Trigonalidae

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Trigonalidae: a rare family of parasitic wasps

Trigonalidae are a small, worldwide group of parasitic wasps. They are the only living members of the superfamily Trigonaloidea and are split into two subfamilies: Orthogonalinae and Trigonalinae. Despite being rare, they are surprisingly diverse, with more than 90 species in 16 genera.

These wasps may be the sister group to all Aculeata (the stinging wasps, bees, and ants). They have a very unusual life cycle. Females lay thousands of tiny eggs and attach or inject them onto the edges of leaves. The eggs must be eaten by a caterpillar. Once inside the caterpillar, the trigonalid egg either hatches and attacks other parasitoid larvae inside the caterpillar (including its siblings) or waits until the caterpillar is eaten by a wasp larva (a vespid) and then attacks that larva. If no host eats the caterpillar, the trigonalid larva does not develop. So they act as parasitoids or hyperparasitoids, with the remarkable requirement that their eggs are swallowed by a host, and sometimes they may involve an intermediate host. A few species directly parasitize sawflies.

The name is sometimes spelled Trigonaloidae or Trigonalyidae, but those spellings are considered incorrect under naming rules. The fossil record for Trigonalidae is poor, and there are no confirmed members before the Cenozoic. The oldest knowns come from the Ypresian Eocene of the Okanagan Highlands in western North America. A possible mid-Cretaceous specimen from Burmese amber may exist, but it differs from modern Trigonalidae and may belong to the stem group. Related groups within Trigonaloidea include the extinct family Maimetshidae from the Cretaceous.