While the male mosquito is content to live on juices he gets from plants, the female is not.
She gets her nourishment from the blood of people and animals.
The female mosquito has a sharp beak which she uses to prick human or animal skin. Then she pokes a hollow tube, her mouth, into the hole and sucks out one or two drops of blood.
Ordinarily, when the skin is broken, as with a pin prick, the blood begins to clot.
But a liquid in the mosquito’s mouth prevents that clotting. This liquid, which most people and animals are allergic to, is left under the skin, causing the skin to swell and itch.