This family is closely related to the running crab spiders Philodromidae. There are 26 British species. They are broad-bodied, with the front two pair of legs are much longer and stouter that the others and furnished with stout spines, which they often hold wide open in a manner similar to the way a crab hold its front legs open defensively. This, together with their widely spaced eyes placed on tubercles and their ability to walk sideways has given them their common name. There can be substantial colour and size polymorphism, with smaller and more cryptically coloured males.
Ambush predators on flowers
Crab spiders do not build a hunting web. Many species of crab spiders use a sit-and-wait strategy to hunt insects. They sit on a flower immobile, front legs open and rear legs holding them to the substrate, relying on their cryptic colours to avoid being seen by visiting pollinating insects (top shot shows Misumena vatia. Credit: Alan Shearman, CC BY-NC-SA 2.0). As soon as an insect lands on the flower the spider will swiftly bite it catch it. Other species can also stalk prey, cat style, while on the ground, or low vegetation. More on a BugBlog post here.
Xysticus cristatus stalking a fly.
Xysticus cristatus on dandelion, using the sit-and-wait strategy.
Crab spiders are very diverse in shape and colour and some species are very colourful, but yet, remarkably camouflaged in their habitats. Some species, such as Misumena vatia are able to slowly change colour to match the flower where they are sitting. Adults can change colour from yellow to white or greenish with or without pink lateral markings, and most of the time it matches the colour of the flower it is sitting on.
Female Xysticus cristatus
Male Xysticus cristatus