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Dick Kuhl, age 11, of Sioux City, Iowa, for his question:    

WHY DOES A MOSQUITO BITE ITCH?

A mosquito has no teeth. Even if she had, they would be useless because she cannot open her jaws. She is a born bloodsucker, and her so‑called bite is an elaborate device to extract a liquid meal. This part of the process is amazingly skillful and usually quite painless. The itchy bump that follows is merely a side effect of the operation.

The mosquito carries her knives and forks in a long slender beak. Her delicate little tool kit is enfolded in a bag of skin that rolls back as she gets ready to stab. This tool bag is actually an extension of her lower lip, called the labium. This kit includes a remarkable assortment of tools used to stab and saw, squirt and siphon.

Only the female mosquito dines on human blood, for the male's beak is too weak to pierce the skin. He must make do with juices from tender plant tissues. When you hear her buzzing around, she is homing in on your supply of nice warm liquid blood. She uses her feelers to locate a suitable spot and settles down very softly.

Then the tool bag rolls back as she stabs in her stylets. This is done so gently you don't feel a thing. She is all ready to siphon up her meal. However, this may take a few moments‑‑and when blood is taken outside the body it clots and becomes thick. The mosquito can sip it up only when it is thin and running. So she takes precautions.

When she stabs in her stylets, she lets a small sample of her saliva run down into the tissue. It contains a chemical substance that stops the blood from clotting. The mosquito can take her time while she sips up her liquid formula. When her meal is finished she flies away, usually before her victim notices her.

The trouble comes later, maybe several minutes or even an hour after the attack. The flesh around the stab wound swells up, stings and itches and may turn red. The itchy bump is caused not by the wound or by loss of blood, but by the anti‑blood‑clotting chemicals that were present in the mosquito's saliva.

There are about 2,500 mosquito species and only a few of them feast on human blood. However, some of these carry dreadful diseases, including malaria, yellow fever and encephalitis. These germs are injected with the saliva that also causes the itchy bump. Modern science strives to wipe out these dangerous types, though they still exist in some parts of the world, mostly in the torrid tropics.

 

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