The leaves your caterpillars eat need water to stay fresh.
I usually make a container by taping together three plastic water bottles. That’s nice and stable, and it works especially well in classrooms. You can also use any kind of vase or jar.
Make sure your caterpillars can’t get down into the container. Caterpillars don’t understand water. They will crawl down a stem into the container and then sit there and drown.
When I use bottles, I stuff a paper towel around the leaf stems. If you have something with a wider mouth, you can use aluminum foil or plastic wrap, and then poke holes for the stems.
If you tape bottles together, be sure to put pieces of paper over the spots where the sticky part of the tape is exposed, to to make sure that caterpillars don’t get stuck to it.
Leaves in water will usually stay fresh for two or three days. Change them regularly. If you add fresh leaves to the cage, the caterpillars will find them. You can take out the old leaves after the caterpillars move on.
If you have to go foraging for leaves, pick a lot at once. Keep the extras in a bag in the refrigerator. They should stay fresh for about a week.
Be sure to avoid trees that have been sprayed. This is most likely to happen around malls or apartment complexes. Also steer clear of yards or other areas billed as mosquito-free.