Anybody can visit insectlore.com and buy a cage, but most of us have the materials to make a cage at home. Here is one method. This is simply a gallon jar with netting on top held on with a rubber band, a paper towel in the bottom to make frass (poop) clean up easy, and a small vase with foodplant (nasturtium in this case). A fish bowl could also be used. The insects don't really need much air, so a paper towel can be used on top instead of netting. Sometimes the larvae crawl into the water and drown, so it is nice to cover the vase with a bit of saran wrap and poke holes in it to put the plant stems through. I used a pretty vase, but an old spice bottle will do. The main concern, however, is fresh host plant. Take a vase with water with you when collecting more food plant, and put the stems in water directly after cutting them. The caterpillar depends upon the plant for food and water, so it must be fresh.
Another easy cage can be made out of an old Easter basket and netting. I get netting at Long's Drug Store, but any fabric store will have it. Again, put a paper towel in the bottom to catch the frass. Then put in a vase with foodplant. And last, use ribbon or string to tie the netting to the basket. Very simple and very effective. I like this cage because the caterpillars tend to pupate on the handle. The butterflies can cling easily to the handle when they emerge, and I don't have to worry about them falling. With the gallon jar, the caterpillars can cling to the glass and sometimes form the chrysalis on the glass. But since the butterflies can't cling to the glass, they sometimes fall and are unable to expand their wings. I tape long strips of paper towel next to the chrysalids on the glass to remedy this situation, but the basket seems like a better solution.
A tomato cage and one gallon nusery pot make the framework for this cage. If the foodplant is growing in the pot, that is best, but it also easy to put dirt in the pot and put a vase with foodplant on the dirt. Insert the legs of the tomato cage into the soil and cover with netting. The netting is overlapped and tied to the pot at the bottom and tied shut at the top. No paper tower to catch frass is necessary with this cage, the poop just falls into the dirt. If you want to get fancy, sew the netting into a tube before using it to cover the tomato cage. This cage works best for tall foodplants, like fennel.
No matter what cage you use, I recommend uncovering the caterpillars regularly to get a good look at them because it can be hard to see all their markings through the netting or the glass. So take down the barriers and enjoy the beauty of metamorphosis every day.
