Processionary caterpillars

Several British news sources posted the above photo of caterpillars crossing a road -

This incredible convoy is the ultimate commuter train - a trail of tiny caterpillars heading off into the bush. An amazing 136 caterpillars wriggled top-to-toe across a road on their migratory route in Kruger National Park, South Africa. The delicate insects were linked by a single thin silk thread which set their path out into the bush, where they will eventually turn into butterflies. Jamie Rooney, from High Wycombe, Bucks, was visiting South Africa for the World Cup when he snapped the crawling convoy.

They didn't offer any details about the biology, which took me a while to track down.  They appear to be "processionary caterpillars."  They are the larvae of moths (not butterflies), have nasty urticating hairs, and are capable of defoliating their host plants.  The silk on the trail is for grip, not for guidance, which is determined by pheromones and by tactile stimulation by the setae of the preceding caterpillar.  More info and pix at this link.

Photo credit.

Addendum: Found quite a bit more info at this Catalonian site, including this cautionary observation -

Veterinary services have many emergency calls at the time when the caterpillars are migrating to the ground as inquisitive dogs can get too close to the intriguing procession and may pick up the hairs onto their paws, these irritate and so they lick them. Once the hairs are on the lips/tongue it will induce itching, swelling and possibly vomiting. Look out for the symptoms of : small white spots in the mouth and on the tongue, excessive drooling and chomping. In some cases partial amputation of the tongue is the only course of action.

Addendum:  Reposted from 2010 to add this video via Nothing To Do With Arbroath.

YouTube link.