There were a number of these fragile-looking butterflies fluttering around the backyard. This is the Dog Face Sulphur.
They are indeed fluttery, constantly in motion. Those wings are always going, so it was really hard to get any good pictures of them. They just wouldn't pose.
They are called Dog Face because of the pattern on their wings. That dark spot represents the dog's eye.
I was able to get some fairly clear ventral views of the small (2 - 2 3/4 inches wingspread) butterfly, but...
...this is typical of my dorsal shots. Very blurry.
Image by arizonensis.org.
So, I resorted to this image from the internet to show you the dog face pattern. Think of a poodle with its mouth slightly open. See the image on each wing?
These butterflies are supposedly around this area from March to December, but I really only see them in my garden in the fall.