Crab spider

Article and images by Dr Julie O'Connor, senior conservation partnerships officer

How well is a crab spider really camouflaged?

The spectacular crab spider (Thomisus spectabilis) is an Australian spider in the family Thomisidae. Also known as a flower spider, it can most often be found sitting motionless and camouflaged in a flower, often with front legs outstretched waiting to seize unsuspecting prey. But there’s a lot more to this little spider than meets the eye.

Firstly, she has the amazing capacity to change her colour relatively quickly (within a couple of days) to match her chosen flower. For example, in her white form she is perfectly camouflaged on the white native hibiscus (Hibiscus heterophyllus) or the introduced marguerite daisy (Argyranthemum frutescens). Similarly, in her yellow form she blends perfectly into the flower of the introduced Singapore daisy or the native Wedelia spilanthoides.

But here’s the interesting thing. One would assume that as an ambush predator she is camouflaging herself from potential prey to increase her hunting success. However, what appears to be camouflage to the human eye may be the exact opposite to the insects she preys on. Humans only see the visible light spectrum with wavelengths from 400 to 800 nanometers. Insects, however, perceive higher frequency wavelengths in the electromagnetic spectrum from 650 to 300 nanometers, which includes the ultraviolet range. The significance of this is that a white crab spider sitting on a white flower is quite cryptic to the human eye. Insects, however, with vision at the ultraviolet end of the spectrum see dramatic contrast that is completely invisible to us.

A 2003 study published in the journal Nature looked at the relationship between crab spiders and foraging bees (Apis mellifera). The researchers discovered that not only were bees undeterred by the presence of a resident crab spider, they were in fact significantly more attracted to flowers with a spider than to flowers without one. While white flowers that reflect ultraviolet light are very rare in nature, the spectacular crab spider does reflect ultraviolet light. To an approaching bee coming into fairly close range of the flower, the ultraviolet light reflecting from the spider creates a striking contrast with the non-reflective flower. You might be thinking this would serve as a warning, but bees are particularly attracted to flowers with strongly contrasting patterns.

So, it appears that this amazing little spider has evolved to exploit the honeybee’s preference for bold patterns. This entrapment is also likely to ensnare small native bees, which share a similar vision system to honeybees. And if her ingenious art of deception works, as it frequently does, she also has a potent and fast acting venom in her arsenal that is capable of immobilizing prey many times larger than herself.

References

• Heiling AM, Herberstein ME, Chittka L. Crab-spiders manipulate flower signals. Nature. 2003 Jan;421(6921):334.
• Uetz GW. Foraging strategies of spiders. Trends in Ecology & Evolution. 1992 May 1;7(5):155-9.