One reason to keep a photo journal of insects like this is so you can record what you find the insects feeding on.
This week, however, we found a caterpillar munching on our cats-claw vine.
My son suggested that because both plants have tubular flowers, long seed pods and winged seeds, that perhaps the plants are related. I looked it up, and it turns out that both desert willow and cats-claw vine belong to the family Bignoniaceae. They are related!
From now on we'll keep an eye out for rustic sphinx caterpillars on both plants.