Teeeeny tiny parasitoid wasp! The fly above it is about 1.5mm long to give you an idea of the size of this thing. Pencil to scale in the second photo (fly still above the wasp).
Parasitoid wasps lay their eggs directly into their hosts, which is often a caterpillar. The eggs hatch and the larvae feed on the host while it’s still alive. The larvae pupate, and end up killing the host when they emerge as adults.
These little guys are so efficient at finding their hosts that they’re often used as biological control agents in agricultural settings.