The goldenrod crab spider gets its name from its ability to change yellow. It uses this camouflage to hide on yellow goldenrod flowers and ambush butterflies, bees and other nectar eating insects that visit the flower looking for food. Since goldenrod is not always available it can also wait on other color plants, in this case thistle, in its white form.
Since this type of spider does not spin a web they must rely on their ambush ability. This spider has caught a skipper butterfly. The spider will grab its prey with its front legs and then inject it with its venom. Once the prey has become immobilized it will suck out all of the bodily fluids. This goldenrod crab is a female. You can tell the difference because males have a brown thorax with a white abdomen where the female is entirely one color, other then the red stripes. Females are also about three times larger which makes mating very dangerous for the males.