Crab spider stalking fly

I spotted this crab spider in my local wildlife garden, it had this strange clock-work movement and grabbed my attention. Then I spotted the fly. The crab spider, a female Xysticus cristatus, legs outstretched, on the left of the photo. The large fly is on the right, having fed or drank from the base of a teasel leaf. If you want to know what happened next see this video.

The sunny side of the fence

The outside of our garden fence is east facing. Many insects and spiders like to bask on it: even normally ground loving invertebrates climb up on it to get the sun a bit earlier. In the last few days I have been watching (mainly) spiders on the fence early in the morning. The male Nursery Web spider (Pisaura mirabilis) on the top shot, warms up on the side of the fence, in almost the same spot every day.

After warming up yesterday he went down to the weeds at the base of the fence to hunt, and caught a greenbottle fly

A crane fly resting, legs outstretched on the fence.

A male Xysticus cristatus, much darker in tone than the female. This is a ground dwelling crab spider and he was basking about 10 cm from the ground, but it is much easier to spot on the fence.

And an eager male Wolf Spider displaying to the female. Alternatively lifting each palp, trembling and getting closer to her very slowly on tiptoes. She kept tapping the ground with their front legs, and didn't miss a movement. When he got very close, almost touching her, she retreated cautiously, tapping regularly.

Ground Crab Spider

Until a couple of weeks ago, I was under the mistaken impression that there were no crab spiders in the north of the U.K. Although this is true for flower crab spider, Misumena vatia, a chamaleon-like hunter that changes colour to match the flower is sitting on, there are many other species that are widely distributed, in the U.K. as I found out through a thread in Wild About Britain. All crab spiders have some ability to change colour to match their surroundings and become invisible to their unsuspecting prey. They do not build a web to hunt, instead, they rely on their superb camouflage and their front two pairs of legs, larger than the rest and furnished with forward facing spines, which they keep open when hunting, forming like a living trap. Their's is a stalking strategy, sitting with its outstretched legs waiting for invertebrates to turn up. The name crab spiders suits them well, as their oversized front legs, divergent lateral eyes and sideway walking makes them look like miniature crabs. One of the most widespread crab spider is Xysticus cristatus. This crab spider is found on the ground or on low vegetation, although it can also hunt in flowers. The female above was sunbathing on my conservatory frame on an awkward corner for photos. I captured it and took a few shots in the while bowl. I released it on a dandelion (above) - before I found out about its preferred hunting grounds - but she didn't like it and scuttled away relatively quickly.

Xysticus cristatus female on her crab-like stance

Unlike flower crab spiders, which feed mainly on pollinators - bees, hoverflies and butterflies - ground crab spiders feed on a wide range of prey, from small prey such as ants aphids and springtails to the larger bees and butterflies, and other spider species, even earthworms! (see Figure below from Nyffeler & Breene). Birkhoffer and coleagues found that Xysticus cristatus - together with wolf spiders - eat larger proportions of aphids when compared to web building spiders. Their common aphid prey might make them a suitable biological control, retarding the population growth of aphids during spring so the authors suggested to encourage habitats favourable to these spiders near crops.

References
Nyffeler, M., & Breene, R. (1990). Spiders associated with selected European hay meadows, and the effects of habitat disturbance, with the predation ecology of the crab spiders, Xysticus spp. (Araneae, Thomisidae) Journal of Applied Entomology, 110 (1-5), 149-159 DOI: 10.1111/j.1439-0418.1990.tb00108.x
Birkhofer K, Gavish-Regev E, Endlweber K, Lubin YD, von Berg K, Wise DH, & Scheu S (2008). Cursorial spiders retard initial aphid population growth at low densities in winter wheat. Bulletin of entomological research, 98 (3), 249-55 PMID: 18439342