Our Dependent Guest: The Housefly

The Common Housefly
Photo from How Stuff Works

Spring is here! And with it comes cute baby bunnies and chicks, a wide array of beautiful flowers, and of course, houseflies.

They are not the most appealing animals to emerge this time of year, but they are just about everywhere. A single female housefly can lay up to 900 eggs in her lifetime [1]. And where to lay those eggs but in a delicious, moist pile of manure, giving her baby larva something to snack on when they hatch. Sound disgusting? Fascinating? You don't know the half of it yet.

Houseflies probably first evolved in the Africa, later hitching a ride with us to all the corners of the world [2]. In fact, houseflies are surprisingly dependent on us. They eat our food, use our animals' waste as a nursery and use our structures to take shelter so that they can make it through cold winters, ready to emerge when the weather warms up [3]. And emerge they do. Undoubtedly you will begin to see them land on your wall, ceilings, and food.

But the housefly has one big disadvantage when it comes to eating solid food: they have nothing to chew with. In fact it is physically impossible for a housefly to eat solid food. But they are able to get around this problem by liquefying our otherwise solid meals. With a straw-like mouth piece called a proboscis the housefly will drink up any liquid food that it can. When the food is to tough though, it will use tiny hairs at the end of its proboscis to scrape off some food particles. Then, the fly will vomit. It vomits digestive juices onto the food, dissolving them into a liquid and slurping it all back up [1]. Gross? Maybe, but it is exactly what we do, only we do it inside our bodies, chewing food into particles and dissolving them in our stomachs. Really we and the housefly have a very similar process, but our's takes place inside our bodies, and their's, out.

Sorry Mr. Larson, but you got this one wrong.
picture from pbase.com

But while we might use a new straw for every Slurpee, flies do not have that convenience when it comes to their proboscis. Inevitably some old food gets stuck in the tube, only to be vomited back out again, mixing with what was going to be your delicious dinner. Not all of it gets slurped back up again, leaving you with a glop of partially digested food and fly vomit. And thus, flies have gained a not-undeserved reputation as dirty, disease-spreading pests.

They are almost the perfect animal for spreading disease. They lay eggs in wet organic matter like dung, or compost or trash, picking up potentially deadly diseases like typhoid fever, salmonella, dysentery, even anthrax [4]. They then land on your food, or you, delivering diseases to your front door.

But simply carrying such diseases wouldn't be so bad if they weren't so hard to kill. After all we can't all be the Karate Kid. But what makes swatting a fly so difficult?

Flies are extremely well adapted to sensing motion of any kind, and are great at aerial maneuvers. The eyes of the housefly are extremely complex. If you have ever seen a picture of a "fly's eye view" with a thousand of the same picture seen in every widow, forget that. Seeing the same image over and over and over and over again makes very little evolutionary sense. In reality, each lens on a fly's eye acts like a one of ours. You can think of a fly having not two eyes, but up to 12,000 that are arranged in two semi-circle bundles [1].

A (probably) more accurate picture of a fly's view
Photo from flickr

This creates an image for the fly that severely lacks resolution, but is exceptional at sensing movement.

But the housefly doesn't stop with the eyes. It's body is covered with tiny hairs, known as tarsi, that are extremely sensitive to air movement. They can feel the air being pushed by that flyswatter coming at them, and they can get out of the way fast [1].

And once in the air they have a myriad of tricks they can perform. Tight turns, zigzags and loop-de-loops all help a fly get out of the way in a fast and unpredictable manner. These tricks are due to two extra wings that the fly has just below it's large main ones, and the ability to process information at astounding speeds. While its main wings beat furiously (up to 20,000 times per minute) [5]  two small wings, called haltere, are stabilizing the fly in mid air and the fly is taking in, processing information, and reacting to it in a few hundred milliseconds. They react in literally the blink of an eye [6].

And they need those reaction times. Birds, lizards, spiders, insects and swatters are a constant threat. So before you pick up that bug spray, just remember that these amazing animals live in world that is dangerous enough as it is. Just, maybe don't let them land on your food.