Insect of the week: 7 August 2023

Megaprosternum sp. nov. (Hymenoptera: Bethylidae: Scleroderminae)

If Baeus sp. (our first insect of the week) is the living embodiment of roundness, then Megaprosternum is flatness itself. A rare genus with only 3 known species, Megaprosternum nonetheless has a very wide distribution. Single species are known from Australia and Fiji, and Megaprosternum cleonarovorum was described from Bangalore, India in 2016. More recently, this species was found in the Seychelles, on Cousine Island. This was the first record of the genus from the Afrotropical region, but until now there have been no records of Megaprosternum from continental Africa. We collected an undescribed species of Megaprosternum from Kasigau mountain, near the Taita Hills in southern Kenya. Like the Taita Hills, Kasigau is one of the ancient Eastern Arc Mountains. The Eastern Arc Mountains are famous for their great number of endemic species, both plants and animals, so it was no surprise that our sampling in Kasigau produced yet another endemic.

There is evidence that Megaprosternum species are gregarious parasitoids, with single adult females laying many eggs on a much larger host. The Indian species, M. cleonarovorum, was found in tunnels excavated by the long-horned beetle Cleonaria bicolor Thomson. The parasitoid larvae were feeding as ectoparasitoids on larvae of the beetle.

Regarding the collection of Megasternum cleonarovorum from the Seychelles, it is likely that the species was introduced accidentally, probably from India. The Seychelles broke off from India in the late Cretaceous, about 85 million years ago, and in the time since then it is highly unlikely that either, or both, of the widely separated populations of this species should not have evolved into separate species.


Credits: Dr Robert Copeland