A new species of Megarhyssa Ashmead (Hymenoptera: Ichneumonidae: Rhyssinae) from tropical Mexico with a key to Mexican species.

A new species of ichneumon wasp, Megarhyssa gratiosa sp. nov., is described from Chiapas province in the tropical part of Mexico. This is a southern most record of the genus in the New World. The new species is one of the largest and most spectacular ichneumonid species in the Mexican fauna. Megarhyssa gratiosa sp. nov. differs from its North American congeners primarily by the conspicuous colour pattern of the metasoma which is yellow with broad dorsal black stripe extending along its entire length, and the longer ovipositor. A key to three Mexican species of Megarhyssa is also provided.