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Giving new meaning to the term ‘buzzy’. (Image: Getty/Tina Tiller)

ScienceJanuary 31, 2024

Flies might be annoying, but they’re just doing their best

an orange background wwith a black housefly surrounded by blue question marks
Giving new meaning to the term ‘buzzy’. (Image: Getty/Tina Tiller)

These under-appreciated insects bring all sorts of benefits – they’re the reason we’re not surrounded by rank carcasses, for one.

In summertime, Julia Kasper is asked a lot about flies. As Te Papa’s lead invertebrate curator, she specialises in the order Diptera, which includes all two-winged insects, including flies. Most of these insects breed in summertime, when warmth and humidity makes it easier for them to reproduce.

“People associate flies with filth and disease,” Kasper says. There is a reason for this: because of how easily they can move around, flies are mechanical vectors for, well, potentially gross stuff. “We don’t like them to visit our houses after they’ve been on dog poop or whatever,” she says. There’s just no telling where a fly has been, and it’s easy to assume the worst. 

Some members of the Diptera order are a real nuisance, and at times a threat to humans, as anyone who has ever been kept awake by the whine of a fly can attest. Malaria and other mosquito-borne diseases have killed nearly half of everyone who has ever lived. Finding maggots in your food can be concerning, as can the idea of bacteria travelling onto your food from flies landing in the kitchen. That’s all the more reason to use mosquito repellent, keep surfaces clean, food and bins covered, and to trap flies if you are finding lots of them. 

the ouline of a weta in red but no weta is there it is just empty
Insects are among the most threatened native species (Image: Tina Tiller)

But before you reach for the fly spray and scorch the earth (and the kitchen counter) where a fly might have walked, Kasper wants you to stop and think. To her, fly species are an endless source of curiosity and amazement – and, she points out, humans are dependent on flies in many ways too. 

“Fly larvae are decomposers, in water and on land,” she says. “Everyone hates sandflies, but without them our beautiful streams would look horrible.” Houseflies and blowflies are associated with our rubbish, especially meat, but without them carcasses from different species would pile up. Then there are the specialised flies that pollinate food, including a midge that allows cocoa beans to form. Flies are also a crucial part of the food chain; many of Aotearoa’s iconic birds would go hungry if they didn’t have flying insects to feast on. 

While Kasper is able to rattle off a long list of the benefits of flies, she’s also frustrated that she has to do so in the first place. “These benefits are just from the perspective of humans,” she says. “In nature, there aren’t benefits; it’s about balance, and every organism has a right to be there.” 

Maybe part of the reason that flies are so easy to despise is that they are so different from humans, including in how they sense and reproduce. Like humans, they detect scents by identifying molecules that enter their bodies. Unlike humans, those molecules reach their brain through tiny holes in their antennae. Despite operating in miniature, they are much more sensitive to smells than humans. “They orient mostly by smell, which they can detect even in low concentrations from far away.” Many flies also have compound eyes, and their brains can change their visual sensitivity depending on what kind of environment they’re in. These sense organs also remain mysterious: scientists know that mosquitoes can detect carbon dioxide and the oily sebum on our skin but it’s still not totally clear how these factors combine to make some people more delicious to mosquitoes than others. 

a whole lot of insects surrounded by hearts
Flies are among other unloved insect species (Image: Tina Tiller)

The fly species that humans most dislike – blowflies, mosquitoes – are widespread mostly thanks to humans. We’re the ones who throw away thousands of tonnes of food waste each year, offering plentiful nutrients to enterprising scavengers, while covering water-absorbing soil with tarmac and concrete. “As we clear habitat and urbanise, we will have more of those species that adapt to those circumstances. We create habitat that works for introduced species,” Kasper says. “The [introduced] mosquito is cosmopolitan – it can live in any old dirty water, so it is increasing and it is biting us, while the native ones in the bush might be in decline.”

New Zealand has at least 3,000 species of native flies, although Kasper says there are probably many more that haven’t been identified yet; there’s much less funding and research about insects than there is for, say, birds. “We have a high rate of species that aren’t found anywhere else,” Kasper says. But the coolest native flies aren’t hanging out in your kitchen. The batfly, which has evolved to clean pollen off the fur of short-tailed bats, is a scientific celebrity; glow worms, the luminescent larvae of a species of gnat (and contenders for 2024 Bug of the Year) light up caves around the country; native mosquitoes get their blood from birds and can live their whole lives without getting near a human. There are even native blowflies, completely indistinguishable from the introduced ones if you don’t have a microscope. “Flies are so amazing,” Kasper says. “There’s still so much we don’t know about them.” 

There are frequent concerns about the so-called “insect apocalypse” – evidence that points to plummeting numbers of insects worldwide, which would have devastating effects on human food systems and other animals. “It’s not that we’ll necessarily have less biomass,” Kasper explains. “But there will be more homogenous biodiversity worldwide.” Fewer wingless, pollen-munching flies; more of the same maggots and houseflies, wherever you go. While it’s known that some native insects are threatened, none are protected – except the batfly, by proxy, because of the protection extended to the short-tailed bats. “They are so unloved, there is still so much we need to learn,” Kasper says sadly. 

It’s not entirely clear how much a warming climate will affect sensitive native flies, but there could be issues with, for example, species that have adapted to pollinate particular flowers going hungry because the plants are flowering earlier. “What we can see is the increase of insects adapted to a warmer climate coming over the border.” If this is the case, it’s likely that Kasper will have to answer a lot more questions about mosquitoes and blowflies in the future. “When we attack their habitat, we only invite something that is more adaptable and therefore not so healthy for us to take their place,” she says. 

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