A Hitchhikers Guide

On Wednesday I arrived home from a long day working in Birmingham to a plastic box by the back door. Being known as the village entomologist (or possibly the village weirdo) means I get given a wide variety of interesting invertebrates by neighbours (and sometimes inanimate objects that resemble something living), so this in itself was not a surprise.

Jo also collects anything interesting she sees in the garden, and I soon discovered this was in fact a welcome home present from her. She had found a beetle in an old plant pot whilst gardening and thought that I would want to see it. The beetle in question was this Dor Beetle Geotrupes stercorarius. 

Dor Beetle Geotrupes stercorarius

This species looks almost identical to the Common Dumble Dor G. spiniger, although Dor Beetles tend to have more distinct ridges on the wing cases. To check the identification you really need to look at the ventral surface of the abdominal tergites, or in laymans terms you turn it on its back and look at its bottom. G. stercorarius has hairs and punctures across the width of the tergites, as can be seen in the picture below, whilst these are much less extensive in G. spiniger.

Dor Beetle Geotrupes stercorarius with a species of phoretic mite

Dor Beetles are alternatively known as dung beetles. They will dig a tunnel under a suitable piece of dung and line this tunnel with the dung for the larvae to feed on. The horse field opposite my house is a potential breeding ground for Dor Beetles, though the horse dung tends to be collected up religiously every morning to rot down into horse manure! Apparently Dor Beetles have also been recorded using decaying fungi and rotting plants in woodland habitats in a similar manner.

The other noticeable feature of Dor Beetles, as the pictures show, is that they are often infested with mites. This individual was no exception, and was living up to one the best colloquial names of all insects, the Lousy Watchman. It would be easy to assume that these mites are parasites that cause the beetle harm, perhaps they are eating it alive? In fact the truth is far more interesting and remarkable.

These are a species of phoretic mite, and so belong to the same class as ticks, spiders and harvestmen - in other words they are Arachnids. Phoretic mites use the strategy of hitching a lift on the bodies of beetles and bumblebees to travel between food sources. They do not harm their host, this would not be a sensible evolutionary strategy, unless perhaps they are so numerous that they weight it down. This seemed to only be a mild infestation, so are probably not causing this beetle too much of an inconvenience.

Dor Beetle Geotrupes stercorarius with a species of phoretic mite

These mites belong to the Acari taxon, a large group comprising mites and ticks. The Acari are comparatively little studied. In fact, according to the Natural History Museum, there are at least 54,000 species worldwide, with many hundreds surely yet to be classified. They have roles in agroecosystems and potential uses as archaeological indicators of change. There are no easily available resources for identification, but NHM are currently producing these, which will surely make this group more accessible.

Given this, getting an identification for the Dor Beetle's travelling companions is a bit of a challenge. However, after a bit of research these mites appear likely to be Peocilochirus carabi. A check on the NBN gateway shows just how under recorded this group is. For this species, which is surely widespread and common, there are only records from two adjacent 10km squares in East Anglia, and no others at all in the UK. This makes these mites a potential VC40 first. I have contacted those working on the NHM project to see if they can confirm identification, and will provide an update if they can help.

This goes to show that sometimes a closer look at one insect, can reveal something else unexpected, but often quite fascinating.