How to Make It
In a medium glass or stainless-steel bowl, whisk together the vinegar, mustard, 1/4 teaspoon of the salt, and 1/8 teaspoon pepper. Add the olive oil slowly, whisking.
Wrap the body of each crab with two slices of bacon without overlapping, leaving the legs exposed. In a shallow bowl, combine the flour, the remaining 1/2 teaspoon of the salt, and 1/4 teaspoon pepper.
In each of two large frying pans, heat 1 1/2 tablespoons of the cooking oil over moderately high heat. Dust the crabs and bacon with the flour mixture and shake off the excess. Put the crabs in the pans, upside down. Cook until the bacon is browned and crisp, about 3 minutes. Turn and cook until the bacon on the other side is browned and crisp and the crabs are just done, about 3 minutes longer.
To serve, toss the arugula with the vinaigrette and mound on four plates. Top the salad with the crabs.