Bug’s Color is Warning for One Predator, Invisible to Another
The Hibiscus Harlequin Bug (Tectocoris diophthalmus) would seem to be in a bit of a pickle. It has to avoid two different predators with two different defenses that seemingly contradict each other. But the bug has an elegant solution and makes its brilliant, beautiful shell pull double duty in a way that we can’t easily see.
One predator the bug has to deal with is birds, which will try and eat them. But birds learn that's a bad idea pretty quickly: The harlequin bug is related to stinkbugs and is loaded with chemicals that birds seem to find disgusting once they actually get a taste. After just one or two run-ins, a bird will be conditioned to avoid the bugs. This isn’t the case for one of the harlequin’s other predators, though. Mantises seem to have no problem with the way harlequins taste, and will eat them with no second thoughts.
To deter birds, the harlequin would do well to advertise that fact that it tastes bad with a warning signal. This is called aposematism, and often shows up in animals in the form of conspicuous colors or patterns (for example, the various bright shades of poison dart frogs). Since their flavor isn’t a turn off to mantises, the harlequin's best bet is to hide from them entirely with camouflage. There’s the rub: To stay out of both birds’ and mantids’ stomachs, the harlequin bug needs to be both aposematic and cryptic, conspicuous and inconspicuous, at the same time.
How can it do that? Well, what’s conspicuous and what’s not is in the eye of the beholder. The same thing can help the harlequin stand out or hide, depending on the animal that’s looking at it and how that animal’s visual system works.
For the harlequin bug, the trick lies in the color orange. The bug’s shell is pale to bright orange all over and sometimes its back is spotted with iridescent blue-green patches. To birds, with their sharp eyes and good color vision, an orange bug sitting out on a green leaf is hard to miss, and the speckles help them stand out even more. At the same time, an orange bug on a green leaf is actually hidden from mantises. This is because the mantid visual system is much different than a bird’s or a human’s. Their color vision isn’t very good and the world as they see it is pretty monochromatic. They’re sensitive to green, but orange and red don’t stand out all that well. They rely on movement and differences in brightness to differentiate objects. Thus, an orange harlequin bug not doing much else to draw attention to itself won’t stand out, to the mantid’s eye, from a leafy green background.
By donning the right color, the harlequin bug can have it both ways and advertise its defenses to one predator while camouflaging itself from another.
A pair of Australian biologists, Scott A. Fabricant and Marie E. Herberstein, found this out when they modeled how plain orange and spotted harlequin bugs would look to a mantis’ eye. Their model showed that orange bugs were basically indistinguishable from the background, while the speckled bugs stood out a little more. Live mantises, meanwhile, showed that they could detect the spotted bugs from about a foot away but had to be right on top of the plain orange ones before they noticed them. In a second experiment, mantises were placed on a branch that forked and led to two leaves, one with a plain bug on it and one with a spotted bug. The mantises went after the spotted bugs more often, but when the choice was between a plain orange bug and an empty leaf, the mantises acted like there was no food at at all.
While orange hides the harlequin bug from mantises, its shiny spots can still give it away. The researchers think that this flaw in its camouflage explains the variation in the bug’s appearance across its range in Australia. In areas where mantises are its main predator, the bug may be under pressure to maintain a pale orange with little or no spotting, but is free to sport a brighter shell with more, larger spots in areas where birds are the bigger concern. Where the two predators overlap, a little spotting and the right shade of orange can help it both be seen and hide in plain sight.