Appearing crystalline and utterly unreal, this spun glass slug caterpillar, Isochaetes beutenmuelleri, looks like something created by Swarovski rather than a living, crawling caterpillar. We can see right through its feathery appendages, and it appears to have balls of molten frosted glass attached to its dorsal (top) surface.
In marked contrast to this beautiful - at least to my eyes - caterpillar, the adult is a rather Plain Jane little brown moth - certainly nowhere near as extravagant as its fascinating larva.
Looking down on the animal, we can see right through the body and to the innards. That dark stripe running the length of the caterpillar is the gut. Leafy matter enters the anterior end, gets digested as it passes down that dark tract, and what's left emerges from the posterior end as frass - caterpillar poop.
Spun glass slugs consume the foliage of beech and various oak trees, so that's where you'll want to look. They're normally going to be found on the underside of leaves, and like most other caterpillars, are most likely to be found at night.
Thanks to John for sharing this incredible discovery and his great photos.