Aposematic coloration is the technical term for the coloration phenomenon found in such illustrious animals as poison dart frogs (Family: Dendrobatidae) and skunks. The rationale is that the animal makes itself very conspicuous in an attempt to signal a ‘warning sign’ to potential aggressors. In order for it to work, the aggressor must be able to recognize the signal and exhibit a corresponding avoidance response; often as a learned behavior based on past unpleasant experiences.
But what happens when the aggressor is colorblind? In this case, the aggressor would likely have to rely on recognition of the coloration pattern rather than the actual colors. But let’s take things a step further and ask what happens if the aposematically-colored organism has two types of predatory aggressors and one of the predators is colorblind and the other is not? Even better still, what if the colorblind predator isn’t affected by the aposematically-colored organism’s defenses at all?
Enter the curious case of the harlequin bug Tectocoris diophthalmus, a shieldback stinkbug that can exhibit a coloration anywhere between solid bright orange to bright orange with iridescent blue patterning. Life is difficult for this brilliantly adorned Hemipteran. It must contend with both avian predators and formidable Hierodula majuscula mantids in its environment.
Birds, which possess the visual capabilities required to detect aposematic coloration, avoid the flashy harlequin bug like the plague. Birds seem to find the taste of T. diophthalmus to be absolutely repugnant and it only takes one or two exposures before the avoidance behavior is engrained. H. majuscula mantids, on the other hand, are unfazed by the same defenses that make T. diophthalmus unpalatable to birds and will happily consume these harlequin bugs if given the chance.
To properly defend itself against the birds, T. diophthalmus must exhibit aposematic coloration yet if it has any chance at all to avoid detection by the mantids, it needs to rely on cryptic coloration (i.e. camouflage). But how? The answer is: the color orange. Orange is, of course, a profound aposematic indicator with respect to avian aggressors, but it also retains a high degree of utility in terms of confusing mantid vision.
Mantids possess monochromatic vision with peak sensitivity in the ‘green’ region of the spectrum. Thus, to H. majuscula, a flat orange harlequin bug likely appears as though it is a sort of ‘yellow green’ color with little contrast with respect to the surrounding foliage. Behavioral experiments conducted by Scott A. Fabricant and Marie E. Herberstein seem to support this theory. The researchers determined that H. majuscula would approach iridescent T. diophthalmus harlequin bugs more often than solid orange ones, suggesting that the solid orange coloration has a cryptic effect from the perspective of the mantids. The researches did, however, also note the cryptic effect seemed to diminish when the mantids got into a closer range, yet the would-be predators certainly had difficultly distinguishing between unoccupied leaves and leaves occupied by orange harlequin bugs at longer search distances.
By employing a solid orange coloration, T. diophthalmus is able to simultaneously exhibit aposematic and cryptic coloration when the sum of their potential aggressors is considered. This phenomenon also suggests that T. diophthalmus populations in areas heavily influenced by avian predators may exhibit a more ‘patterned’ coloration whereas those populations subject to predation pressures from mantids may exhibit a more ‘uniform orange’ morphology. Fascinating!
References and Further Reading:
- Fabricant, S. A., & Herberstein, M. E. (2014). Hidden in plain orange: aposematic coloration is cryptic to a colorblind insect predator. Behavioral Ecology, aru157.
- Fabricant, S. A., & Smith, C. L. (2014). Is the hibiscus harlequin bug aposematic? The importance of testing multiple predators. Ecology and evolution, 4(2), 113-120. (Open Access)