Pastel predators, fearsomely floral

Those of you who enjoy gardening, or simply looking at flowers closely, may well have seen crab spiders (family Thomisidae) lurking in them waiting for prey. You know the ones - they're often fairly flat compared with more bulbous orb-web spinners, and tend to wait with their legs spread out and bent crab-wise (they can walk sideways) until prey comes and they pounce - none use webs to catch their prey. Many are also well camouflaged, and this is certainly true of Misumena vatia which is found fairly commonly in Britain, particularly in the south. Males are tiny (3-4mm) and quite uniform in colour (mainly brown), but females (the focus here) are larger (9-11mm) and variable in colour - usually whitish, yellowish or greenish, and for this reason are particularly associated with white or yellow flowers, although I have seen a small white female crouching very obviously on a purple Buddleia davidii flower-spike - as a youngster, she obviously had a lot to learn!

Waving, not drowning - M. vatia in a typical posture showing the first two pairs of legs which are used to trap prey.

As this photo above shows, the rear pairs of legs (pairs III & IV) are used to grip the substrate with claw hooks and tufts of hairs ('scopulae'), while the front pairs (pairs I & II) act as traps for prey.

M. vatia hiding in a yellow flower. Note the elongated leg pairs I & II used for trapping prey.

M. vatia lurking in a flower being visited by a bee.

Ambush specialists such as M. vatia have venom which is particularly toxic to prey such as bees, even bumblebees, which are much larger than themselves. To us they are quite attractive and engaging little spiders, but to many other invertebrates they are fearsome foes. When suitable prey arrives at a flower, the spider may do nothing for some time, or may open leg pairs I & II in preparation for attack, sometimes also making subtle realignments to ensure it is accurately facing the potential prey item. When, and only when, the prey is definitely within its grasp is the ambush launched, then once gripped it is bitten and quickly dies. Although I did not see one of these spiders attack a bee, I was lucky enough to witness a similar attack on a hoverfly - the common Marmalade Fly Episyrphus balteatus.

After a quick snatch, the fly is bitten on the back of the neck and feeding commences.

Feeding may go on for some time leaving a dry husk as crab spiders have small jaws/fangs so can't mash their prey.

However, if disturbed this species often darts beneath its flower to hide. In this case, the disturbance was accidental by a benign ecologist, and the spider responded true to form - in this case still cluching its prey which dangled from its jaws, showing the considerable strength often seen in invertebrates.

Although I have looked solely at M. vatia here, crab spiders are an interesting and varied group. Some such as those in the genus Philodromus may chase their prey more actively while others show adaptation of shape as well as colour to aid camouflage, such as the angular-bodied females of Thomisus onustus which is often pink, found on heather and can change colour slowly to match its surroundings. Should I manage to get any good shots of T. onustus, they are likely to appear here...

Here's lookin' at you...

Reference

Roberts, M.J. (1995). Spiders of Britain & Northern Europe. Collins, London.