Nicholas Brandley, a teacher at the Colorado College, ran an experiment to find out why the black widow spider sports a bright red belly.
Bright red coloration is one way animals scare off predators. Black widow spiders are usually fully red with black spots. But the North American black widow, on the other hand, is the opposite. These spiders are usually fully black and have an “hourglass” red spot on their bellies. Some species of black widows also have red spots all over their bodies. Brandley thinks these spots are to ward off birds, but he said he wonders why the spiders aren't redder or fully red.
For the first part of his experiment, Brandley went to Duke University to print out 3D models of black widows. With some of the models, he replicated the red hourglass on their bellies. With others, he left them plain black. He put both types of spiders on bird feeders and awaited the results. Those without spots got attacked by birds more often than those with spots. He hypothesized that the spots were there so that birds wouldn't attack, because they could see the spot better than insects.
For the second part of the experiment, Brandley put real spiders in a terrarium and studied how they made their webs. Black widows with more red spots spun webs higher up than the other spiders. Brandley said he thinks that's because these spiders have predators, both high and low. Spinning webs so high, they might miss out on prey. Brandley figures they are willing to lose a potential meal rather than to become one.
Even though it was dangerous to work with black widows, whose fangs contain chemicals toxic to humans, Brandley reassured himself and others that he wouldn't get hurt because “Their fangs don't even usually pierce human skin!”
[Sources: Duke Today; The Smithsonian]