I always look at milkweed plants for monarch caterpillars. My success rate is about .001 percent, but today I got lucky and found one on a plant right next to the trail. Monarchs lay their eggs on the underside of milkweed leaves and the caterpillar eats the leaf when it hatches. Milkweed is toxic and the caterpillar becomes toxic to would-be predators.
There were a ton of monarchs flying around too, as seen below.