When I first saw these crawling on some cardboard I have in the bin I thought what the hell! After looking at the pick it appears aphids mating? Not sure
If they are soldier flies you want them. Their maggots are great in your worm bin. I freaked out at first when I found a ton of them crawling in my bin but after some research I learned they are good. They mate, lay eggs and then starve to death.
I was getting gnats from my bin so ended up putting a piece of fine silk screen mesh over the top of the worm bin secured by a bungee cord. Now they fly around in there trying to find the way out until they give up, die and, their carcasses soon become tiny morsels of food for worms/fungi. The circle of life...