What are flies doing when the bite you?

Question

I thought they were sucking blood, but I don't really know.

Answer 1

The great majority of flies do not bite.  There are only a few species that do.  Deer flies, horse flies, black flies and a few species of biting midges.

http://edis.ifas.ufl.edu/IG081

Answer 2

I used to go swimming in the river and this would happen.  They are trying to get blood out of you.  

They are deer flies.

Answer 3

Most of then are trying to digest you.  Most flies can't bite.  They squirt out a digestive fluid (like an acid) then suck it back up.  But it usually doesn't work on people, so they get bored and fly away.

There ARE some the feed on blood, and they kinda poke you like a big mosquito would.  That HURTS, so you show them away really quick.

And there are some that are laying eggs.  They are not biting, but poking you with their "ovipositor" (egg laying device).  If the egg hatches you will have a little worm (maggot) living inside you.  These kinds of flies live almost entirely in tropical countries, near the equator.