Eupelmidae is a diverse family containing more than 900 species that can be found around the world, with 119 of those species occurring in Canada and the United States. Members of this group have diverse life histories and currently are divided into 3 subfamilies: Calostinae, Eupelminae, and Neanastatinae. While most Eupelmidae are tiny, the largest members of the genus Metapelma can be up to 6 mm in body length (figure F below). Species vary in color from dull brown to brightly colored and even metallic.
Most are internal or external parasites of the immature stages of moths, butterflies, true bugs, beetles, lacewings, grasshoppers, cockroaches, gall midges, and other wasps. Some are egg parasites on eggs of spiders with the egg case; as they attack multiple eggs, they can be thought of as predators rather than parasites (figure E below). Other species are hyperparasites, parasites that attack a host that is also a parasite.
Members of the subfamily Eupelminae have a unique jumping mechanism which is powered by the contraction of the large acropleural muscles. These muscles stretch energy storing blocks made of resilin. When triggered, the energy is released distorting the thorax, forcing the head and abdomen to bend over it causing the midlegs to snap backward propelling it into the air. These modifications will often cause Eupelminae to die in a bent shape with the head and abdomen held upwards (figure A and E above).
Species of Eupelmidae have been used as biological control agents of agricultural and forest pests. For example, Anastatus disparis is an egg parasitoid used to control the gypsy moth (Lymantria dispar), one of the most destructive insects in the eastern United States. It can defoliate over one million acres of forest a year causing millions in damage.