Genus Megarhyssa Ashmead, 1900

Thalessa Holmgren, 1859: 122 .

Megalorhyssa Shulz, 1906: 115 .

Eurhyssa Derksen, 1941: 721.

Diagnosis (updated from Townes 1969)

Large to very large insects (fore wing length 10–30 mm); clypeus small, transversely rectangular, ventral margin laterally and sometimes medially bluntly produced; occipital and hypostomal carinae joining above mandible base; mandibular teeth subequal, lower tooth pointed and upper tooth more or less chisel shaped; propodeum of moderate length, without carina dorsally but pleural carina distinct; fore wing with areolet closed except in occasional dwarf males, receiving 2m–cu within its apical half, pterostigma about 5.5x longer than wide; hind wing with distal abscissa of Cu present, joining Cu&cu–a near or at junction with M; mid trochantellus with a ventral longitudinal ridge; tarsal claws simple, large; tergite 1 fused with its sternite and without glymma; tergites 3–6 almost smooth to finely and sparsely punctate and with isolated aciculate areas; female with sternites 2–4 each with a pair of tubercles near anterior margin, and with an apical truncate horn-like process on last tergite; male with metasoma strongly depressed, gonosquama lanceolate, strongly depressed, with a sharp piliferous groove along inner lower margin and a short subapical piliferous groove on outer face.

Species richness and distribution (Gauld 1984; Yu et al. 2012)

Megarhyssa is mostly a Holarctic and Oriental genus, with one species introduced into Australia and New Zealand for biocontrol purposes, and a single species reported from the Afrotropical Region.