The scientific name of this species is Naryciodes posticalis, and it can be found only in Japan. It belongs to the Limacodidae family of moths, which are commonly known as slug moths, because their caterpillars often have a gelatinous, slug-like appearance. That name seems a little ironic in this case, because Naryciodes posticalis actually mimics a snail.

Above: Naryciodes posticalis

Each caterpillar has a bulge in the center of its body, with colorful markings that form three concentric layers on its back. Those features mimic the appearance of a snail shell, which might help to deter predators. The caterpillar is also equipped with two short, stubby appendages that resemble eyestalks, but they're actually located at the back of its body.

Above: this photo shows the underside of the caterpillar's body, with its mouth slightly visible on the right

According to this article (translated from Japanese):

Even during the final instar, the caterpillars' body length remains quite small—approximately 8 mm—and because their morphology resembles that of a snail, I believe that they are often overlooked. Since their host plants are abundant along mountain streams and similar areas within the Kanto region, the larvae are most frequently discovered in such environments.

Above: a close-up of the caterpillar's head

Despite my best efforts, I couldn't find any other information about these strange little creatures, and I get the sense that the entire genus is still poorly-studied.

Sources & More Info:

iNaturalist: Naryciodes posticalis

Kawasaki Organization for Nature Research and Conservation: A Record of Naryciodes posticalis in Ikuta Ryokuchi Park (PDF)

Zootaxa: A New Species of Naryciodes from Korea

i've spoken at length about how immobile and unresponsive Charaxes jasius tends to be but they do adopt this funny tiptoeing stance when nobody's around for some reason (note how not only are the legs not touching the leaf, even the anal prolegs are raised) and quickly abandon it if you approach. normally you have all the time in the world to take photos of them but if you want to capture this specific behavior you practically have to sprint towards the host plant