Glen, and everyone else, Io Moths here in North America do the same thing. And they are also one of the few caterpillars here that sting. I assume everything in Australia is dangerous. I never knew they did this until I raised Io Moths from eggs. The ones I had found before were nearly full grown, and it's only the smaller ones that do the train thing. I woke up one morning and they were like this in the container. I was afraid they might start spelling me messages a la "Charlotte's Web".