For male Habronattus pyrrithrix spiders, the best way to attract a mate is by doing an elaborate, tail-shaking dance. The spiders wave their legs and shimmy their backsides, showing off their brilliant colors, which include rainbow markings on their bodies and a bright-red face.
The spiders have light-detecting photoreceptor cells in their eyes that allow them to see a range of colors. They can even see some things that are invisible to humans—like high-energy ultraviolet (UV) rays of light. However, the spiders don’t have cells that detect the color red. That puzzled scientists. Why would males have red faces if females couldn’t even see the markings?
To solve the mystery, Daniel Zurek closely examined the eyes of H. pyrrithrix spiders under a high-powered microscope. Zurek is a neuroscientist (brain scientist) who conducted his research at the University of Cincinnati, in Ohio. He located a tiny red spot on the spiders’ retina, a structure that lines the back of the eye. The red spot sits on top of photoreceptor cells and acts as a filter that allows the spider to identify red objects. “It’s as if you were wearing regular glasses with a red spot on the lens, so when you look through that spot you’d receive only red light,” explains Zurek.