A still flower, a bee settles on it, and a moment later the bee no longer moves. Thomisus onustus, known as the crab spider or chameleon spider, does not spin webs: it waits motionless on flowers, often the same colour as the petals. If you are not afraid of spiders, it is a beautiful subject to photograph when it shows off its colours.
How to recognise the crab spider
It belongs to the family Thomisidae, the crab spiders. The name comes from its shape: the front legs are long and held open like those of a crab, and these spiders can also move sideways. The abdomen is squared off at the back, while at the front there are forward-pointing tubercles, like small horns, with the eyes set to the sides.
Male and female: two very different spiders
The difference between the sexes is striking. The female measures between 7 and 10 mm and the male between 2 and 4 mm, so much so that she can outweigh him by dozens of times. The colours differ too: the male is dark, brownish, while the female is far more eye-catching and can be white, yellow or pink.
Why does the chameleon spider change colour?
The female can gradually take on the colour of the flower she is sitting on. It is not an instant change, as the name might suggest: it usually takes a few days and is reversible. The male of this species can also change colour, though he remains much more discreet.
The most common idea is that this camouflage hides it from both prey and bird predators, but the question is still being studied. Some research indicates that pollinating insects also react to other signals, such as the spider’s size and movements, as well as the ultraviolet light reflected by its body. In some Australian spiders this reflection attracts bees, whereas Thomisus onustus tends rather to keep them away.
How it hunts
Unlike many other spiders it does not build a web. It lies in wait on a flower with its front legs open and seizes any insect that comes close, even one larger than itself, such as a bee. Its bite injects venom that immobilises the prey, then digestive juices dissolve the inside and the spider can suck out the contents, leaving the shell almost intact.
Its relationship with plants is curious. A 2018 study observed that it can reduce bee visits to a flower, but can also benefit the plant by catching the insects that feed on it.
Where and when to find it
It lives in Europe, North Africa and parts of Asia, especially in warm areas and in lowland vegetation. You can find it in flowering meadows, on flowers and on shrubs, from May to August. It is not hard to spot: just check the flowers calmly, especially those with a strangely motionless insect.
Reproduction
The female lays her eggs in a silk cocoon. Their number varies enormously, from fewer than 10 to more than 400, and is generally higher in cocoons laid early in the season. It takes about a month for the young to leave the cocoon, and at first males and females cannot be told apart.
Is the crab spider dangerous?
No, it is not considered dangerous to people: it is a tiny spider that does not seek contact. Just watch and photograph it without touching it. You can find other small inhabitants of flowers in my gallery of the macro world.
