Strictly speaking Wagner never wrote an aria, or an opera either. Aria and opera are Italian terms, and Wagner hated Italian opera.
Are we throwing Rienzi
(to say nothing of the early works Die Feen
and Das Liebesverbot
) out? I think most musicologists, and Wagnerians, would have to agree that those are operas - strictly speaking. In fact, I thought that the transition from grand opera to music-drama happened over the Holländer
sequence (c. 1840-1850).
But did he not write operas before he devised the term "music drama"? (Which maybe he borrowed from Italian sources, dramma per musica and all that.)
Though of course, revisionism was a favorite pastime of Wagner's . . . .
Ah, leave it to Karl to beat me to the punch. Well, my answer has a nice pedantry to it.