Over the holidays I learned of the tiniest winged wasp, Megaphragma mymaripenne.
It is parasitic on the eggs of a mite that lives on other insects. As you can imagine, each of these is smaller than the predecessor.
That’s the wasp there, next to the single-celled Paramecium. You may have looked at these in pond water through a microscope.
Dylan Miller has more info in his post, A tiny wasp’s big brain survival strategy.
Their brains have 4600 neurons, but none of their cells contain DNA or a nucleus. The cells can’t reproduce and replace themselves, but the wasp only lives five days so that isn’t the primary function of the cell.
Look at it, it’s feathery wings and tucked legs!