Crab spider

I have been fortunate enough to be able to watch two crab spiders from my conservatory window over the last few weeks. It concerned me that these bright white crab spiders should be sitting on the red flowers of our Spiraea shrub where they are clear for all to see.

Why 'crab spider'?

Crab spiders are so-named because they walk sideways and their front two pairs of legs are longer than the others giving them a crab-like appearance. They don't make webs, instead they hide in a flower waiting for insects to come along looking for nectar. Once they are within striking distance they attack and kill their prey with a deadly bite.

The crab spider, Misumena vatia, (photo) comes in various colour forms. Individuals can be yellow, green or white and often have thin red lines or dots.

Colour-changing properties

It seems logical to assume that a crab spider would choose a flower of the same colour as itself so as to ambush more prey.  

Crab spiders can even change colour to some extent, so a white crab spider which finds itself on a yellow flower can pump a yellow pigment into the outer layer of its body to change colour from white to yellow. If subsequently it finds itself on a white flower it can then excrete the yellow substance to revert back to white.

However, recent studies suggest that crab spiders on flowers of their own colour don't catch any more prey than those on flowers of other colours. A similar study has also discounted the possibility of the spiders being hidden in the UV spectrum (in which bees and some other insects see).

So, this begs the question why do they bother to change colour at all? It might be that spiders which match the flower on which they hide are less prone to predation themselves but even this still hasn't been proved. Another theory is that when these spiders evolved they caught types of insects which were fooled by their cryptic colouring but that these prey species have since become extinct.

So I probably don't need to worry about my crab spiders being on the wrong-coloured flower, but I still feel they should know better!

Interested in encouraging more creatures into your garden? Read about encouraging biodiversity here.