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Sep. 08, 2014

Picture of the Week: ‘Sequined’ Spider

by Becca Cudmore

Click to enlarge images
This spider’s decorative patterning may seem like a dead give away to predators. But for those looking to feast on the arachnid, a species in the genus Thwaitesia, the hunt might result in confusion and failure. “Like a disco ball with lots of different mirrors,” reflective splotches on this spider's abdomen probably “scatter light and make it difficult for predators to see it,” says Robert Whyte, an honorary researcher in arachnology at the Queensland Museum, who’s currently co-authoring a field guide to Australian spiders.
 
Thwaitesia species are colloquially called “mirror spiders” or “sequined spiders,” although several other spider families include members with similar silvery or white abdominal markings. Based on what scientists know about some of its better-studied cousins, this spider’s shimmery spots are probably crystalline deposits comprised of guanine, a waste product produced by gut cells called guanocytes, according to retired biologist Ron Atkinson, who maintains the Find-a-Spider Guide for the Spiders of Southern Queensland. Those cells are located right under the transparent “skin,” or cuticle, of the spider’s abdomen.
 
A better-studied species that sports guanine dots, Floronia bucculenta, seems to hide its white patterns by contracting fine muscles that pull the guanocytes into a tight mass, according to Atkinson. “The stimulus for these muscle cell contractions is usually [something] external,” he says—such as a predator. As the guanocytes shrink, other pigmented cells become more prominent, and the spider’s coloring appears to change.
 
There are at least 21 known spider species (including F. bucculenta) that can rapidly change color, which they do by involving guanocytes, according to a chapter in Spider Ecophysiology. The spider featured in this article appears to be able to contract its spots, too—compare the picture below with the one above—but the mechanism is unknown.
 
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Markings such as these can stymie would-be diners. For instance, “reflections from the [silver] plates may resemble those from droplets of water in the green vegetation,” in areas where they live, says Atkinson, “and the colored bits of the spider could easily be confused with flower parts.”
 
While there are multiple species in the genus Thwaitesia, there are likely more that remain unknown to science, says Whyte. In fact, while photographer Nicky Bay photographed the spider pictured above in Singapore, The World Spider Catalog currently has no documentation of a Thwaitesia species living on the island country.
 
“There are several Thwaitesia in China and Vietnam, as well as one each in Myanmar and New Guinea,” adds Atkinson. “I am confident that more Thwaitesia species will be found and described in Southeast Asia in the near future.”
About Becca Cudmore

Becca Cudmore is Science Friday's web intern and a graduate student in the Science, Health, and Environmental Reporting Program at New York University.

The views expressed are those of the author and are not necessarily those of Science Friday.

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