I found a dead hornet on my deck today. Was looking at it under a magnifying glass. It is covered in hair on all three body parts, even its legs. Why? Seems like the hair would just be a drag.
Considering it has an exoskeleton, maybe it is for “feeling” the air. So it can feel its own wind-speed and direction?
This doesn’t specifically mention hornets, but their uses for butterflies could apply: Micscape Microscopy and Microscope Magazine
The setae play an important role in helping the butterfly sense the relative position of its many body parts (e.g., where is the second segment of the thorax in relation to the third segment). This is especially important for flight, and there are several collections of specialized setae and nerves that help the adult sense wind, temperature, and the position of head, body, wings, legs, antennae, and other body parts.
I’m not an entomologist, but I think, with a hard exoskeleton, the “hairs” are what the hornet has to sense its environment with.
