Flies are not always the pests we imagine them to be. In fact, some of them are beneficial to the ecosystem. Abré J Steyn takes a closer look at these underdogs of the insect world.
As we pulled into the patch OF miombo woodland Simon, my long-time right-hand-man, went berserk in the back of the Land Cruiser. He jumped around and hit the roof and side-panels with a tekkie he’d ripped off his foot. A second later, my wife yelled out and started the same bizarre behaviour and then “it” hit me too – in the back of my neck.
Although it was only mid-September, it was already searing hot. All the windows of the Cruiser were wide open, even the drop-down windshield installed for driving in the bush. We’d been navigating slowly through open woodland over a potholed track towards the great Bosanga plain in the top end of Zambia’s Kafue National Park, and we’d been craving a bit of respite from the merciless sun that bore down on us. At last we’d reached that patch of dense miombo and pulled into the dark, cool shade only to be attacked through the open windows by hordes of hungry tsetse flies. Like Kamikaze pilots, they flew straight into us with needle-sharp, stabbing proboscises that pierced even denim with ease.
You can forget about killing them with a fly-swatter – they’re as tough as the buffalo whose blood they drink. You have to catch them and crush them between your fingernails. Their bites burn like red-hot coals but had no further effect on me, while causing swollen weals wherever they bit my wife. Although millions have been spent and enormous environmental damage done in efforts to eradicate them, they are, in a sense, the guardians of the wild and the animals of Africa.
By transmitting the infectious disease nagana, they effectively keep cattle out of game country and in some parts they also cause sleeping-sickness in humans.
But they aren’t the only flies that make a nuisance of themselves. Apart from the well-known house, stable and fruit flies, many people don’t know the most notorious of all insects, one of man’s last unconquered natural enemies, the mosquito, is also a fly. Like all flies, they belong to the order Diptera (di=two, ptera=wings), because unlike other insects, they have only one pair of well-developed wings. In this case less could be more, as flies are masters of flight and there is probably no creature on earth that can fly as well as that honeybee look-alike – the hover fly.
They do it so well that “fly” became their name and it’s the only thing which is called a fly when it actually sits.
Not all flies can fly, though. Many louse flies, especially those that parasitise mammals like cattle, antelope and bats, look more like small flat crabs and have lost all vestiges of wings. Those living on birds like pigeons can, however, fly well.
The most common and typical fly is the cosmopolitan housefly species Musca domestica and the 150 blowfly species, or “greenbottles”, the maggots of which develop under moist sheep wool causing large, sometimes fatal ulcers.
Houseflies spread many serious diseases such as poliomyelitis, salmonellosis, typhoid and leprosy and, apart from the contaminated water in our rivers, flies that walk and feed on infected human excrement and later settle on utensils and foodstuffs are superspreaders of cholera. Polluted rivers and flies are a recipe for a cholera epidemic.
There is an enormous variety of flies in South Africa. No fewer than 18 000 species are recognised and, although quite a few are harmful, many are highly beneficial. Apart from the bad ones already mentioned, there are a few really nasty ones that live on blood, such as the minute biting midges whose larvae develop in water bodies with high organic content like sewerage.
Some species are the vectors of “blue tongue” disease in sheep and African horse sickness, as well as filarial worms and viruses in humans.
Black flies, of which some species along the Orange River emerge in clouds of billions that often send humans and livestock running for cover, are more than a nuisance in tropical Africa. Their nematode worms transmit cause elephantiasis and river blindness in humans.
Most of our more than 200 species of horsefly with their huge, sometimes beautiful eyes are voracious bloodsuckers that can, like tsetses, transmit nagana to cattle and the parasitic loa loa worm to the eyes of humans and monkeys. They prey on anything from frogs to large mammals. Huge black and yellowish hippo flies can drive sunbathing hippos back into the water. Some needle-nosed horseflies with their elongated proboscises can even suck blood from their hosts while hovering.
But not all flies are harmful – the overwhelming majority are entirely harmless or even beneficial to us, including some that we loathe.
Blowflies are the most important scavengers in the world. By consuming corpses and other waste products, they prevent the spread of life-threatening diseases, including anthrax. Blowfly maggots are used in medicine to heal badly infected wounds because they consume dead flesh and bacteria. Forensic scientists use the successive populations of blowflies in a corpse to determine time of death in homicide investigations. The science of genetics would still be science fiction if not for Drosophila – the vinegar fly on which most DNA experiments were done. Besides honeybees, flies are the most important pollinators on the planet. Without them, many plant species would go extinct.
An enormous number of flies are also predators and parasites of other insects, of which many are pests. The most well-known are robber flies, which capture other insects in flight and whose larvae, together with those of many bee flies, parasitise the egg capsules of locusts. Whether we like it or not, life is a package deal where the bad comes with the good, and so it is with flies.
Contact Abré J Steyn on 083 235 4822
or e-mail [email protected]. |fw
