Fairyflies are the smallest living insect. 

Fairyflies — which, despite their dainty name, are in fact a family of parasitic wasps — are the smallest known insects on Earth. The smallest fairyflies measure around 0.139 millimeters long, meaning their total body length is about the width of a human hair. The tiniest member of the family, Kikiki huna, is found in Hawaii and is so minute that scientists don’t even know whether it’s physically possible for an insect to get any smaller. 

Having such a tiny body comes with certain sacrifices: A fairyfly’s brain is almost devoid of nuclei, freeing up critical space inside its little head. And yet, it still has room for a functioning pair of compound eyes — the tiniest eyes of any insect, and some of the smallest eyes of any living creature known to science.