After I finished walking the River Trail at DGCA and had gotten back to my pickup truck, I took off my Camelbak and laid it in the bed of the truck. When I came back around a moment later, a butterfly had landed on the pack and was giving it lots of attention. I had no idea what it was then, but the photo opportunity was excellent because the butterfly was hardly moving.
I did some searching this morning and have determined that this butterfly is a Hackberry Emperor. One of the interesting things I read was that this butterfly likes to "take sweat from humans", which probably explains why it was so interested in my pack.