The black widow is among the most famous species of spiders for good reason: it is notorious for its deadly, toxic bite which can easily bring down its prey.

Just like the comic book spy who bears its name, this dangerous spider is also an expert slayer and a virtuoso at sending cryptic messages.

How so? Apparently, the black widow's red hourglass-shaped mark stands as a warning signal that is obvious to some animals but invisible to others.

The special mark on the female spider's underside alerts potential predators such as birds to keep away from the insect, unless they have a death wish, a Duke University study revealed.

But if black widows can broadcast their red mark on birds, how do they do it without blowing their cover to prey? According to researchers, the spider's secret lies in the differences in how insects and birds see color.

Scare Tactics

To test this idea, scientists first tried to figure out how birds respond to models of black widows with the special red mark and without.

Researchers had set up eight 3D printed, fake spiders one at a time, belly-up, at seven different backyard feeders in North Carolina.

Four of the fake spiders were painted all black, while the other four were painted with the red hourglass-shaped mark to match the reflectance of true black widows.

This first investigation found that fake spiders with red hourglass marks were less attractive to hungry birds. The birds were thrice less likely to grab or peck these 3D printed spiders than the all-black ones.

What's more fascinating is that bird species such as house finches and chickadees were easily spooked when they saw the fake spiders.

"The birds would see a spider model with red markings and get startled and jump back, like 'Oh no man, get me out of here,'" said Nicholas Brandley, lead author of the study.

You Can't See Me

The research team proceeded to estimate how the specific black and red hues of the black widows appeared to birds compared to insects such as beetles, ants and crickets, all of which are scrumptious meals for the lethal spider.

Birds possess photoreceptors in their eyes, enabling them to see long wavelengths of light that are invisible to some animals. Insects, in particular, can detect red light, but not as well as birds and mammals.

Brandley and his team used a spectroradiometer to determine the wavelengths being reflected by two North American black widow species. They calculated how much of that light could be absorbed by the eyes of birds and insects.

In the end, scientists concluded that birds can see the black widow's special mark better than insects. Brandley said the color contrast is more than twice as great for birds.

Look Up!

Additionally, the team suspects that variations in viewing angles also played a role.

Black widows typically hang from under their webs with their bellies face up. With this position, the special mark is more visible to birds that fly overhead, but are partially invisible from creepy-crawlies below.

Again, Duke University scientists tested the idea by letting two species of North American black widows build webs in cages that were 20 inches tall over the course of three days.

The only North American black widow species with red hourglass-shaped mark on its belly built its web furthest away from the ground matched. The mark was indeed visible to birds hunting from above or below.

Animal studies often focus on how well colors of animal warning deter potential predators, but scientists said only few have looked into whether the warning helps bearers avoid being detected by prey, competitors or parasites.

The team's findings are featured in the journal Behavioral Ecology.

Photo : T.W. Buckner | Flickr 

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