There are three (3) closely related Emperor butterflies in the United States, the Asterocampa butterflies.
The most commonly seen Emperor is the Hackberry Emperor, Asterocampa celtis. It found in 40 states or more, mostly absent from the northwestern USA. Had one, a fresh one, in my yard, yesterday.
Less common is the Tawny Emperor, Asterocampa clyton, usually seen east of the Mississippi River, ands in 4 states west of the River.
Less common again is the Empress Leilia, Asterocampa leilia, known in 3 states bordering Mexico.
This one seen here is an Empress Leila. One of the amazing butterflies that I saw in that certain arroyo (boulder strewn dry creek bed). We played tag for quite a while until it finally relented, and agreed to allow me a handful of camera clicks. The Leilias I saw on those several trips to the arroyo never opened their wings for me, preventing me from sharing whether or not they were male or females.
Spending any time in an arroyo is not a good idea. A flash storm miles away can send a wall of water crashing towards you, and . . . . Now that I quietly reflect on that, I kinda feel like . . . .
White Tank Mountains Regional Park, Arizona.