These true bugs hang out in our native desert willow and mesquite trees, where they are hard to spot on the tree bark.
They are called shield or stink bugs. The "stink" name comes from the fact they have scent glands that release an odor when they are disturbed.
This one is probably Brochymena sulcata. It has an almost pleasant cinnamon odor when you pick it up.
The shield name comes from the shield shape in the middle of its back. We'd like to learn more about them, but they tend to keep quite well hidden.