Ragni camaleonte

Eight-legged chameleons

Piero Fariselli and Brizio Cesare

Thomisus onustus female

If you are among those people that find pleasure walking in the countryside, especially in the meadow, and you also enjoy gazing at what happen on the flowers, well, you probably have met one of the most wonderful European spiders: Thomisus onustus or Misumena vatia. These stunning spiders belong to the so called crab-spider family (scientifically known as Thomisidae), because they are crab-like-shaped and walk laterally.

Thomisus onustus female

Thomisus onustus eating

They are formidable and patient hunters, waiting for hours for their preys, they can capture and eat insects several times heavier than their body-weight. Their astonishing performance is due to lethal venom they can inject, that has to work very fast! But, don’t worry, they are (like the majority of the spiders) completely harmless for the humans, since their mouth is very small and their chelicerae (their fangs) are too tiny to cross the skin. Actually, differently form other spiders, their chelicerae are smooth without teeth so that they cannot chew they preys, but they have to inject in the prey bodies also a digestive fluid containing different types of enzymes. Thus, the internal prey body become a nice soup that they can drink.

Thomisidae body is a perfect ambushing machine, with their four legs highly differentiated. The four rear legs are short and strong to keep the body very well attached to flowers or leaves, while the anterior legs are very long and raptorial. The muscles of the raptorial legs stretch them open, while the ultra-rapid closing movement is obtained using a hydraulic flux toward the abdomen, similarly to fast industrial joints. Actually a poor bleeding spiders cannot stretch its leg because it lack of the body hydraulic pressure.

One of them most amazing trait of Thomisus onustus and Misumena vatia is the ability of changing colour in time. Other spiders like the garden spider (Araneus diadematus) can change colour, but this capability is only restricted to tonality variations (darker, paler, etc). On the contrary, these small chameleons are real champions. They can change from white to purple, including pink and yellow!

Being not fast runners, nor web builders, these spiders captures preys waiting for them on top of flowers, then their ability of matching the surround is critical. Moreover, the chameleon spiders have to be invisible for predators, like birds, and attractive for insects (its food). Birds, have a nice vision quite similar to the ours (sometimes more extended), while insects, especially bees and bumble-bees (their main preys) have higher sensitivity in the ultraviolet region. For this reason our crab-spiders have to find a compromise that can be a reasonable trade-off between, not being visible for birds and bees at the same time. Usually the spiders are really good in this camouflaging jobs, however some times they tend to prefer being more suited for their predation activity. Thus, Tomisus onustus can be at home both on white, yellow, blue and pink flowers. But, how can these “small chameleons” change colour and how much time does it take to them?

Thomisus onustus female

Thomisus with crawling ant

To answer the question, we have to remember the spider colour is determined by two distinct factors: pigmentation and selective reflection. In the case of our small ladies (actually is far easier coming across a female) the colour is due to both concomitant processes. So that in the oldest studied case of Misumena vatia, she can be white or yellow. She is “normally” white because of the total reflection of the light due to guanidine crystals contained in the cells just below the transparent skin. However, in case of need, for instance there are only yellow flowers available, she can produce yellow pigments and in about ten days, she is able of matching the surrounding again. Naturally, the process is reversible and in a white environment she can turn white another time. In Thomisus onustus, the chromatic variation is faster (few days) and even more ample. Our eight-legged chameleons can go from yellow to white like Misumena, but also pink, red and violet, and sometimes combinations of them.

Recently I got the impression that Thomisus onustus differently from other spiders, seems also unnoticed by ants. Actually, I came across some Thomisus on top of flowers with crawling ants on the back moving on the spiders as it was part of the plant. Thomisus of course remained still, but nothing bad happened to them (all were nice yellow-wearing ladies).

The males are really undergrowth blokes as compared to their highly painted ladies, and, at least in Thomisus onustus, can change colour too. For sake of clarity, this does not mean, as it usually thought, that the males are also part of the diet of their spouses. On the contrary, their small dimension help them to get closer to the object of their desire and “crawling” on the female body to mate, usually without a fatal destiny.

Thomisus mating

Thomisus protecting

the eggs