Mosquitoes are annoying, but that's partly because they are so amazing. Mosquitoes are like little robots. These robots are great at finding warm-blooded people. A mosquito's head is loaded with a system of "people sensors." It starts with chemical sensors. With their antenna, mosquitoes can sense carbon dioxide and lactic acid up to 100 feet away. They can also detect chemicals in sweat. Since people and animals give off lots of carbon dioxide and these other chemicals, it makes it easy for mosquitoes to find us.

The parts of a mosquito

Mosquitoes also have good eyes. When you move, mosquitoes zero in on you -- especially if you are wearing bright, loud colors. Since much of what moves is animals, this is a good strategy. And mosquitoes can sense heat too. Using the chemical sensors and the movement sensors to get close, they can then zero in on the heat of your body. They find a nice warm place to land, say on your arm, and then the fun begins.

Female mosquitoes are the only ones that bite. They do this because they use blood as a source of protein. Without a protein snack, a female mosquito cannot lay eggs. So when the mosquito lands, her first thing to do is to stick her long, thin proboscis into your skin like a needle. She injects a fluid that keeps your blood from coagulating (thickening). Then she sucks up a drop of blood -- about five microliters -- and stores it in her abdomen. Once she is full, she flies away to lay her eggs.

Photo courtesy Centers for Disease Control & Prevention, photographer Jim Gathany A female mosquito (Anopheles gambiae), feeding

People probably wouldn't mind this so much - it's not a lot of blood -- but there are two problems with mosquitoes. First, the fluid the mosquito injects does not belong in your body. Your immune system makes the area swell so that it can clean out the fluid. The whole clean up process is what makes your skin itch after the bite.

The second problem is diseases. When the mosquito injects this fluid, germs and parasites can enter your body. You could get malaria, yellow fever, dengue fever or West Nile encephalitis. And each of these can be fatal!

Photo courtesy Centers for Disease Control & Prevention, photographer Jim Gathany This female Anopheles gambiae mosquito is feeding. You can see the blood swelling her abdomen.

Since mosquitoes cause all these problems, most people would like to keep them away. That's where mosquito repellants come in. The best repellant that scientists have found so far is called DEET, also called NN-diethyl-meta-toluamide. Scientists think that DEET blocks the mosquito's sensors for carbon dioxide and lactic acid, making it much harder for a mosquito to find you. Apparently, the carbon dioxide also starts a chain of events in the mosquito's little brain that starts the biting sequence. So without carbon dioxide, the mosquito doesn't know to bite you.

You can find spray-on repellents at most stores. These sprays often contain DEET.

Now you can see that mosquitoes are amazing little bugs. But because of all that itching and the threat of serious diseases, it's good to avoid being a mosquito's protein source. So don't forget the bug repellant on your next picnic!