New to Nature No 87: Meziomorphum montagu

Beetles of the family Ptinidae are small, 1-5mm, brown or black, and many are globular in form with long, thin legs leading to the group's common name: spider beetles. All spider beetles are scavengers feeding on dead plant and animal remains. Few are more or less worldwide in distribution and common pests, such as Ptinus fur, the white-marked spider beetle, frequently found in stored flour and grain in British households and occasionally attacking museum specimens, from pinned insects to stuffed birds. Most of the approximately 500 species of spider beetles, however, are found in the wild where they are beneficial, helping to dispense with nature's leftovers.

The spider beetle fauna of South Africa is particularly diverse. Its nearly 50 documented species represent 17 genera of which 13 are found nowhere else on Earth. And the list is growing. For nearly a century, from its description in 1898 until 1996 the endemic genus Meziomorphum consisted of a single South African species, M echinatum. That year, everything changed for Meziomorphum which was about to become controversial. Three new South African species were formally described and named in 1996 and it was further noted that specimens in the South African Museum included as many as four more species. These, however, were either inadequately sampled or in such poor condition that they could not be named at that time. These new but as yet unnamed species were referred to simply as A, B, C and D.

Four years later, in 2000, a different beetle expert concluded that they merely represented variation within M echinatum. Such is the nature of species work. An expert examines all the specimens and data available and hypothesises how many species exist, how much genetic variation exists in each, and where their respective boundaries lie. Subsequently the evidence may be re-evaluated or, better still, additional specimens may be collected or new evidence introduced leading to a critical testing and re-evaluation of the species.

In this case, Professor T Keith Philips of Western Kentucky University revisited the cave where unnamed new species D had been discovered. In a paper published with SR Trimboli in 2011 the species finally got its name, M montagu. In order to determine its status, however, it was necessary to revisit M echinatum as well as its three recent synonyms. After careful comparative study it was concluded that the three species named in 1996 were indeed valid and resurrected them to species status as well as making D an honest beetle. M montagu differs from all others in its genus by its golden setae, brown integument, three rows of spines on each of its elytra or hardened forewings, and a single row of spines that originate on the humeral angles of the elytra.

Montagu Cave is located in the Western Cape region east of the village of Montagu, and is the only place that M montagu has ever been seen. There is a large outer cavern that is dry and a much smaller, inner cavern that has high humidity and a significant accumulation of bat droppings. A different spider beetle of the widespread genus Mezium was found in the outer cavern, but M montagu was restricted to the moist inner chamber where it could be found in cavities that it excavates in the guano, under stones, and walking on the surface of deep faecal deposits. Philips and Trimboli believe that the new species is a true trilobite restricted to the moist inner chamber and point out that its rounded shoulders are a telltale sign of its flightlessness.

The species shares a number of conspicuous anatomical features in common with its congeners that make it unique among ptinids or beetles in general for that matter. These include rows of large spines on the elytra and legs but most of all a truly bizarre pronotum, the segment between the head and elytra. Like other ptinids, the head is concealed by the pronotum and not visible from above. Unlike any others, Meziomorphum has long pronotal setae that are fused, perhaps by a wax or cement layer, to form a weird inflated shape.

Quentin Wheeler is director of the International Institute for Species Exploration, Arizona State University