If you ever have a chance to visit a humid, tropical area in South or Central America, make sure to bring a good flashlight or a headlamp with you, because if you go out into the forest at night, you may be rewarded with the discovery of one of the most amazing animals that this part of the world has to offer – the True leaf katydid. Don’t bother looking for them during the day, though – they will be sitting all around you, but you will never find them. They only way to notice them is to look for the movement of their long antennae and legs as they slowly munch on leaves at night.
True leaf katydids, which belong to a group known as the Pterochrozini, are probably the best mimics of plants among all insects. Their mimicry is by no means restricted to resemblance of plain, green leaves – that would be too easy. No, their bodies are perfect replicas of leaves that have been chewed up, torn, rotten, dried up, partially decayed, or covered by fungi. Some even have fake holes in their wings (fake, because the holes are in fact thin, translucent parts of the wing membrane.) Their mimicry is so exquisite that it once fooled a famous evolutionary biologist into clipping a part of the katydid’s body to prove to me that it was lichens growing on its abdomen. Alas, it wasn’t, and hemolymph was spilled.
But the best part is that no two individuals are alike, and within a single population you can find individuals whose appearance is so dramatically different that one would feel justified to place them in different species. Not surprisingly, entomologists did – South American Peacock katydid (Pterochroza ocellata) was described under thirteen different names, until a captive breeding program, initiated in 1999 by entomologists Serge Xiberras and Pierre Ducaud, incontrovertibly showed that all of these “species” can appear in the same brood. The situation is not much different for other species in this group of insects: nearly 22% of all species of Pterochrozini were “discovered” and described more than once under different names. Why such polymorphism? There are plenty of plant-mimicking insects, but none show a similar diversity of forms within a single species.
The answer, of course, has to do with the predators who target these insects. They are primarily monkeys, especially tamarins, who actively search for katydids by unfurling leaves and systematically combing through the vegetation. In some species, katydids can constitute over 80% of their diet. Primates are very smart animals, and even the best mimicry would not help if monkeys were able to form a search image for a particular type of a fake “leaf.” But when every individual in the katydid population looks slightly different, then the task of finding them is much more difficult.
Some true leaf katydids have a secondary line of defense and if attacked by a predator they suddenly reveal brightly colored hind wings, which often seem to form an image of a head of a much larger animal. This type of display is incredibly effective in repelling at least some primates – when the first Peacock katydid I ever saw flashed its wings at me and started jumping in my direction, I stumbled back and fell over. Of course I knew that the animal was harmless, but the hard-wired fearful response to a sudden appearance of a big animal is something that we simply cannot control, and these katydids take full advantage of it.