I spotted this lovely white Caterpillar crawling along the deck railing the other day and tried to get a decent picture with my cell phone. Something told me not to touch it. I was never squeamish about picking up caterpillars, but have found that I have a nasty reaction from the slightest touch of a Gypsy Moth Caterpillar - so I am more careful now. Besides those long black guard hairs sometimes indicate the 'stinging' type and as I tried to get close enough to snap a pic, it seemed quite aggressive and kept rearing up towards my hand. Pretty feisty for a little guy.
Upon investigation, at Wikipedia, I find that it is indeed one of the top poisonous caterpillars. Caterpillars represent the larval life stage of a Moth, and acquire chemical defenses from their host plants on which they feed. This caterpillar primarily consumes hickory, pecan and walnut leaves, but will also eat those of ash, elm, oak, and willow, among others. Their feeding occasionally causes local defoliation of nut trees, but usually doesn't cause lasting damage.
The long black hairs, two near each end, cause itchy rashes in some people. And these caterpillars can also bite. Mature caterpillars are found from July to September, and feed in groups of 100 or so in the early period, skeletonizing the leaves. They become solitary later.
It's cocoon in the pupal stage is woven loosely with hairs in it, is dark in color and is found in the leaf litter over the winter. In spring Lophocampa caryae emerges, also called the Hickory Tussock Moth or Hickory Halisidota. It's Fore-wings are yellowish-brown, marked with white blotches with a stained glass appearance. The hind wings are mostly white. The body is 'hairy' and pale brown or tan. They fly as moths through May and June.
(This photo from Wikipedia)
From North American Moths, I read that "The adults of most species of moths don’t eat – they don’t even have mouth parts, and for the duration of their adult life they live off of the fat reserves they built up as a caterpillar. In these species, the adult moths will live for only about a week or so, during which their main purpose is simply to seek out a member of the opposite sex and reproduce."
They lay their eggs on the undersides of leaves, where the larva can find nourishment upon hatching.
At the American Journal of Emergency Medicine you can find information on the treatment of poisonous reactions to this caterpillar.
Also check out The 20 Most Toxic Caterpillars, accompanied by beautiful photographs of each.