Peel beets and cut into 1/2 inch wedges. In a steamer set over boiling water steam beets until tender, about 10 minutes, and transfer to a bowl. Discard coarse stems from arugula. Wash arugula well and dry. In a bowl whisk together vinegar and salt and pepper to taste and whisk in oil until emulsified. Pour half of vinaigrette over beets and toss well. To vinaigrette remaining in bowl add arugula and toss well. Arrange beets and arugula on plates.