Flies

Article and images by Dr Julie O'Connor, senior conservation partnerships officer, Sunshine Coast Council

Some wildlife is so ubiquitous that we may be blind to it until it crosses our path directly. An example of this that comes to mind are the little Aussie flies, all 20,000+ species of them. Despite the large number of species, only one species of fly is actually attracted to humans, the bush fly Musca vetustissima, while its close relative, the house fly Musca domestica, is attracted to our dwellings.

Australia has always had a love hate relationship with flies. Well OK, mostly hate. Explorer Ernest Giles wrote in 1876:

“The flies at the camp today were, if possible, even more numerous than before. They infest the whole air; they seem to be circumambient; we can’t help eating, drinking, and breathing flies, they go down our throats in spite of our teeth, and we wear them all over our bodies, they creep up one’s clothes and die, and others go after them to see what they die of…”

Anyone who grew up in Australia’s southern states or out west, can probably relate to Giles’ lament. I remember walking to school in Victoria with my siblings with hundreds of flies hitching a ride on each white school shirt. So where did all these flies come from? And why so many?

Well, bush flies received a huge boost when cattle were introduced to Australia. When cattle arrived, depositing around 9 big moist pats per day, bush flies found themselves in the nearest thing to fly heaven. With no native dung beetles equipped to deal with the type of dung produced by cattle, the lucky flies had free reign over a mounting pile of delectable cow pats. The resulting density elevated them to an iconic status in the Australian landscape. The latest agricultural commodities census puts the cattle population of Australia at 22.3m, so why don’t we still have swarms of flies hitching a ride to school these days? In urban areas, the reduction can probably be partially due to insecticides and better waste management practices. But, at a national scale, the biggest impact on their numbers can probably be attributed to the introduction of imported dung beetles, started by the CSIRO in the 1960s.

Love them or hate them, flies are definitely fascinating creatures. Sure they have some anti-social habits, like vomiting on their food. But that little bubble of vomit you can see coming from the mouth of a common green bottle fly Lucilia sericata (image 1) actually contains a cocktail of digestive enzymes. Humans also possess a range of similar enzymes to facilitate digestion of the food we eat. The only difference is we send our food to our enzymes, and flies send their enzymes to the food. In terms of etiquette, that is possibly a vital difference you might be thinking.

Despite the nuisance that the bush fly used to be in Australia, its global impact pales next to the potential of the common house fly to wreak havoc. House flies have been considered by some, along with mosquitoes, to be one of the biggest threats to human health emanating from the animal kingdom. And as the name suggests, it has long been associated with human dwellings and will actively seek to enter your home.

And once in your home you can witness another amazing capacity of the fly, which is its ability to walk upside down on the ceiling. You may not have ever given that much thought because we've just always seen flies doing that, but it is actually a wonderful bit of leg design that enables this to happen. Flies have a pair of adhesive pads nestled between the claws at the tip of each leg. Each pad, shaped like a suction cup, is covered in hundreds of extremely fine hairs. The hairs receive an oily secretion from glands based in the pads, which help the fly to stick to any surface. So effective is this arrangement that the fly is able to stick upside down to smooth surfaces and still support double its own body weight.

In addition to their unusual eating habits and acrobatic abilities, flies possess fascinating vision systems also shared with many other insects. Have you ever had an annoying fly buzzing crazily around your bedroom while you are trying to read, only to have it instantly stop when you turn the light off? In addition to their large compound eyes (image 2), flies also have a light sensitive organ called the ocellus. In fact, they have three ocelli that do not see true images but respond strongly to changes of light intensity (image 3). When all three ocelli are exposed to bright light, the fly will fly and walk faster. If you cover its ocelli (you would need a very steady hand) or when darkness falls, the fly will not move at all.

Flies, of course, are not without their enemies. As well as being swatted by humans they are eaten by a range of predators, including birds, praying mantis (image 4), dragonflies and spiders (image 5), to name a few. If a fly is unlucky enough to get caught in a spider’s web, as you can see in this video, it is difficult to escape because even a small spider will move quickly to immobilise it.

While we may love to hate flies, the Diptera family actually play exceptionally important ecological roles on the planet. They excel as nutrient recyclers, pollinators of crops and native plants, pest controllers, soil conditioners, water quality indicators, and as a food source for a range of birds and beneficial invertebrates. And if all that doesn’t make you love them, then consider this - they are the sole pollinators of the cocoa tree Theobrama cacao, which is the plant used to make chocolate. Yes, that’s right. Flies and chocolate are intrinsically linked. No flies, no chocolate. I love flies.

References

• Larson, B. M. H., Kevan, P. G. & Inouye, D. W. 2001. Flies and flowers: The taxonomic diversity of anthrophiles and pollinators. Canadian Entomologist 133(4): 439-465
• McCalister, Erica, The Secret Life of Flies, CSIRO Publishing, Australia.