It is a seed, no it is an ant, no it is a leaf, no it is a stick,… Actually, it is all of the above through a temporally spaced sequence of disguises (*): It is the Australian walking stick (Extatosoma tiaratum). This insect is indeed a master of deception. It is a fascinating example of a series of adaptations that maximized its survival by multiple versions of mimicry successfully fooling predators at every stage of their life cycle.
(*) The author wants to say: along a developmental timeline.
Mimicry is a ubiquitous survival strategy. It evolved repeatedly in so many plant and animal lineages. Nature is full of wolves in sheep clothing or vice versa, that alters perception of the other party. Plants may disguise their seeds as dung and fool insects to disperse them. Here, the walking stick gives it a different spin: its eggs mimic a plant seed with full set of adaptations to make it really convincing to ants. Adaptations include an elaiosome-like tissue (capitulum) encasing them. Ants are particularly fond of carrying seeds with elaiosomes which are rich in lipids and proteins. Once inside an ant nest (first wave of dispersal) one may think all is nice and safe. However, in an X eat Y world even that strategy can take a negative turn. Take the Alcon blue butterfly (Phengaris alcon) for instance, which adopted almost an exact kind of mimicry also known as the “cuckoo strategy” to benefit from ant defenses. However despite this elaborate effort to remain hidden, this butterfly has a cunning predator: An ichneumon (hunter of dragons in Greek) wasp has evolved pheromones that create confusion among ants to bypass their defenses. The wasp can find and parasitize the caterpillars of this butterfly before they move into the next stage in their life cycle.
ichneumon (hunter of dragons in Greek) wasp
Ants make up only 3 percent of animal diversity yet they may constitute up to half of the total animal biomass on land. Therefore forming a biological alliance with ants can be quite beneficial, but it is not a guarantee of success in survival. The Australian stick insect emerges as an ant mimic nymph (second wave of dispersal) in so many ways that even its walking patterns, posture and antenna movements all resemble a spider ant.
For instance the story of the Lord Howe Island stick insect (Dryococelus australis) is quite remarkable. These massively large insects were thought to be extinct due to rats introduced to their island by a British ship that crashed to the shore in 1918. Fascinatingly, it turned out a few individuals survived in a rather unexpected place, a rock wall cliff. They literally clung onto life under a Melaleuca howeana bush located on the World’s tallest vertical sea wall (550m high) on Ball’s Pyramid for 80 years until they became rediscovered by scientists.
The natural history story of the stick insect has been covered a few times in the past. The following section from BBC’s “Life in the Undergrowth” presents two types of deceptive mimicry. In the first, Australian stick insect eggs look like a seed encased with elaiosome-like tissue. Ants are particularly fond of carrying seeds with elaiosomes which are rich in lipids and proteins. Once inside the nest the eggs are safe and ignored by ants probably because they change chemistry. The second example has a bit more complicated natural history involving blister beetles of California deserts. Blister beetle larvae deceive male digger bees by emitting sex pheromones and use them as carriers. When male digger bees try to copulate with a female bee the larvae hop onto females and get transported to her nest where they devour on pollen and also the larvae of the host bee.
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