The mere mention of the name black widow strikes fear into most humans, although such reputations carry little weight in the animal kingdom, which is why methods to signal one’s identity are so important. Accordingly, the iconic red and black coloration of black widow spiders appears to be specifically designed to catch the eye of predatory birds in order to ward them off, while remaining invisible to "eavesdroppers" from which the deadly arachnids prefer to disguise their identity.
Black widows display a visual signal in the form of a distinctive red hourglass-pattern on either the ventral or dorsal side of their bodies – meaning the underside or the back. Called an aposematic coloration, these are bright, conspicuous markings that warn predators of an animal’s lethality, advising them to keep their distance. However, it has never been scientifically established whether these markings are intended exclusively for would-be predators, or are designed to catch the attention of other creatures as well.
Should these colors be visible to prey, for instance, it would most likely place the spiders at a disadvantage by warning potential victims not to come too close. Therefore, a team of researchers from Duke University devised an experiment to test this possibility, and to gain a better understanding of how these colors are used to communicate with other animals.
To do this, they first measured the wavelengths of light reflected by the bodies of two species of North American black widow. Based on the types of photoreceptors present in the eyes of birds and insects, they were able to conclude that the wavelengths of light reflected by the spiders’ red patches are too long for insects to be able to detect, but are visible to birds.
Reporting these findings in the journal Behavioral Ecology, the study authors conclude that the red markings on black widow spiders’ bodies are indeed exclusively aposematic, as they carry important signals to predators while remaining invisible to prey.
Interestingly, some species of North American black widow carry dorsal markings, while others have ventral markings, and others have both. To investigate the effect of these variations, the researchers set up an experiment whereby fake black widows – some of which contained red spots on their backs while others did not – were placed in a bird feeder.
Noting that birds were 2.9 times less likely to enter the feeder when faced with red-stained spiders, the study authors were able to confirm that these crimson patterns are indeed effective aposematic signals. However, this raises further questions about why some species of black widow have markings on their backs, while others only have them on their underside.
To solve this riddle, the researchers paid attention to how the spiders build their webs, and found that those with markings on both sides of their bodies tend to spin their webs much higher above the ground than those that lacked markings on their backs. Because spiders normally have their backs facing the ground when they are "at home," it makes sense to have ventral colorations in order to ensure that birds are able to see these markings from above.
However, spiders that live close to the ground are less likely to be attacked by birds from below, thereby removing the need for dorsal aposematic markings. In contrast, those that spin their webs high up can be attacked from both above and below, and therefore display deadly flashes of red on both sides of their body in order to make sure all predators know exactly who they are dealing with.