Your pictures certainly look as though you must have the species, but you may well find that an actual specimen will be needed to be certain about your harvestman. In any case I suggest you contact Mike for opinion on your photo/s (details in SRS News articles).
Images on the Leiobunum tisciae page.
However Achaearanea really need adults under a microscope to identify with reliability and even then they are rather difficult to separate. Whilst it might seem reasonable to assume from the location and position in the habitat that this must be A. riparia, there are other species in Europe which could potentially be present in Britain and for which microscopical examination is needed. We have already had a number of spiders new to Britain recently which would not have been recognised without voucher specimen/s and microscopical examination.
Achaearanea riparia is a scarce spider, so it would be very good to gain definite confirmation of your spider. Can you provide information on the location with grid reference etc.
Could anyone suggest what this retreat might belong to? It was about 5 cm long, 1 cm wide at the entrance and positioned above a Lasius flavus nest. As well as soil from the nest, it had several dead ants incorporated into it. It seems to match the general description for Achaearanea riparia, but I don't know if anything else makes a similar structure? When I found it (18.9.12) it was occupied by a female spider and several young: I haven't been able to get a decent photo of them because of their size, but they did have the high abdomen which is apparently typical of the genus. Any thoughts much appreciated.
Ill try to revisit. It was not hard to find. My money is on you being reflex right as usual!
The problem with many spiders, and especially most linyphiid spiders, is that there is variation both in adults and even more so potentially in juveniles, and so whilst it may be possible to make educated guesses as to the species from appearance (pattern, colours, size, shape etc) it is ALL TOO EASY to make mistakes, even after a lifetime of experience making preliminary identifications in the field. Without confirmation of identification of genitalia under a microscope (and in quite a few cases where id is especially difficult and even experienced arachnologists can have problems, the opinion of other specialists) then an identification will not be reliable.
It is also necessary to appreciate the very real possibility that there may be other species waiting to be recognised in the British fauna, not necessarily even from the nearby European fauna. We have at least two new examples from recent events this year.
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