40 mm length, Arches tail upwards in curve when alarmed in the same way as a devils coachman.
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I believe several terrestial beetle larvae have this threat posture reaction, some times emiting brown fluid from the mouth
Many thanks for the comments. I can see the resemblance but was thrown by its size - considerably larger than black adults I have seen. Just measured it again and it is 47 mm long
I would venture to suggest that the larvae of most insects tend to be larger than the insect itself as they require food stores to provide the energy for metamorphosis the dormancy period.
Lat/Lng: 50.9, 0.1
OS grid ref: TQ5216