Otter, water wolf and funnelweb spiders in Assynt

July 6th 2019

Spiders are a frustrating group for the amateur naturalist. Some 670 species have so far been recorded from the British Isles, inhabiting particular niches in habitats from seashores to mountain tops, where they are major predators of other invertebrates.

Regrettably, however, only a minority can be identified with certainty from photographs.  Some 280 species belong to just one family, the money spiders or Linyphiidae, most of which are quite tiny.  They, like many members of other families, can only be named by microscopic examination of their genital structures, the male palps and female epigynes, which is not possible on live specimens.

Happily, there are exceptions, and David Haines has recently taken close-up photographs of two of these.  On 30th May 2019 he spotted, on the old curling pond in the middle of Culag Wood (NC089216), one of the otter spiders, or water wolf spiders, Pirata sp.  They are ambush predators that skate about on the surface film.

Two members of this genus have been recorded from Assynt, P. piraticus and P. hygrophilus. David’s photograph enabled me to confidently assign this one to the former species.  Diagnostic features are lines of white hairs along the margins of the carapace, and the backwardly-pointing V-shaped marking in the centre of the abdomen, with lines of white spots along its sides, both also composed of white hairs. From the clarity of these markings and the simplicity of the palps, it was a female.

On 3rd June 2019, David photographed another specimen in his house at Culkein Drumbeg (NC1133). Then the following day a similar species was photographed on the external door surround.  By sheer coincidence (or was it?), these were a male and female respectively of the funnelweb spider Textrix denticulata.  This species resembles some of the wolf spiders, but the long spinnerets at the end of the abdomen and striking pattern give it away.  It normally occurs on stony ground, where it spins a small sheet web containing a tubular retreat.  Tripwires associated with this web probably alert it to potential prey.  In Scotland it is recorded as ‘common in houses’, with the more adventurous males venturing inside in search of mates, as in this case, perhaps?

Britain’s Spiders: a Field Guide, by Lawrence Bee, Geoff Oxford and Helen Smith (2017), provides a superbly-illustrated introduction to this fascinating group of creatures.

Ian Evans

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