These delicate, ephemeral little black “flies”, about 6-8mm long, are actually bark lice — Cerastipsocus venosus — with the appallingly banal common name of tree cattle. (They actually share this common name with a number of other species of bark lice.) Both adults and nymphs of C. venosus form crazy-large groups (like herds of cattle, hence the common name) on trees. When disturbed, the “herds” of insects will move en masse, like sheep or cattle, separating and then rejoining once the event has passed. The huge groups (up to several hundred individuals, depending on species) can look like an infestation which will have to be removed; in reality, bark lice eat lichen, fungi, dead bark, and other growths, and are harmless to the tree itself.
Despite the “louse” moniker, bark lice are not parasitic at all, and are considered beneficial insects to have in your backyard. They are sometimes called “maids of the forest”, which sounds fitting as a description of the tiny, black, iridescent-winged adults.
C. venosus is one of the larger species of bark lice. “Most males” have a white spot on their wings. The nymphs have ridiculous, spherical, black and yellow striped abdomens, and look like something out of Beetlejuice. In groups, they look like a series of interlinked hypnotic spirals, or, more alarmingly, a very small swarm of bees. I love them.