Wednesday 7 October 2015

Philodromidae: running crab spiders

This family has over 500 species worldwide, but just 15 in the UK. The most abundant genus is Philodromus, relatively small spiders, which do not build webs and bear a superficial resemblance to crab spiders (Thomisidae), although they are not closely related to them. Some species are very similar and require microscopic examination of female genitalia and male palps for identification. The genus Tibellus, includes a couple of British species that live on grass and have an elongated body and legs that stretch forward and back.

Ambush or pursuit hunters
Philodromus have long, but robust legs, specially the first two pairs, and like to bask with legs curled up like wolf spiders or nursery spiders. They can be ambush predators, often sitting atop flowers or foliage in trees or bushes, legs outstretched, awaiting prey, although they can also run fast pursuing prey or escaping predators.

The strategy of waiting on flowers pays off for this Philodromus, who got a juicy fly.
Although Philodromus does not build a web, it uses silk as a safety line - which you can see in this photo of one in ambush on a flower.

Colour change
These spiders can be strikingly well camouflaged on their habitat, which can include lichens on tree trunks, heather or sandy beaches. Some species even have the ability of changing the depth of colour to match their surroundings. A species that lives in lichensPhilodromus margaritatus, is hard to spot when sitting on them, and it can also change colour from green to white or grey depending on her surroundings.

Iridescent males
The males of some Philodromus species can look quite different in color pattern to the females. In some species males are iridescent.
The beautiful iridescent male of Philodromus aureolus. The female looks like the individuals in the photographs above.
This is another male Philodromus with iridescence, and just six legs.

Spiders are not particularly known for their ability to regenerate body parts. But it is not unusual to see Philodromus with legs missing. The specimen with the fewest legs I've seen is four, only one left on one side, and yet it seemed to move about freely over flowers. It looked like it used its remaining palp for balance.
If leg loss happens in young individuals they can be regenerated as they grow a new leg, thinner and shorter than the original, in the next moult. 
This Philodromus sp with its aphid prey has regenerated the two middle legs on the foreground, which is thinner and shorter than the other and has a plainer appearance.
Philodromus basking on a sunny spring day.

 And to finish this family's post, a photo of an individual of the genus Tibellus, with a very different appearance to Philodromus.


Ragged Robin said...

A wonderful series of posts - am so enjoying and learning a lot :) Thanks so much and I look forward to the next one.

Africa Gomez said...

Than you so much Ragged Robin, I'm glad you are enjoying it, it is being quite a lot of fun to do the research for the blog posts and write them up too.