My wife and I experienced two separate encounters with large quantities of the insect shown in the attached photo. Both times we were having lunch on a picnic table in Georgetown. The bug in question is 2-4 mm long and occurs in groups of 20 to 50 individuals. My wife's pants were covered with about 40 by the time we were done with our lunch. No biting or obnoxious behavior other than being a nuisance. What are they and are they harmful?