UV light reveals mating secrets of jumping spiders

Kevin Robinson

For humans, ultraviolet light can be something of a nuisance. It fades our furniture and drapes and gives us sunburns and cataracts. However, for other members of the animal kingdom, such as insects and birds, UV light is key to their survival because it allows them to forage for food and to navigate and communicate. Recently, researchers at the National University of Singapore discovered that jumping spiders use UV reflectance and fluorescence to find a suitable mate.

According to Daiqin Li of the university’s department of biological sciences, scientists knew that the jumping spider Cosmophasis umbratica has UV-sensitive photoreceptors in the movable retinas of its principal eyes. However, little work had been done to determine the role that color and UV light plays in the life cycle of the spiders.

The Cosmophasis umbratica spider has excellent eyesight that is sensitive to UV light, and the males and females have UV-reflective body parts. Images courtesy of Matthew L.M. Lim and Daiqin Li.

Many of these spiders, especially the males, are brightly colored and have elaborate ornamentation, but the discovery of UV-induced fluorescent communication took the researchers by surprise, Li explained.

“When we were running experiments to look at whether females used UV for choosing mates, we turned off the full-spectrum light but left the UV light on. We saw a pair of very bright moving palps in the female only. This led us to look at the importance of fluorescence in jumping spider communication.”

Male jumping spiders have patches of UV-reflective scales on their faces and bodies, which they show off when posturing during courtship. The females do not have UV-reflective scales but instead have palps that fluoresce bright green when excited with UV light at around 375 nm.

The experiments were relatively simple. Li, Matthew L.M. Lim and Michael F. Land observed the courting behavior of male and female jumping spiders under lighting conditions with and without UV wavelengths. Without UV light, the spiders didn’t court successfully. The researchers even set up the experiment to illuminate only one of the pair. The results were the same: Most spiders have trouble courting without the UV clues, even when the courting behavior remains the same.

For the work, they used a Hitachi dark tube for UV illumination and a Voltarc Ultra Light 110-W fluorescent tube for full-spectrum light. They imaged the UV reflectance and fluorescence with a Nikon D1X digital camera with a UV lens and filter. To determine the emission and excitation wavelengths for the females’ fluorescent palps, they used a Leica inverted light microscope equipped with an Ocean Optics miniature fiber optic spectrometer.

These color images show the UV-induced fluorescence in the female C. umbratica spider.

Li said that the group does not think that the spiders can control the fluorescence or reflectance. They can simply see it and use it in much the same way that birds use bright colors to identify and choose mates. Another interesting finding is that in this jumping spider species, juveniles don’t reflect UV light.

The researchers are already planning follow-up experiments. Li said they are considering identifying the UV photoreceptor proteins and genes to look at the evolution of UV vision in the spiders. They also will be looking into which proteins produce the fluorescent palps, how the UV reflectance and fluorescence help the spiders adapt, and what implications this may have for industrial materials.

Science, Jan. 26, 2007, p. 481.