IS THIS A DAGGER THAT I SEE BEFORE ME?

I spotted this beastie this afternoon, which turns out to be the caterpillar of the much less colourful Grey Dagger moth (Acronicta psi). My insect book has the intriguing information that "this moth is impossible to separate with certainty from the Dark Dagger (A. tridens) without examining the genitalia" but doesn't go on to explain why or, more interestingly, how. Perhaps moth sexing is a job that some of my friends in the Arts world could consider retraining for in these crazy times.