Aleocharinae – the coleopterist’s Everest, the biggest and most challenging subfamily of beetles. Like Everest, it’s not a challenge you can really take on all by yourself. I am pottering slowly towards base camp and all I can offer here at the moment are some tips on the essential identification characters for these beetles.
Number of tarsal segments.
The tarsal formula is expressed in the form “4, 5, 5” where 4 is the number of fore-tarsal segments, and both mid- and hind-tarsi have 5 segments. Aleocharines have tarsal formulae 3, 3, 3 or 4, 4, 4 or 4, 4, 5 or 4, 5, 5 or 5, 5, 5. Thus the fore-tarsus should be examined first, and only if it has 4 segments is it necessary to examine the mid-tarsus. It shouldn’t be necessary to examine the hind-tarsus.
This can be a difficult character to see but it is much easier to count the number of tarsal segments before carding a specimen. I move the tarsus around with forceps or a fine needle until I find an angle of lighting which allows me to see the divisions between segments clearly. I sometimes card the specimen at this point, add a label with the tarsal formula, and defer the rest of the identification process. To count the tarsal segments on a carded specimen, it helps to wet the tarsus with a blob of water or alcohol. In all cases, a good high-power microscope with good lighting makes tarsal segments easier to count.
There is some contention about the value of tarsal formulae in the identification of Aleocharinae. Certainly some of the tribes as currently defined are polymorphic for tarsal formula (e.g. species in the tribe Oxypodini are predominantly 5, 5, 5 but include some 4, 5, 5 species). Presumably the ancestral state would have been 5, 5, 5 and various species have secondarily reduced tarsi. If one were to dissect an apparently 4-segmented tarsus and examine it under a compound microscope (i.e. up to 1000×) it is likely (?) that a rudimentary 5th segment would be discernible, either as a tiny segment or fused to another segment. However, as long as one doesn’t take segment-counting to ridiculous extremes, it works!
Pronotal pubescence patterns, or Pronotal Behaarungstypen.
Pronotal pubescence patterns provide an excellent set of characters for identification of Aleocharinae to genera and to species. I prefer to use the German name from FHL: Behaarungstyp (singular) or Behaarungstypen (plural). Types I to V should be memorised. The keys also refer to a type VI found in a small number of species, and there are other patterns which occur even more rarely, e.g. male and female Aloconota gregaria have pronotal pubescence patterns unlike each other and unlike any other British aleocharine.
In order to use Pronotal Behaarungstypen, clean specimens are essential. Otherwise, the pronotal pubescence can become matted and it can be very difficult to discern the directions that the hairs would naturally lie in. Collecting aleocharines with a pooter helps in acquiring clean specimens, whereas picking them up by hand will often get them dirty and may even rub the pubescence off. It doesn’t seem to matter if the specimens get wet, whether in alcohol or in dilute acetic acid. As long as the fluid is clean, it will evaporate off and the hairs will more-or-less spring back into position.
Seeing the Behaarungstyp through the microscope requires some practice. The specimen should be lit from a very low level, probably c. 90º to the angle of observation, i.e. from the horizontal, with no top-lighting. The aim is to light up the hairs against a dark background of the pronotal exoskeleton. By turning the specimen to face towards, away from and side-on to the light, it should be possible to work out which way the hairs lie.
The key feature of the Behaarungstypen is the direction of hairs on the pronotal midline: entirely forwards in I, entirely backwards in II and V, diverging in III and converging in IV. The clear and unambiguous nature of the pronotal Behaarungstypen is what makes them such good objective key characters, most of the time. However, the distinction between types II and V is not always clear-cut. There can also be intermediates between other types. In the diverging or converging types, the point of divergence/convergence may be referred to as the ‘nub’. In types I, II and V there may also be a nub but it will be very close to either the front or hind margin of the pronotum. This can occasionally lead to potential confusion, e.g. the only specimen I have seen of Acrotona troglodytes (a species which keys out as having Typ III) had the nub only 20% of the way back from the front margin, thus approaching Typ II.
Neck-marginations and temple-marginations (Halsrandung and Schläfenrandung respectively)
These are often difficult to see from the side on carded specimens because they are either deeply shaded, obscured by glue or both. These photos at least show what the marginations look like in a clear ventro-lateral view.
Freude, H., Harde, K.W. and Lohse, G.A. (eds). (1974). Die Käfer Mitteleuropas, Band 5. Staphylinidae II (Hypocyphtinae und Aleocharinae), Pselaphidae. Krefeld: Goecke & Evers.