Melanotus click beetles

Don Stenhouse: I wouldn't rely on pronotum / elytra proprtions to separate these two species. In lieu of the aedeagus I find the form of the pronotum is best. That of castanipes is less elongate, more indented and flatter with a depressed front margin, which some people regard as a distingushing feature in itself. You can recognise a good example in the field. But it ain't always that certain.

Pictures of male aedagus available here