Itchy scratches:

>Why do mosquito bites sting?
>
>From an evolutionary standpoint I would have thought that the mosquito has 
>nothing to gain and much to lose from the victim of a bite feeling pain 
>afterwards and thus trying to avoid mosquitos.

It's a perfectly reasonable question. Some blood-sucking animals such as
leeches do secrete an anasthestic and analgesic to suppress the pain reaction,
although even in these animals, the initial puncture is not painless. The
analgesic takes a moment to take effect. Mosquitos have not apparently evolved
a similar mechanism, but that may be simply that they do not cause everyone
pain, or that by the time the pain reaction has set in, the mosquito itself has
left. They do however secrete an anticoagulant to prevent the blood from
clotting, just as leeches and bloodsucking bats do..

The "sting" you perceive is more of an itch, and that itch is an
immunological
reaction -- and it varies greatly from person to person and by prior exposure
of any single individual. Further information can be found at:

     http://www.drgreene.com/21_60.html

Wirt Atmar
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