The smallest known adult insect is a parasitic wasp, Dicopomorpha echmepterygis.  These tiny wasps are often called fairyflies.  Males are wingless, blind and measure only 0.005 inches (0.127 mm) long.   As larvae, they feed inside the eggs of other insects.   Their body is smaller than a single-celled paramecium.

                                                                                           Photo credit: John Huber