Re: what causes the blood sucked by the mosquito not clot?

There are a number of  reasons, but the major reason is that the mosquito,
like other blood feeding animals from lice to vampire bats, produces a
powerful anti-coagulant that is injected into the bite site and which
prevents clotting. If you think about it, the mosquito absolutely does not
want the blood to clot whilst it's feeding; so a powerful blood clot
preventing substance is pretty much necessary for mosquitoes to blood feed.
Furthermore, there are potent proteases in the mosquito gut and bacteria
(that produce their own substances) that help in breaking down the proteins
in the blood meal.