A tale of two pincers

In case you haven't heard of iSpot, it is a UK-based online species identification site, linked to the Open University and Opal (the Open-air Laboratory), where anyone from beginner to expert can post photos of wildlife either with or without an identification, for others to agree with or amend as appropriate. Some groups are relatively straightforward to identify from photos (many birds and butterflies for example) whereas others are more difficult (or need detailed close-ups of key features) such as lichens, ichneumons and many flies. So, when a set of close-ups of a pseudoscorpion appeared on iSpot with a suggested identification of the rare species Kewochthonius halberti, my interest was piqued and I decided to investigate...

The specimen had been found inland whereas K. halberti is considered coastal, associated with the high-water line. However, like many pseudoscorpions (they form an order of arachnids), their small size (up to 4mm) and secretive habits, being found under stones, logs, debris etc., this species is poorly understood and it is always possible that it is associated with other habitats as well. So, I consulted the key in Legg & Jones (1988), the standard work on British species (which is very good value from here by the way), bearing in mind that even such a well-respected text may have omissions given the poor level of knowledge regarding the group covered.

The pseudoscorpion in question, preserved in ethanol

Having worked through the key, I too came to K. halberti, but it didn't look right - the shape of the cephalothorax (the front segment with a plate covering the head and thorax) narrowed towards the front for example, and so I posted a comment on the observation on iSpot and began a discussion with the original observer, Kris Hart (who kindly gave permission for me to use his photos here).

The movable finger of the pedipalp pincer showing small blunt teeth along the edge.

Some features were clear such as the tergites (plates running across the top of the abdomen) having no longitudinal split down the middle. However, whether or not the chelicerae (the jaws attached to the front of the head forming small pincers rather than the large scorpion-like pincers on the pedipalps) were more or less than 2/3 the length of the cephalothorax was a less clear decision. I measured them as 0.7 of this length but given the margin for error, this was a borderline result. The teeth on the edge of the pedipalp finger (above) match those of K. halberti, but this species is only 1.2mm long, whereas the specimen is twice this length - definitely outside the plausible range of variation.

So, I returned to the key and chose the 'smaller chelicerae' branch which took me to the genus Neobisium. This immediately looked promising as the species were of the same size as the specimen, and also showed more or less the same shape of cephalothorax. Indeed, keying it out soon led to the 'common neobisid' N. carcinoides - probably Britiain's commonest pseudoscorpion. The final feature indicating this was the shape of the galea - the small bump on the outside edge near the tip of the 'finger' of the chelicera.

The movable finger of the chelicera of N. carcinoides. The galea is the small round bump near the tip to the bottom left of the photo and is a low, rounded structure rather than being more highly raised.

So, a possible rare species turned out to be a very common one. This often happens and may seem disappointing, but not so - apart from being a more accurate record, it illustrates the process of using fine structural or morphological details to aid identification. It also provided an opportunity to look at a specimen from a group that isn't seen very often (without finely sieving leaf litter or other suitable debris) and become more familiar with it. Variaous possibilities were considered - was it an adult? (juvenile pseudoscorpions are very poorly understood); had the preserving fluid had an effect on colour? It also showed how a resource such as iSpot can be used to collaborate in the identification of difficult specimens as I asked Kris for extra photos, further details and so on through the comments facility. Lastly, it has highlighted an area of difficulty in the key - something that can only be done by using it. So, yes it's a common species, but also very much a worthwhile piece of work. Thanks Kris!


Reference

Legg, G. & Jones, R.E. (1988). Pseudoscorpions. Synopses of the British Fauna (New Series) 40: i-vi, 1-159.