Opiliones in Belgium

(published VIII.2004)

While the Belgian community of eight-legged animals shudders with fear in view of the next excursion of the flourishing Arabel (Arachnologia Belgica), one little group keeps his peace of mind: the Harvestmen. The majority of arachnologists has lost interest since some simple youth experiments with the legs. I couldn't beat the knowledge of the Arabel-people, so I took the Opiliones as my territory.

Dr. Baert (Royal Institute of Natural Sciences) kindly put the relevant collections at my disposal. Although Opiliones were never the object of a dedicated sampling session, the bottles were rich enough to keep me happy for several years on row. I added some material from my own entomological studies during the 1980s and from the former Laboratory of Oecology of the State University of Ghent. Another important source of information, mainly about the western part of the country, were the data obtained by W. Slosse with his pitfall campaigns during 1994-'96. Maps based on all this observations were drawn and made available on the internet in June 1999.

For the current set of maps the samples of several more pitfall campaigns were included. I've also walked through the old dataset, checked suspicious observations and made corrections if necessary.

The previous version of this site (published in June 1999) is made available as a zip-file (opibel1999.zip; size = 1.3Mb).

In the previous version of this web-article (published in June 1999) I wrote some words about the (the lack of sufficient) samples then and in earlier times making it impossible to draw any firm conclusion about the species, their distribution and possible trends.

The present maps are based on 31581 individuals coming from 170 UTM-squares. That is better than before, but still only about half of all the Belgian squares. Our statement that we badly need more samples from the southern part of the country is still valid. All recent sampling effort comes from the Flemish (northern) part of Belgium. Several interesting Opilionids (e.g. Amilenus aurantiacus, Lacinius horridus, Platybunus pinetorum) are only known from the south and more samples would yield a better knowledge about their occurrence and perhaps reveal still more species.
Read the introduction to the maps.

It doesn't necessarily mean that a species is rare, when she is only occasionally present in our samples. We learned from the Introduction to the maps that this supposed rarity could be an artifact caused by an inadequate sampling method. This means that we must take proper care no to jump too fast to conclusions when we compare the abundance of different species. Quantitative analysis of the maps shouldn't cross species boundaries. It's probably safe to work with presence/absence information though.

Rilaena triangularis is present in more than half of the squares from which we have samples (93 out of 170) and is therefore the most widespread Opilionid in Belgium. Oligolophus tridens comes next with 71 squares. At the other end of the scale we find Platybunus pinetorum from which we found one specimen in one square and Lacinius horridus with 23 individuals also in one square.
Read more about the different species.

I would be nowhere without the collections of the Royal Institute of Natural Sciences and the people submitting their samples. If you ever donated even only one sample to the R.I.N.S., your name could be on this list.