What is a tiny black insect found in a woods that cause welting when they sting?

my husband sat down against a tree in the woods behind our house. He was suddenly overtaken by a swarm of tiny black insects that appeared to be stinging him. His whole chest, neck, and top of shoulders was covered in welts. six hours later the welts are still there and he says he feels like someone slapped him in the chest with the flat side of a board. He was fully clothed at the time of the attack. Could someone give us some idea as to what they were? They were NOT ants or bees.