Length: Wasp in photo 0.85 mm to wing apex
Members of the family Platygastridae are truly minute wasps, black and shiny. They are notable for having almost no visible wing venation. The family is named for another important trait: Platygastridae means "flattened abdomen."
Those armed with a microscope can separate Platygastrids from similar wasps by counting the antennal segments; Platygasterid wasps have 9 or 10 antennomeres.
Larvae are parasites of Gall Midges (Cecidomyiidae), or in some cases white flies or mealybugs (Borror and White, 1970).
In North America about 200 species of Platygastrid wasps have been described; about half of these are in the genus Platygaster (Arnett, 2000). The photos on this page are from North America, but note that this family is also well-represented in the Neotropics (Muesebeck, 1951).
What was once called family Scelionidae is now considered subfamily Scelioninae of family Platygastridae. An example from this group:
American Insects site