ARACHNIDA > ARANEAE > Philodromidae > Philodromus

The Philodromidae family is represented in Britain by 18 species in four genera; Philodromus (11), Rhysodromus (2), Thanatus (3) and Tibellus (2).

The collective common name of 'running crab spider' is primarily used to describe the Philodromus and Rhysodromus species due to their typical crab-like body shape and rapid and erratic, occasionally sideway, movement. They are active hunters of other invertebrates and can react very quickly. They all have eight eyes and a rather flattened shape with a near circular carapace and oval, sometimes elongated abdomen. Their legs are quite long with the two front pairs being slightly longer than the back four.

Although a few species have distinctive markings that can assist with identification, there is a fair amount of variation because of cryptic colouration and camouflage. In respect of the Philodromus species only the rare Philodromus margaritus, can be reliably identified from its overall colouration and general shape.

The most common and widespread member of the genus is thought to be Philodromus aureolus, but due to misidentification of other species, particularly P.buxi, P.cespitum, P.collinus, P.dispar, P.longipalpis and P.praedatus, accuracy of recording may be in doubt, such that without detailed examination all these species should be regarded as a group or complex.

However, general appearance coupled with habitat can often provide a 'likely' or 'probable' species, which has been the case with the following that have all been looked at by experts from the FB Spiders Group.

Philodromus albidus is a common, regional species - the smallest in the genus - typically found on the lower branches of broadleaved trees along woodland rides. The carapace is greyish-brown with a broad, paler median band, and the abdomen a pale yellowish-brown with a vague pattern. It is indistinguishable from Philodromus rufus, but that species is rare and has only been recorded from a few sites in the south-east. It is also found in open scrub, rather than woodlands.

Philodromus dispar is another common, regional species, occurring in a wide range of wooded habitats including mature woodland such as Garston Wood. The specific name dispar comes from the disparity between the sexes. Whereas the female is basically brown and similar to other species, particularly members of the P.aureolus group, the male is easily distinguishable with an iridescent bluish-black carapace with a thin white marginal line and a near colour-matched abdomen all contrasting with its pale brown legs. As noted, the second photo of this species was identified as a subadult male.

Philodromus cespitum is probably the most frequently observed species in my area and, although officially part of the aureolus group, it's considered the most 'likely' candidate for a few of the individuals that I've photographed and posted to the FB Spider Group for confirmation of identity.

Having said that, the last two photos that I've added to the page have been labelled in accordance with the advice I received; the individual photographed at Longham Lakes was undefined so it was suggested that I leave that one as Philodromus aureolus (group), rather than Philodromus cf.cespitium; and the one found in scrub bordering Martin Down was considered to be a very likely candidate for Philodromus praedatus (note the spiderlings, just visible bottom left).

Finally, I was pleased to be able to update this write-up with some photos of Philodromus margaritatus, the 'nationally rare' Lichen Running Crab Spider that I only briefly mentioned when the page was first published. This attractive species has very local distribution in just a small number of scattered locations across the south and in central Scotland. Fortunately, one of the country's hot-spots is around my area so, despite being rare, it was not a massive surprise for me to find not one, but two in the garden. The first, photographed on the side wall of the house, is the darker brownish-grey form. The other, found earlier in the year, initially on some timber screening before it dropped down onto some wood before disappearing into a flower bed, was the more typical pale greenish-grey variety.

Updated, Jan.25 (v.5)