Earlier today I was checking in at blogger pal hardworking hippy's La Ferme de Sourrou. She has some interesting photos of Processional Caterpillars in the woods near her home in France, and also explains the extreme dangers to animals, children, and some adults: serious life-threatening danger from just touching the caterpillars or even inhaling their hairs. Read her post Processional Caterpillars in the Woods.
She also gives a link to her blogger pals at Chez Francois et Danielle, whose post Poisonous Caterpillars goes into great detail, along with photographs of their poor dog's mouth after he ate some of the caterpillars (his tongue and mouth swelled). They rushed him to an emergency vet, and he lived; however, part of the dog's tongue rotted out within a couple of days (photos of this, as well), and he now has some difficulties eating and drinking.
hardworkinghippy was also kind enough to steer detroit dog to a SUNY Cortland website about a variety of social caterpillars, including photos and a link to the Pine Processionary caterpillar.
The caterpillars, which live in host Pine trees, are found in Europe, the Near East and North Africa. They are called Processional because they follow each other literally "head to toe" from one tree to the next.
I will try to find out if they also populate the Americas, and will update this post if so.