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Thanks Cosmin, Jorge, Chris. Sensory seems to be one function but I can't see why they would need such large spines. There has to be a 'more exciting' explanation. This species is a parasitoid of Arctiid caterpillars which in many species are excessively hairy.
Fly is probably Hystricia abrupta
In line with my earlier posts could I venture to say "Our Tachinids may not be bigger than your Tachinids, but they are a lot more spikey":D:D:D