Nemastoma bimaculatum species account

Nemastoma bimaculatumNemastoma bimaculatum is found everywhere in the British Isles, including the Channel Islands and Orkney and several other small islands. It also occurs on the tops of mountains at heights above 2,500 feet; and abroad it extends over the whole of Europe from the Arctic to the Mediterranean.

Nemastoma bimaculatum @NBN: Taxonomy; Grid map; Interactive map;

The general appearance and colouring makes the species immediately recognizable and distinct from other genera. The small rotund body, 2.5mms. long, with rather short legs, and its prevailing black colour with two large white patches on the cephalothorax, are quite distinctive. In some specimens the patches are pale yellow, sometimes they are cream-coloured and sometimes silvery.

Of both N. bimaculatum and the very similar N. lugubre (see below) very rare variations without the patches (hence totally black) occur, which on the continent could lead to confusion with other european species such as N. dentigerum, N. bidentatum or N. triste. Only very few records of these all black N. bimaculatum specimen are known, Meidell & Stol (1990) list only one each for France, Britain and Norway, nevertheless the website of the European Society of Aragnology shows a black N.lugubre in their gallery. If an all black specimen is found it might be equally likely that it's an imported specimen of one of the continental all black species, especially if more than one are found at once, so great care should be taken to assure proper ID.

The Lugubre confusion

Two species - Nemastoma bimaculatum and Nemastoma lugubre - have been dreadfully confused for some two centuries and the situations has only been cleared up (sort of?) in the past few decades. The fact that both species, as interpreted now, are credited to ancient authors (N. bimaculatum (Fabricius, 1775); N. lugubre (Müller, 1776)) reflects the hard work that has gone into interpreting old literature and collections, mainly by Gruber & Martens (1968), not any sense of clarity since the animals were named/described.

In continental Europe N. bimaculatum has a (south-)western distribution and N. lugubre a (north-)eastern one with fairly little overlap. In this picture Britain would fall into the bimaculatum range and indeed it is assumed that this is the only species of the two present in Britain. Incidentally that is part of the reason for leaving the credit for the species with Fabricius, as his original description of Phalangium bimaculatum lists "Anglia" as the locality.

Nevertheless, following some early 20th century publications by Roewer that handled them as one, many authors have listed and described N. lugubre for Britain. Interestingly some of these even give characters in the description that would fit N. lugubre much better than they do N. bimaculatum. This can mean one of two things:

• • The authors copy-catted the descriptions from other (eastern) European authors and didn't look at the British material closely enough.

  • The authors indeed did accidentally have (an extremely rare?!) N. lugubre from Britain at hand when they wrote/checked their description.

To date the only infromation about the occurrence of (the true) N. lugubre in Britain we could find is this remark from the newsletter of the Opiliones Recording Scheme Ocularium No.1, 1998 That's all. Neither the checklist that John Partridge kindly made available to us, nor the NBN Gateway list the species for Britain.

So, it's probably best to assume the presence of Nemastoma bimaculatum only in Britain, until proven beyond any doubt that N. lugubre has a solid base somewhere too.

If anyone has any knowledge about (publications on) the occurrence of N. lugubre on the British Isles please do let us know!

General description

Just for reference, this section includes some distinguishing features for both Nemastoma bimaculatum and N. lugubre even if occurrence of the latter in Britain is questionable at best.

The abdomen is very hard above and bears transverse rows of small blunt denticulae or granulations, marking the hind edges of the tergites (but notably more so in N. lugubre than in N. bimaculatum. The legs are strong; in colour they are dark with paler metatarsi and tarsi. The dorsal side of the femora of the 4th pair of legs bears strong denticulations on N. bimaculatum females and only very few denticles on N. lugubre.

The sexes can be fairly easily distinguished by a close look at the chelicerae, as on the males these carry extra characters. On N. bimaculatum the top of the first segment of the male chelicerae has two apophyses, a bluntish process on the outside and a little sharp tooth/denticle sticking out on the inside, on N. lugubre it's a single slightly bigger blunt process.

Weak characters for field ID

The shape of the white/silver spots is quite variable, nevertheless Dutch and Belgian experts have worked out that the indentation of the outside edge is a good "field character" - quite reliable for quickly separating the two in the field (but not a dead-certain qualifier for scientific ID!):

• • N. bimaculatum : More rectangular-ish (sharp corners) all over and clearly indented on the outside/lateral edge

  • N. lugubre : More rounded all over (corners) and not indented on the outside/lateral edge, which is however somewhat jagged.

Please note this is a weak character and based on continental European specimen to boot, so use with some care for British animals.

Also, there is some slight difference in habitus, especially for the females, as the 'abdomen' is dorsally flatter on N. bimaculatum and more rounded on N. lugubre, but it requires ample experience with both species to clearly recognize this at all, and even then it is quite hard to interpret from most perspectives that photos will provide.

Left: N. bimaculatum; Right: N. lugubre.

Proper characters for ID

Unfortunately, the characters needed for "proper" ID are quite hard to photograph properly, let alone to recognize from 'general purpose' shots of the animal:

These lists of characters are UNDER CONSTRUCTION still and NEED CHECKING, so please do not take these as "fact" just yet

MALES

• • Shape of the palpal tibia, best evaluated laterally. Images needed! (see Meidell & Stol (1990) linked below).

  • Shape of the apophyses on the distal end of the 1st chelicera segment, best evaluated dorsally (see image below and here): N.bimac. has the small extra denticle on the central side which N.lug. lacks.

  • Shape of penis Images needed! (see Meidell & Stol (1990) linked below).

Double apophysis on chelicerae of male N. bimaculatum

FEMALES

• • Habitus of opisthosoma ("abdomen"): Quite flattened dorsally on N.bimac., more rounded/bloated on N.lug.

  • Denticulation on opisthosoma (hind edges of sclerites): Weaker on N.bimac..

  • Denticulation on dorsal side femora of 4th legs: Stronger on N.bimac.)

  • Length of setae on ovipositor. Images needed! (see Meidell & Stol (1990) linked below).

References

• • Meidell, B.A.; Stol, I. (1990) Distribution of Nemastoma bimaculatum (Fabricius, 1775) and N. lugubre (Müller, 1776) (Opiliones) in Norway, with a discussion on “east-west pairs of species”. in Fauna norvegica, Ser.B., Vol.37, pp.1-8. - PDF (good read!).

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