Spiders of the genus Thwaitesia are often described as mirror-ball spiders, because their bodies are lined with reflective panels that make them look like tiny disco-balls. These arachnids are also known as dewdrop spiders or sequined spiders.

Above: two different species of mirror-ball spider

The shimmering panels on the abdomen are produced by a digestive secretion known as guanine, which seeps out through the lining of the spider's gut and forms a mosaic of crystalline cells just beneath the surface of the abdomen.

Above: Thwaitesia argentiopunctata and Thwaitesia margaritifera

The crystals shrink and expand as the muscles in the spider's abdomen contract; they often shrink when the spider is agitated, and they expand as it begins to relax.

Above: the panels expanding and contracting

The adaptive purpose of the shimmering effect is unclear, but researchers believe that it mimics the glistening appearance of raindrops or beads of dew, which could allow the spiders to blend in with their environment.

Above: another view of the guanine crystals shifting just below a thin layer of cuticle

The reflective panels may also help to confuse predators, as this article explains:

The spider’s decorative patterning may seem like a dead give-away to predators, but for those looking to feast on this arachnid, the hunt might result in confusion and failure.

“Like a disco ball with lots of different mirrors, the reflective splotches on the 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.

Above: Thwaitesia affinis

There are at least 22 known species of mirror-ball spider, and their physical features can vary significantly. In some cases, the silver panels on the abdomen are accompanied by colorful, iridescent spots and scales, and the abdomen itself may have a red, orange, green, yellow, or beige appearance.

Above: spiders of the genus Thwaitesia

These spiders are widely distributed throughout the southern hemisphere. They can be found in the tropical and neotropical rainforests of South America, Southeast Asia, Africa, Australia, India, Sri Lanka, China, Singapore, and Madagascar.

Above: Thwaitesia nigronodosa

Mirror-ball spiders are not aggressive toward humans, and there are virtually no recorded cases of humans being bitten by the spiders of this genus. They're also tiny, with most individuals measuring just 2-4mm long (roughly 1/10th of an inch), which means that their fangs are often too small to penetrate human skin.

Above: Thwaitesia margaritifera

Sources & More Info:

iNaturalist: Mirror-Ball Spiders

Australian Geographic: This Sequined Spider Glistens in the Light

My Modern Met: The "Mirror Spider" and its Changing Mosaic of Reflective Panels

A Field Guide to the Spiders of Australia: Tiny Forest Jewels: Spectacular Thwaitesia Spiders

Science Friday: How is a Spider Like a Disco Ball?

Land for Wildlife: Spider Sampling Points to a Potential Indicator Genus (PDF)

Royal Society Publishing: The Spider Cuticle

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