The Trigonalidae (sometimes spelled Trigonalyidae but the former spelling has formal precedence; see the Trigonalidae Website) are one of the more unusual families of hymenopteraninsects, of indeterminate affinity within the suborder Apocrita (though sometimes believed to be related to the Evanioidea), and presently placed in its own superfamily, Trigonaloidea. The Trigonalidae are divided into two subfamilies, Orthogonalinae and Trigonalinae. These wasps are extremely rare, but surprisingly diverse, with some 90 species in over 15 genera, and are known from all parts of the world.
What little is known about the biology of these insects indicates a remarkably improbable life history: in nearly all known species, females lay thousands of minute eggs, "clamping" them to the edges or injecting them inside leaves. The egg must then be consumed by a caterpillar. Once inside the caterpillar, the trigonalid egg either hatches and attacks any 'other' parasitoid larvae (including its siblings) in the caterpillar, or it waits until the caterpillar is killed and fed to a vespid larva, which it then attacks. Therefore, they are parasitoids or hyperparasitoids, but in a manner virtually unique among the insects, in that the eggs must be swallowed by a host. A few species are known to directly parasitise sawflies, however.