up all sorts of the biological world's detritus, from dead wood to the slime in drainpipes. Drain flies, or sewer gnats, are actually cleaning up human mess. Occasionally, however, they may have a population boom that sends the adults into the air, which is annoying; if the bodies disintegrate into tiny particles in the air, they are potentially harmful to human health.

And, of course, there are the flies that feed on dead bodies - the 1,100 species of blow flies, favourites of forensic detective shows. The maggots of these flies, like the very attractive bluebottle larva, devour corpses of mice and men and everything else. Within science, flies are one of the great subjects of laboratory study.

Or rather, the fly: Drosophila melanogaster, commonly known as the fruit fly, although Erica points out it actually belongs to a group called the vinegar flies. They are easy to work with and share the same
basic DNA as all life. Historically, they have provided much of the foundation for modern genetics. And now, they may provide deep insights into neuroscience and other fields.

Flies can be startling in their appearance as well as their behaviour. One Middle Eastern fruit fly has patterns on its wings that look something like spiders. No one knows why. Another fly, Achias rothschildi, must swallow air to inflate its eye stalks when it first emerges as an adult.

There are, Erica notes in her book, limits to even her affinity for flies. Houseflies, for instance, may be affected by climate change. According to one projection, the population could increase by 244% by 2080. "That's a lot of flies," she writes, "even for my tastes."