Processionary Caterpillar train 5, Noosa National Park
I watched this group of caterpillars stop, and then reform and proceed onward.
Here is another instance of these caterpillars from a previous days tramp
Excellent reference from Don Herbison-Evans and Stella Crossley here is what they say about the procession:
"Each Caterpillar when it walks, leaves a trail of silk from its spinneret near its mouth. When a Caterpillar of this species encounters such a silken trail, it starts to follow it.
Thus the Caterpillars when they walk are inclined to follow each other, nose to tail, like a minature freight train."
Ochrogaster lunifer (Herrich-Schaffer, )
Update 9 May 08 - Here is a great reference on social caterpillars from Terrence Fitzgerald and another reference on Pine Processionary caterpillars that I got from a blog by Andy Dean of the NC State Insect Museum on cycloalexy. Thanks Andy.