It’s summer. Which means swatting away pesky flies, swiping at buzzing mosquitoes and scratching the red welts from the ones you didn’t see.
An evening’s relaxation is often punctuated with “Why, why do flies and mozzies love me?” Professor Mark Elgar has a simple explanation.
“Flies taste things with their feet and are attracted to human sweat,“ says Professor Elgar, from the University of Melbourne’s School of BioSciences. “They feed on the secretions that contain protein, carbohydrates, salts and sugars, dead skin cells and the bacterial growth that blooms in our sweat.
“Otherwise flies will seek out rotting items to eat the bacteria.”
Professor Elgar says the bush fly, rather than the house fly, is more readily attracted to humans. It often rides around on our back as it’s one of the sweatiest parts of the body.
But why do flies appear in summer and where do they lurk at night? Professor Elgar explains: “Adult flies increase in number in summer when warmer temperatures speed up the lifecycle, meaning 100 to 200 eggs may hatch within a few hours of being laid and the larvae develop quickly.
“The reality is that an increase in insects in spring is no more surprising as the profusion of spring flowers. There are fewer flies in winter because the cooler temperatures prevent the eggs from developing.”’
If you were able to stalk a fly for some of its two-week summer lifespan, you would find it active during the day and resting at night, in room corners, ceiling fixtures and even cellars.
Professor Elgar says flies, like other insects, are important for ecological processes — they are composters and even pollinators, as well as lunch to birds and lizards.
The other uninvited BBQ guest is the mosquito, sparking dark thoughts about powerful cordless vacuums or flamethrowers.
So why do mosquitoes like me more than others? Professor Elgar explains: “To mosquitoes, humans are essentially plumes of CO2 , lactic acid from perspiration and perfumes. Mosquitoes use these chemical odours to find food and mates and their elegant antenna have receptors for these odours.
“The biting mosquitoes are the females. They must secure a blood meal in order to nourish the development of their eggs. They also use their antennae as thermal sensors to locate blood near the surface of the skin.”
So individual variation in odours explains why some humans are more enticing than others to mosquitoes. Professor Elgar also suggests our plumes of CO2 and odours is best detected at dawn or dusk, when there is drop in temperature and the wind is generally slighter.
“High humidity vastly improves conditions for blood hunting too. Depending on conditions mosquitoes can detect humans from 10 metres,” he says.
“Mosquitoes don’t venture out in hot dry conditions as they can desiccate in direct sunlight. So during the day they are found underneath foliage and plants and near standing water.
“In winter, the larvae remain dormant in pools of water ready to spring into action next summer.”
Banner image: Asian tiger mosquito Aedes albopictus obtaining a blood meal from a human under experimental conditions. Picture via Creative commons