Okay, most everyone know the black widow;s red abdomen tattoo is a double warning. But why?

it does appear that the red marking is there for a purpose. The marking is on the belly, or underside, but the spider normally walks around upside down on its web so the red markings face upward and potential predators from above can see the markings fairly easily. Further, when the female spider is a juvenile the red markings are found only on its back but not the belly. Juvenile spiders spend a lot of time moving around out in the open, looking for a suitable place to spin its web. While they walk, the red markings are therefore visible to potential predators. Indeed the placement of the red markings do suggest that they serve to enhance the survival of the spiders. As the spider matures, the red markings on the back are lost and the red markings on the abdomen appears.

Data also suggests that avian predators tend to avoid them because they can get sick eating the spiders because of their toxin. Therefore a good case can be made that the black widow spider evolved its red marking to warn potential predators that it is not good to eat and should be avoided. As to how such markings could have evolved from presumably nothing, we simply do not know. a mutation is probably how it first evolved. The first spider that evolved such a marking may not have any advantage over others of the same species that are less conspicuous, because they are probably more visible to predators. gradually, however, as predators eat the spiders and get sick or die, then the markings may become advantageous because those predators that survived will (hopefully) learn to avoid them. Those that did not survive will also not be a threat. Therefore as time goes by, the spiders with the red markings will become more common because they are preyed upon less often by predators. The shape of the markings will also evolve. Less effective or less conspicuous markings may be eliminated by natural selection so that the markings, through random mutation, may finally become their current shape.

Just a color pattern that adapted genetically to ward off possible predation. It does make identifying them easy being a consistent pattern.

Ever see the very pretty orchard spider, some people think it means danger, but they are harmless.

Google peacock spider images, a way cool spider.

Wanda, my pet western black widow:

https://www.flickr.com/photos/32083154@N02/3965620...

http://www.dailymail.co.uk/sciencetech/article-347...

It's a mark distinctive enough to warn off bird predators, but still difficult for insect prey to spot.