Ask A Scientist© Biology Archive

Question:  Why do mosquito bites itch?
Burbank School

Answer:  I believe itching is understood to be a very small pain feeling, 
so then the question becomes why do mosquito bites hurt.  The simple answer 
is because the skin is damaged by the bite.  But I think it's important 
that it is damaged chemically more than physically, by a substance in the
mosquito's saliva that it injects to prevent the blood clotting while it
tanks up.  There are beasts that can suck your blood without the process
hurting, including leeches and vampire bats (I think).  Leeches are used in
surgery to drain excess blood for this reason.  You'd think that over time
mosquitoes would evolve painless anti-clotting spit, so they can drink in
peace without alarming you by the hurt and making you swat them.  But, on
the other time, perhaps *we* have evolved the pain as a warning sign that
we are being bitten, and nasty diseases (encephalitis, malaria) may be
being transmitted.
Christopher Grayce