Camouflage, part 3

To my delight, it was an oblong-winged katydid nestled in amongst the leaflets. It’s a rare occurrence to see these slow, awkward ambling insects during the day, but I frequently encounter them on the screens of my windows on warm nights in the late summer. Thousands of times every night, the tireless males stridulate by rubbing a lobed scraper on the right forewing against a toothy plate on the left forewing in the hopes of attracting a mate. The sound is further amplified by a drum-like cell called a mirror (here’s a great article on the mechanics of their sound: source), resulting in a sound something like a buzzy “Tzzzzip” (video).

In the exhaustive ritual that is courtship, males repeat this rubbing motion every few seconds through most of the night, every night for about a month. As an endurance athlete I can admire their commitment and tenacity. And as a naturalist, I appreciate how easy their courtship ritual makes them to find! I often say that during the breeding season, the pursuit of sex overrides the better judgment of animals. And while some of the other nocturnal insects of late summer are more skittish at the vibrations my feet make (tree crickets in particular), katydids seem to be unbothered by my approach and will happily stridulate while standing on my hand. But during the day without that auditory cue, it’s nearly impossible to find these critters (unless, like me, you accidentally stumble across them).

While not quite as camouflaged as some of the other leaf-mimic katydids (top prize goes to their tropical cousins: link, with special commendations to the dead-leaf katydids), my garage katydid was doing a pretty thorough job of looking quite like a leaf. This particular form of camouflage – looking like some innocuous element of the environment – is called mimesis, and is well suited to insects, who are mostly in the same size category as leaves, twigs, bark, rocks, and even animal droppings (like the black swallowtail “scatterpillar” pictured above). By imitating something uninteresting (or even disgusting) to potential predators, they are easily overlooked by visual predators.

Take for example the animal in the photo below. It’s not until this creature moves (or you get really close) that it’s easy to spot, as in the photo after.

Looks by themselves, however, are not enough. Like many predators, our peripheral vision is quite good at detecting movement, and so for any animals that utilize mimesis to appear like an object, it also needs to behave like an object. This means either not moving at all or moving on a time scale more akin to that of plants than vertebrates. That katydid was quite easy to see while it was flying rapidly through the air, but once at rest it blended seamlessly into the sea of locust leaflets. Once on the branch it was either stationary (about 5 minutes) or moved in a slow, stilted motion similar to that two steps forward, one step back gait of chameleons (video).

Camouflage is good until it isn’t. If you’ve been detected, there’s no point in pretending that you’re still a leaf, and its time for fight or flight. Most prey choose flight. Carolina locusts, for example, spend most of their time on exposed rocky soils or sandy soils, looking quite drab while at rest with a mottled gray and brown body. But get too close to a locust and it will spring to life, exploding up into the air in an erratic frenzy of flapping wings. As it flies, it exposes a dazzling yellow band flanking the rest of the dark gray wing. Along with the loud fluttering sound from its flapping wings (much like the whistling noise of a mourning dove’s wingbeats), this startles potential predators and gives the locust an extra heartbeat of time to escape.