There are wasps that are smaller than one-cell organisms!

When people think of wasps, they generally think of the several millimeter bugs that sting. A normal reaction to a wasp ranges anywhere from careful avoidance to full out screams and running. 

However, in the case of the third smallest insect alive, Alexey Polilov has devoted years not running from wasps, but studying them carefully. Of course, the wasps in Polilov’s study aren’t about to sting him. They are called Megaphragma mymaripenne. They are only 200 micrometers large. 

For a little perspective, a bug called a thirp is a millimeter long, so there eggs are considerably tinier. The M. mymaripenne’s eggs are so tiny they are often laid inside of the thirp’s eggs. It’s actually incredible that the wasp can function at its size. All of its organs have to be microscopic and some organs have to be left out for a bug to be that small. 

Its nervous system is particularly problematic because it can only be shrunk so small. Previously it was that the neurons in the system would have to have a nucleus to function, but in the small wasps only about 95% of the neurons have nuclei. This allows the system to be small enough for the wasps to be the third smallest insect! 

(Source)