The Costa Rican Leaf Moth: when the caterpillars of this species feel threatened, they assume a defensive pose that mimics the appearance of a snake
Above: an Oxytenis modestia caterpillar mimicking a snake above a hummingbird's nest
The Costa Rican leaf moth, Oxytenis modestia, is an excellent mimic at every stage of its development.
Above: the late instar caterpillar of Oxytenis modestia
The newly-hatched caterpillars have a bumpy, brown-and-beige appearance that mimics bird feces (complete with fake seeds) while the older caterpillars have greenish-yellow bodies with markings that allow them to mimic a rolled-up leaf.
Above: the younger caterpillars mimicking bird droppings
As they reach maturity, they develop a flared thorax and a pair of false eyespots that mimic the appearance of a snake. These eyespots are generally hidden when the caterpillar is at rest, and they're exposed when it assumes its defensive pose.
Above: the older caterpillars displaying their eyespots
When the caterpillar feels threatened, it enhances its snake-like appearance by lifting its upper body, inflating the area around its head, and then swaying back and forth in a serpentine fashion.
Above: the defensive pose of Oxytenis modestia
The caterpillar's defense mechanism can sometimes backfire, however. This article describes a "stand-off" that ensued when an Oxytenis modestia caterpillar assumed its defensive pose right next to a hummingbird’s nest:
One of us noticed the hummingbird acting in an agitated and aggressive fashion. We looked for an assumed snake, but found instead the caterpillar, which was feeding on a leaf immediately above the nest.
Above: the caterpillar mimicking a snake right above the hummingbird's nest
In its defensive pose, the caterpillar bears a striking resemblance to a green parrot snake, which is a species that happens to feed on hummingbird eggs and hatchlings.
The caterpillar was mimicking a snake because it felt threatened by the hummingbird, and the hummingbird then became aggressive because it thought that the "snake" was attacking its nest:
When the caterpillar became agitated, it whipped out the false snake head, making the hummingbird feel threatened. And as the hummingbird’s attack intensified, the caterpillar became more snake-like—lifting the false head up, flaring out the sides, and swaying it back and forth.
Above: the hummingbird attempting to defend its nest
Thankfully, both of the animals survived the encounter (not that the hummingbird’s safety was ever in question):
The scuffle lasted for at least half an hour, and it only ended when the seemingly unharmed caterpillar crawled away—not because it took the high road, but most likely because its appetite kicked in.
The genus Oxytenis contains several other snake-mimicking species, all of which assume a similar defensive pose when threatened.
Above: more snake-mimicking caterpillars from the genus Oxytenis
The adult moths engage in mimicry, too, as they have a flat, brownish-beige appearance that mimics dead leaves.
Above: the adult forms of Oxytenis modestia and Oxytenis albilunulata
Some of the other snake-mimicking moths that have been featured on my blog include Hemeroplanes triptolemus, which has a stunningly realistic disguise, and Abbott's sphinx moth, which just looks like a tiny cyclops.
Sources & More Info:
Ecological Society of America: The Ultimate Stand-Off: Hummingbird vs. Caterpillar
Ecology: Antipredator Behavior by a Nesting Hummingbird in Response to a Caterpillar with Eyespots
iNaturalist: Photos of Oxytenis modestia
Audobon: Misunderstanding Between Caterpillar and Hummingbird Leads to High Stakes Face-Off
100 Butterflies and Moths: Oxytenis modestia
Journal of Research on the Lepidoptera: Oxytenis naemia, a Tropical Caterpillar that Mimics Faeces, Leaves, and a Snake