We have always known about Trichogramma Wasps because they lay their eggs inside our butterfly eggs.
They are so tiny (less than 1mm), you need a microscope to see them! To protect our eggs, we collect them every hour and half, so the wasps cannot parasitize them. Recently at our microscope we have had some hatch out of the eggs on display.
The trichogrammatid wasps are parasites of the eggs of other arthropods and therefore all are very small, mostly less than 1 mm in size. Many Trichogramma species have a wide host range and will attack the eggs of many species and even many families of insect hosts.
Common hosts are the eggs of moths and butterflies, beetles, flies, wasps, and true bugs. Several species are aquatic, attacking the eggs of aquatic insects such as diving beetles and dragonflies. Some aquatic trichogrammatids use their wings as oars, to swim through the water in search of host eggs.
Information courtesy of http://www.entomology.wisc.edu/mbcn/fea607.html