Caterpillars of a number of large moth species in the genus Wiseana are commonly called 'porina'. They're important pests of New Zealand pasture, but farmers aren't without a few natural allies. This is one: a big tachinid fly in the genus Protohystricia. Its larvae parastise porina caterpillars, but to the best of my knowledge, no one knows how they affect porina populations. My guess — and I stress it's a guess — is that parasitism by these flies has no discernible impact. Still, if you live in New Zealand and see one of these — don't swat it!
[17 January 2014, Olympus OM-D EM-1, 60 mm f2.8 Macro , ISO 400, 1/500 at f5.6]
All content © 2014 Pete McGregor