The Secret Life of Flies

Few people would count flies as their favourite animal, but, luckily for you and me, there are exceptions. Erica McAlister, the senior curator for Diptera at the Natural History Museum, London, has been enamoured with them since childhood and in 2017 wrote the very successful The Secret Life of Flies. In preparation for reviewing her new book The Inside Out of Flies, I (finally) read the book that started it all to see what the buzz was all about.

The Secret Life of Flies is one of those books you wish that more scientists would write. There must be many more entomologists and other biologists that are treasure troves of knowledge who can give you a renewed appreciation for the mundane. In this handsome and very giftable little book, McAlister takes you on a ramble through the order Diptera – the flies, midges, mosquitoes etc. – with each chapter themed around a certain fly diet and lifestyle.

Your first surprise might be to learn that flies are important pollinators. Bees are the flamboyant movie stars of the insect world who get all the attention, but it is the small Theobroma cocoa fly that exclusively pollinates the cacao trees, giving us chocolate. And, as McAlister tells us here, flies are key pollinators of, for example, mango, chilli pepper, black pepper, carrot, fennel, and onion. But just as some flies make plants grow, plenty of others feed on them. Yes, there are vegetarian flies too, but we tend to be a lot less fond of those because of the global economic impact they have on our crops.

It is a fly-eat-fly world out there, though, and a recurrent theme in this book is that of biological pest control. Not infrequently, there are other fly species that will parasitize the pest species we are bothered by. Another unusual food source for flies are fungi. Colour me surprised, for I did not know this. Some of these are obviously the scourge of mushroom farms. But you will find flies feasting on anything from large forest mushrooms to slime moulds, and some even infiltrate termite nests to plunder their underground fungus gardens!

Of course, a large part of this book delights in its yuck-factor. The flies feeding on decaying matter (the detritivores), poo (the coprophages), dead bodies (the necrophages), blood (the sanguivores), and the brutish world of predators and parasites. McAlister recounts some amazing findings here. The adults and larvae of Mystacinobia zelandica that live in sticky guano and groom each other to stay clean (how sweet!), a behaviour barely seen in flies. Or the scuttle flies that manage to get into sealed coffins six feet under to munch on desiccated corpses.

But there are quite a few passages that made even the biologist in me cringe or cry out; the treatment of gangrene with live maggots who munch necrotic tissue (still an option today, though somehow not very popular). The scuttle flies who use blade-like ovipositors to pierce the back of ants to insert eggs. The hatched larvae then crawl into the ant’s head and slowly eat the contents while they walk around alive until their heads drop off. Death by a thousand cuts? How about exsanguination by 55,000 black fly bites? (The victim in question was a cow.) Or the bumblebee robber fly who have a hardened proboscis surrounding a long, slender tongue-like structure called the hypopharynx, which they use to hunt. By stabbing their prey. In the eye.

Despite being a little shop of horrors in place, McAlister’s goal is foremost to show you how flies, which we often ignore or just swat, are fascinating creatures in their own right. Their tiny dramas may be largely invisible to us, but they are no less intriguing for it, and no less deserving of our curiosity and attention. And in some ways, this is a little book of gratitude – we would be knee-deep in corpses and excrement if it was not for these industrious little insects.

McAlister opens the book admitting that this is not a book on the taxonomic arrangement of flies, which is forever in flux, nor on their structure or workings. Luckily, she is now making up for the latter with The Inside Out of Flies, to which I will turn next.