Together with the ironclad beetle (Zopherus nodulosus), eyed click beetles (Alaus oculatus) were the most iconic beetles for me growing up in Texas, so a few weeks ago in Missouri I was happy to find this click beetle right on my doorstep. This species is easily recognizable from its two large eyespots on the pronotum. Many hypotheses have been thrown around regarding the evolution of adaptive value of false eyespots in insects, e.g. (1) eyespots on nonvital body regions may redirect a predatory attack, (2) the sudden unveiling of eyespots may induce a startle response in a predator, (3) eyespots may increase the bearer’s apparent size, and (4) eyespots may convey to a predator that it has been spotted, reducing the predator’s motivation to continue its pursuit. The last idea has received much less discussion in the literature, but seems promising to me. However, no behavioral data exists for click beetles to help understand functional significance of their eyespots.
For all of these potential explanations, it is advantageous for eyespots to be as noticeable as possible to effectively grab a predator’s attention. Accordingly, eyespots are often dark in the center, strongly contrasting with a bright outer ring. Eyed click beetles are no exception. In fact, the composition of the eyespots differ dramatically from the rest of the beetle’s black body, achieving almost complete absorption of light (~96%). Cone-shaped microstructures are arranged vertically within the eyespots to effectively scatter incident light and increase the amount of light transmitted into the cuticle. This results in a black coloration that appears more black than what would be produced simply from pigments such as melanin— termed “super black.” Not only is super black darker than pigment-based blacks, but it also vastly reduces specular reflection when light hits the surface at an angle. So, the click beetle’s body glints in the sunlight when viewed at an angle, but the eyespots remain uniformly black.
The presence of super black coloration is present across the animal kingdom, for instance in birds of paradise, jumping spiders, butterflies, stick insects, vipers, and deep-sea fishes— all of which are achieved through varying, though similar, structural architectures. In the former three cases, super black is presumed to enhance conspicuousness against adjacent color patches, while in the latter three it likely serves for camouflage. In predatory sea-fishes that hunt using bioluminescence, densely-packed melanosomes in the epidermis minimize reflectance from their own light lure to hide their bodies in the dark depths.. how cool is that!
If you look closely at this beetle you will notice the white ring around each eyespot is incomplete. Normally the white scales fully encircle the eyespots, so it’s likely the beetle has escaped an attack from a predator. While handling the beetle, I noticed it actually couldn’t ‘click’ when flexing its body, and while feeding the beetle it would curiously bend its body repetitively as if trying to click. It’s likely the hinge mechanism between the pronotum and abdomen was damaged. Hopefully it can somehow realign itself, so it can go on to use its click when in peril.
