Long tailed bombyx larvae (Trichiocercus sparshalli)

I had heard about these larvae clustering like sawfly larvae but have not seen it before. I suspect they only do it when very young as older specimens I have found are always solitary. These were in a tight group of about 50. This species was originally described from a single moth found in 1821 in England but has never been seen outside SE Australia since... a single reverse convict.