Question

Why do midges insert "poison" in people's bodies after they stitched them? Is there any evolutional-based reason for that?

Answer 1

the "poison" avoids blood to coagulate when mosquitos and smlilars stitch animals. a stitch is nothing less than a lesion in the wall of vessels. such "injures" start an activation cascade among blood proteins which lead to coagulation. the insect would get stucked in the injure and wouldn't be able to feed on blood. so, evolution gave these insects a natural remedy which they literally "spit" in the vessel to avoid coagulation. of course, this method helps the insect, which doesn't care about the effect this "poison" has on the host. the side effect on the host is itch! note that this mechanism is also exploited by a veeeeery small organism, the plasmodium which causes malaria in people. the plasmodium is clever enough to wait in the salivar glands of the anofele (the mosquito causing malaria). when the insect stitches a host, the plasmodium follows the "poison" into the bloodstream, thus infecting people. inside the body they reproduce in a very complicate way, and once they're ready, they re-enter the blood stream. the next insect which stitches that host will "pick it up" from the blood. at that point the plasmodium will find its way to the salivar glands, and there it will wait for the insect to stitch another host to re-start its life cycle