All known fairyflies are parasitoids of the eggs of other insects, and several species have been successfully utilized as biological pest control agents.

Haliday and two close friends, John Curtis and Francis Walker, respected entomologists in their own right, were influential in the early studies of Hymenoptera in the 19th century.

[5] Earlier attempts of classification by Walker treated the group as a genus, and classified all other known fairyflies under it as subgenera.

[7][8][9] Haliday described fairyflies as "the very atoms of the order Hymenoptera" and remarked on the beauty of their wings when viewed under the microscope.

The beautifully mounted fairyfly specimens of the early 20th century English microscopist Fred Enock are possibly the most famous of the collections.

They are the most commonly encountered fairyflies, followed by Alaptus, Camptoptera, Erythmelus, Ooctonus, and Stethynium, which make up a further quarter of known species.

Peck et al. (1964) proposed the subfamilies Gonatocerinae and Mymarinae based on the number of segments (tarsomeres) in the tarsi.

[18] They can be distinguished from other chalcidoids by having an H-shaped pattern of sutures, known as trabeculae or carinae, below the frontmost ocelli and the inner eye margins.

These are small bristles (setae) which point distinctly backwards on the ventral surface of the wing membrane.

The fully developed (macropterous) wings of fairyflies are normally flat with rounded tips and are several times longer than they are wide.

[20] Forewing curvature, such that it is distinctly convex or dome-shaped, is also exhibited by at least one species of the genera Cremnomymar, Mymarilla, Parapolynema, and Richteria.

Most of these species inhabit particularly harsh and wind-swept environments, and the curvature may help in absorbing and retaining heat or prevent the fairyflies from being blown away.

In rare instances, the hindwings may also exhibit curvature, with a convex or concave anterior and posterior margins.

[18][20][21] Wing reduction or absence is usually exhibited by at least one sex (usually the female) of species that search for host eggs in confined areas (like leaf litter, soil, or the tubules of bracket fungi).

[14][28] Fairyflies include the smallest known insect, Dicopomorpha echmepterygis from Illinois, whose males are only 0.139 mm (0.0055 in) long.

[29][30] Four males, lined up end-to-end, would just about encompass the width of a period at the end of a typical printed sentence.

[16] In Prestwichia aquatica, mating has been reported to occur prior to the emergence of females from the host eggs.

[34] After emerging, females search rapidly for suitable host eggs by tapping their antennae over stems or barks of plants.

When a telltale scar left by egg-laying insects is found, a female will insert her antennae into the recess and check to see if the eggs are suitable.

[14] They are peculiar for insects which exhibit complete metamorphosis (holometabolism) in that they produce two distinct kinds of larval instars before pupation.

Though not pests, D. cruentata proved to be sufficient hosts for A. epos in winter, allowing them to survive into the next year.

[45] In South Africa, Anaphes nitens was introduced very successfully to control an undescribed species of eucalyptus snout beetle (Gonipterus sp.).

[16] In Australia and Israel, members of the genus Stethynium, were being investigated in 2006 as possible biological control agents for gall-forming crop pests such as Ophelimus maskelli.

And in 2011, John T. Huber and Dale Greenwalt described fairyfly fossils from the oil shales of the Kishenehn Formation (Lutetian age) of Montana.

Dating back to the Lower Cenomanian age (about 100 mya) of the Late Cretaceous, it is the oldest known fairyfly (and chalcidoid).

They are surprisingly very similar to modern genera, though with a greater number of flagellar segments and longer forewing veins.

[20] Shillingsworthia is also excluded, as it was a tongue-in-cheek hypothetical concept of a species from the planet Jupiter, "described" by Alexandre Arsène Girault in 1920 to disparage his colleague Johann Francis Illingworth.

These fossil genera are classified under Mymaridae:[8][46] Despite their relative abundance, fairyflies are unpopular among modern insect collectors because of the great difficulty in collecting them.

With this method, it is possible to observe the life history and determine the hosts of particular species of discovered fairyflies.

Mounting specimens (preferably in permanent slides) is also time-consuming and requires a fair amount of practice.