Researcher Stan Gordon wrote a piece for the Paranormal News detailing a sighting which took place on June 27, 2009. A number of motorists travelling near Mt. Pleasant, in Westmoreland County, Pennsylvania, reported to Gordon the sighting of a creature which "looked like a giant caterpillar". It was about 7 inches in length, tapered at the ends and thicker in the middle, and appeared to be segmented. It was a blue-white color but the witness noted that it was not luminescent.
Within a few moments, the four witnesses encountered a second worm... and another... and another. In all, six or seven individuals were seen both in the road and beside it.
In September the same year, Brian Seech of the Center for Unexplained Events forwarded a second report to Stan Gordon, this one originating from Youngwood, again in Westmoreland County. This one was of what the witnesses first took to be a snake, about four feet long which before their eyes "broke" into eight smaller creatures anywhere between six and eight inches in length. The witnesses reported that each of these creatures seemed to have four legs, and was of a dark color, with a "shiny wet" texture.
Then, they claimed, the smaller animals assumed a ring-like position and then reformed into a large snake and slithered away.
Chad Arment has noted that the so-called hickory horned devil, the caterpillar of the regal moth, is the largest variety of caterpillar in Pennsylvania and can grow 6-7 inches long. The larvae have a greenish coloration and segmented appearance much like those in the first sighting. The witnesses did not to my knowledge report any sort of horn-like appendages that these larvae sport. Many people on websites note sightings of hickory horned devils from roadways and also note that they are highly visible from a distance.
As for the second sighting, Arment notes that there are a number of species of caterpillars which migrate in what is called a processional pattern; processionally-migrating caterpillars can at times be difficult to distinguish from one another, possibly giving the appearance of a snake. None of these processional caterpillars are known to exist in Pennsylvania. He does believe it possible, however, that there could be some present. The most famous is the pine processionary caterpillar. It is interesting that another detail of the second sighting claimed that after the snake broke apart, its components formed a ring-like pattern before it reformed; just such a pattern is shown in the photos on this page.
I had at first been thinking that the second sighting, at least, may have had some sort of mammalian explanation, rather than an entomological one; the wet-shiny appearance, four legs and dark color practically screamed otter or weasel (the barely 7-inch least weasel is native to the western half of the state). But these revelations (to me) of caterpillars of a similar size and appearance and that there are similarly-behaving caterpillars to the second sighting leads me to wonder whether there may be some as yet unidentified moth native to the state.
And obviously, such a thing would not be outside the realm of possibility at all!
I am in contact with Stan Gordon and will let you know if anything more develops.
Apologies to E.F. Benson for the title of this post, which is shamelessly ripped from the title of one of his stories.