(This info is from Wikipedia)
Aposematism is a secondary defense mechanism that warns potential predators of the existence of another primary defensive mechanism.
The organism's primary means of defense may include:
UNPALATABILITY: such as from the bitter taste arising from some insects such as the ladybird or tiger moth, or the noxious odor produced by the skunk, or:
OTHER DANGERS: such as the poison glands of the poison dart frog, the sting of a velvet ant or neurotoxin in a black widow spider.
In these particular examples, the organism advertises its capabilities via either bright coloration in the case of the ladybird, frog and spider: or by conspicuous stripes in the case of the skunk. Various types of tiger moths advertise their unpalatability by either producing ultrasonic noises which warn bats to avoid them, of by warning postures which expose brightly colored body parts. (see unkenreflex)
Velvet ants have both bright colors and produce audible noises when grabbed (via stridulation), which serves to reinforce the warning.
Aposematic signals are primarily visual and involve bright and contrasting colors. They may be accompanied by one or more signals other than color. These may be specific odors, sounds, or behavior. Together, the predator encounters a multi modal signal which is more effectively detected.
A Master Blister Beetle is unpalatable and has bright red markings. I took this photo at the Racetrack Playa in Death Valley National Park.
A Tarantula Hawk warns of its painful sting with its bright orange wings. I took this photo at Eagle Point, Grand Canyon West.