Mosquitos
It's a beautiful summer night and you're outside grilling dinner and enjoying the breeze.
Ouch! You look down at your arm and see a painful, swelling mosquito bite. Moments later, you feel another one bite you.
Mosquitoes can sense carbon dioxide and lactic acid up to 100 feet (36 meters) away. We give off these gases as part of our normal breathing.
Certain chemicals in sweat also seem to attract mosquitoes.
On top of that they can sense heat alterations in the environment as well as color contrasts; if your outside, and they're near, you're not avoiding them.
Only females bite, she lands on your skin and sticks her proboscis into you (the proboscis is very sharp and thin, so you may not feel it going in).
Her saliva contains proteins (anticoagulant) that prevent your blood from clotting and then proceeds to suck your blood into her abdomen.