|Photo by Marty Horowitz 11/12/2016|
Empress Leilia butterflies are probably just waiting (not dueling) on this rock. And they might all be males. From the link above (Butterflies and Moths of North America):
Life History: Males perch most of the day to watch for females. Eggs are laid in groups of 10-15 on the top of host plant leaves. Third-stage caterpillars hibernate.
Flight: All year in South Texas, April-November in Arizona.
Caterpillar Hosts: Tree Celtis pallida in the elm family (Ulmaceae).
Adult Food: Sap and dung, occasionally flower nectar.
The host plant for the larvae (caterpillars) is our favorite Desert Hackberry (Celtis pallida). As far as I know from Joan Tedford's most up-to-date list, this plant is in the Cannabaceae (Hemp) family (not the Elm family).
As Fred tells us in this post, the Empress Leilia caterpillars use only this plant as their host.