I think this is how it works: As a rain drop falls, a tiny little pressure wave is pushed ahaid of it and this pressure wave can push the little fly or gnat out of the way, missing the raindrop. I only know about this because years ago I asked why fly swatters were made up of a meshy material. Can you see how these two questions are related? 😛
I know I keep asking you annoying questions but they really just to help you learn and become th ebest scientist you can be!!
When it rains the drop of rain creates a tiny pressure wave ahead and below the raindrop. As the rain drop is falling with the force and wave pushes any light small object on its way sideways and the drop misses it. Same with the gnat, as the rain drops the wave and force pushes it sideways. Most times they do not have control under heavy rain.
Because of the difference in scale, we can regard a collision between a raindrop and a gnat as similar to that between a car moving at the same speed as the raindrop, speed does not scale, and a person having only one thousandth the usual density, for example, that of a thin rubber balloon of the same size and shape.
A balloon is easily bounced out of the way, and would burst only if it was crushed up against a wall.