Although it is not the only example, a common spider with a white dot on its back is Phidippus audax. One of the jumping spiders, it is fuzzy, black and about ¾ inch in length, with a distinct but irregular spot ranging from white to orange on its back.
The daring jumping spider, so-called because it is not afraid to eat things much larger than itself, can leap more than four times its own body length. Its quick pounce enables it to ambush its insect prey, as it does not build a web. Those quick leaps also allow it to escape from predators.