This is called a mirror or sequined spider. These spiders are all members of the Thwaitesia genus. Their most distinctive trait is the reflective silvery patches on their abdomen, hence the common name. The scales look like solid pieces of mirror glued to the spider’s back, but they can actually change size depending on how threatened the spider feels. The reflective scales are composed of reflective guanine, which these and other spiders use to give themselves color.
Thwaitesia spiders occur throughout the world and there are more than 20 described species at the moment. They are harmless to humans and relatively small, with adults being about 2 – 5 mm long, depending on the species and sex. Similarly to most spiders, female mirror spiders get larger than males.