Spider Sex Triggered Only by UV Light

Ultraviolet light might be harmful for many organisms if it is in excess, but some creatures are turned on by it; we literally could say they do not reach orgasm in its absence.

People, and mammals in general, can't detect ultraviolet light, but for a bizarre type of spiders this is essential for their mating. While most spiders are blind or almost blind, jumping spiders have an excellent vision.

These spiders are odd, too, because they do not weave a web. But UV light just triggers them to a sexual frenzy. "It seems that both male and female jumping spiders (here the species Cosmophasis umbratica) have markings on their faces and legs that glow in ultraviolet light", explained Daiqin Li at the National University of Singapore. "Many animals possess UV vision and use it for foraging, navigation and sexual selection," Li explained. "Jumping spiders are known to have good eyesight, many of these spiders are colorful, with the males generally more colorful than females."

"That suggests a possible role of UV vision in jumping spiders, so the researchers decided to explore the possibility."

"But we discovered UV-induced fluorescence communication in jumping spiders by chance," Li said. "We were hoping to find out whether jumping spiders could really detect UV and respond to it, as well as whether jumping spiders use UV signals in their communications," Li explained.

Pairs of spiders were tested in a vast array of light conditions to see their reactions to each other. Full sun-light spectrum, which compasses UV, triggered in males and females the courtship behaviors (which includes bent legs and hunched or flexed abdomens).

Visual spectrum without UV light left both females and males indifferent to each other.

Placing just males in UV light triggered the females' courtship ritual, but 16 of 20 males ignored the females, who could not reflect UV light.

Placing just females in UV light produced the opposite: the males started the mating behavior to the glowing females, but 10 of the 12 females remained indifferent. "We conclude that sexual coloration is a crucial prerequisite for courtship," wrote Li.

Photo credit: Daiqin Li