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We found two of these last weekend - one on the ground next to its hole and one half out. Of course the casings were empty, and we guessed that they had been the pupa of something large, as they were around three inches long.
Friend
Snail came to the rescue and told me they were very likely to be goat moths - or were goat moth pupa before the moth came out and flew away. That sent me Googling to discover that the larva is from the witchety grub family (of edibles!). I can see why a few of these would make a good meal. If you could eat them raw, as the Aborigines do.
Apparently the goat moth larvae like wattles - or acacias - and the larva burrows deeply into the trunk of the tree. When it's ready for the next stage, it burrows down into the ground where it stays until "hatching". Often it doesn't come completely out of the ground, but leaves the casing behind (as in the photo above).
I imagine these are big moths, although I couldn't find anything that told me their average wingspan, but we'll be keeping a lookout for them at dusk.
This might also explain why so many wattle trees up there tend to fall over!