Fairy Wasps: these wasps are the world's smallest insects, and some species can measure as little as 0.13mm long, which is roughly the size of a single human ovum
Mymaridae is a diverse family of wasps that contains more than 1,400 known species, and these wasps are often described as fairy wasps (or "fairy flies") because they are so incredibly tiny. In fact, these are some of the smallest multicellular animals ever described, with most fairy wasps measuring less than 1mm long.
Above: Lymaenon aureus, a species of fairy wasp, depicted on top of a lime
One species of fairy wasp, Dicopomorpha echmepterygis, is the smallest insect known to science. The adult males of that species can measure as little as 0.139mm (139 microns) long, which is roughly the size of a single human egg.
Above: Anagrus fairy wasp
As this article explains:
Fairyflies are not actual flies at all, but chalcid wasps; they get their name because of their fairy-like wings, and for being tiny and "invisible" like the mythical fairies. Fairyflies have the smallest known adult form of any insect: wingless and blind males of the species Dicopomorpha echmepterygis have a body length of just 0.139 mm, about the average width of a human hair. The fairyfly family Mymaridae consists of many species, including Tinkerbella nana and Kikiki huna, the smallest known flying insect species with a body length of 0.16 mm.
Above: the photo at the top shows a fairy wasp of the genus Anagrus, while the photo on the bottom shows another species from genus Polynema
Most fairy wasps have fully-functional wings that are lined with long, feathery bristles:
Fairyfly wings are not only small, but their structure is comb-like and made of several bristles instead of a planar sheet. Their very small size means that the air around them is more viscous than it would be for larger insects. At such small scale, the ratio of inertia to viscosity, the Reynolds number, is so low that any movement provided to an air packet quickly dampens out. It also means that a single wing bristle drags along with it a boundary layer of a comparable size. Thus, if the bristles are sufficiently close, the comb-like wing can act as a paddle to generate the requisite aerodynamic forces, while reducing the overall mass of the wing.
Above: a fairy wasp depicted beneath a push-pin
All of the wasps in this family are parasitoids, and they lay their own eggs within the eggs of other insects. Their hosts include aphids, scale insects, booklice, thrips, water beetles, and other tiny arthropods; the female fairy wasp inserts her ovipositor into the host's egg and then deposits her own offspring within.
Above: this composite image shows male and female fairy wasps of the genus Polynema standing on a pinhead
This seems like a good time to point out that fairy wasps do not parasitize mammalian ova, nor do they parasitize the eggs of any other vertebrates, and they are completely harmless to humans. In fact, they're actually beneficial, since they often parasitize agricultural pests.
Above: a fairy wasp trapped in a tiny guttation drop produced by a fungus
I posted some information about fairy bees a few weeks ago, so I figured that I should share these little fairy wasps, too.
Above: a Mymar fairy wasp with an aphid standing nearby
Current Biology: Fairyflies
Annual Review of Entomology: Small is Beautiful: Features of the Smallest Insects and Limits to Miniaturization (PDF)
Journal of Hymenoptera Research: A New Genus and Species of Fairyfly, Tinkerbella nana, with Comments on its Sister Genus Kikiki, and Discussion on Small Size Limits in Arthropods (PDF)
