The 2 photographs here are Soft Tree Ferns, Dicksonia antarctica. They have fallen or bent into an arch. On reaching the ground, the stems turn vertical and continue to grow and hence having the appearance of giant caterpillars crawling on the forest floor.
Although Dicksonia antarctica is a common species in Australia, seeing them in a caterpillar state is quite rare.
But in Dicksonia Forest Ravine, there is a forest of Dicksonia antarcticas, and many of them are in this caterpillar state. More photosgraphs of these caterpillars, and a trip report to the ravine are in my blog:
http://mntviews.blogspot.com.au/2016/10/dicksonia-forest-ravine-nsw-blue-mts.html
By the way, the ravine is like an enchanted world, like a mythical fairy land. You should visit it if you have a chance.
(Dicksonia Forest Ravine is in the north-western corner of Blue Mountains National Park, NSW, Australia. Click on the above blog link for the precise location.)
Please visit my main page at http://mntviews.blogspot.com/
Perga affinis (Spitfire Sawfly) ...
Are they have mass orgy sex ?
1)
Photo taken in 2006 January
on Staircase Spur trail
while climbing Mt Bogong, Vic, Australia
Trip report is in this link:
https://mntviews.blogspot.com.au/2009/04/bogong-vic-australia.html
2, 3)
Photos taken in 2011 March on Staircase Spur trail
while climbing Mt Bogong, Vic, Australia
Trip report is in this link:
https://mntviews.blogspot.com.au/2011/03/bogong-vic-australia-west-peak.html
Spitfire Sawfly are not true caterpillars.
They are the larvae of a wasp,
or more accurately a Symphytan.
These larvae are gregarious.
They are often found in a mass
of many individuals
on the ground or on Eucalyptus tree trunks.
When disturbed,
they exude a nasty smelling fluid
from their mouths.
This latter habit has given them
the common name of "Spitfires",
although they do not actually spit the fluid,
just dribble it.
The larvae also are inclined to tap their tails
in some form of communication with the group.
Please visit my main page at
https://mntviews.blogspot.com/