Promyrmister kistneri: A new species of myrmecophile Clown Beetle from Cretaceous Burmese Amber.

Clown (of Hister) Beetles, Histeridae, are a diverse group of carnivorous Beetles found across the globe. They are easy to distinguish, the elytra (wing cases) that do not cover the ends of their abdomens and club-like tips to their antennae. They occupy a wide range of ecological niches, with most species feeding on other Arthropods and small invertebrates, though other specialise in the carrion of larger animals (and in some cases are used by forensic entomologists to help determine how long bodies have been dead). The group contains many myrmecophile species; Beetles that have evolved the ability to live within the colonies of Ants, largely by using chemical and behavioural mimicry to fool the Ants generally highly aggressive defence responses. This trait appears to have evolved several times within the Histeridae, with one subfamily, the Haeteriinae, being entirely made up of myrmecophile Beetles  highly specialised for this lifestyle.

In a paper published in the journal eLife on 16 April 2019, Yu-Lingzi Zhou of the Key Laboratory of Zoological Systematics and Evolution at the Institute of Zoology of the Chinese Academy of Sciences, and the Australian National Insect Collection, Adam Ślipiński, also of the Australian National Insect Collection, Dong Ren of the College of Life Sciences at Capital Normal University, and Joseph Parker of the Division of Biology and Biological Engineering at the California Institute of Technology, describe a new species of myrmecophile Clown Beetle from Cretaceous Burmese Amber.

Cretaceous ‘Burmese Amber’ has been extensively worked at several sites across northern Myanmar (though mostly in Kachin State) in the last 20 years. The amber is fairly clear, and often found in large chunks, providing an exceptional window into the Middle Cretaceous Insect fauna. This amber is thought to have started out as the resin of a Coniferous Tree, possibly a Cypress or an Araucaria, growing in a moist tropical forest. This amber has been dated to between 105 and 95 million years old, based upon pollen inclusions, and to about 98.8 million years by uranium/lead dating of ash inclusions in the amber. 

The oldest fossil Ants appear in this amber, and the roughly contemporary Charentese Amber of France, though they are all considered to be 'stem group' Ants (members of the Ant lineage, more closely related to living Ants than to other Hymenopterans, but not descended from the most recent common ancestor of all living Ants) rather than 'crown group' Ants (Ants which are descended from the most recent common ancestor of all living Ants).

The new species is named Promyrmister kistneri, where 'Promyrmister' means 'early-Ant-Clown Beetle', and 'kistneri' honiurs David Kistner, for his expertese on Social Insect symbionts. It is described from a single specimen, which is 3.2 mm long and 2.3 mm wide, roughly oval in shape, black or dark brown in colour, and has visible hairs on its dorsal surface.

Promyrmister kistneri  (A) Dorsal habitus of holotype CNU-008021 with origin of exudate globule (black arrow) and elytral striae (white arrows) indicated; the three lateral striae are complete (top three arrows), the three medial striae appear incomplete. (B) Right lateral habitus with flight wing indicated. (C) Ventral habitus, boxed region expanded in panel H. (D) Head, dorsal and (inset) frontal views, with antennal scapes and frontoclypeal carina (FC) indicated. (E) Right foreleg, laterally expanded femur and tibia indicated. (F) Proventral mesoventral boundary showing proventral keel with posterior incision. (G) Left pronotal margin, lateral view, showing possible origin of putative glandular exudate; arrow in G corresponds to that in panel A. (H) Ventrite one showing proximal leg segments (Trc: trochanter) and postcoxal gland openings (white arrows), with globule of putative exudate emanating from left postcoxal gland opening. Zhou et al. (2019).

The specimen shows a number of traits that lead Zhou et al. to colclude that it is both a myrmecophile and a member of the Haeteriinae. These include broad expansions of the tibiae with spines on the outer margin, short tarsi received on the outer face of each tibia, a triangular antennal scape, pronounced antennal cavities on the prothoracic hypomeron, and a broad proventral lobe to fully embrace the retracted head, all of which are thought to be protective modifications that enable myrmecophile beetles to withstand handling by ant mandibles.

Reconstruction of Promyrmister with stem-group host ant and larva (ant based on Gerontoformica). Zhou et al. (2019).

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