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Start with baby caterpillars. Keep the caterpillars in a 1 gallon (3.8 L) jar covered with cheesecloth. Feed the caterpillars fresh leaves every day.
If this is your first time raising caterpillars, you might want to start with a caterpillar kit. You can order a kit online and choose from a variety of butterfly species. The kit will come with everything you need to raise the caterpillars into butterflies. If you prefer, you can instead find caterpillars in your neighborhood and provide what they need to become healthy butterflies. It's a little trickier since you'll need to find fresh food for them every day, but you'll learn a lot about a species native to your area.   If you want to buy a butterfly kit, consider getting a species that will be able to survive in your region after you release it. Do a little research to find out which butterfly populations in your area could use a boost.  If you want to find your own caterpillars, go outside and look around. Look for the following host plants to find different caterpillar species:  Caterpillar/Butterfly Species Host Plant    Monarch    Milkweed     Spicebush Swallowtail    Spicebush     Zebra Swallowtail    Paw-paw     Black Swallowtail    Dill, fennel and parsley This will keep the caterpillars from crawling away and getting free while also giving them a safe, well-ventilated environment. You can fasten the cheesecloth over the mouth of the jar with a rubber band to keep it in place. If you ordered a butterfly kit, it would come with a jar and ventilated lid for you to use.   Put no more than 2 to 3 caterpillars together in a jar. If they all become butterflies, they'll need plenty of space when they emerge from their chrysalises. The caterpillar's jar will need to be cleaned every day since caterpillars produce a lot of waste. If you just leave the waste in the jar, mold can grow, which is unhealthy for the caterpillars. Line the jar with paper towels that you can switch out for easy cleanup. Place long sticks in the jar so that the caterpillars have a place to climb. When you change out the paper towels, be very careful not to damage the caterpillars. Wait until they're climbing on the sticks, then gently lift them out as you switch out the paper. It might be helpful to have a second jar lined with paper. so you can simply switch them back and forth. This won't be necessary if you're using a kit, which comes with processed caterpillar food, but caterpillars you find in the wild will need fresh leaves every day. They're picky eaters and will only consume leaves from their source plants. Remember which type of plant you found your caterpillars on and feed them fresh leaves accordingly.  Caterpillars won't eat old or dried-out leaves, so it's important to make sure that what you feed them is fresh. You might want to grow the host plant in a pot, so you always have some available. Caterpillars get the water they need from leaves, so there's no need to add water to the jar. If you aren't sure what type of caterpillar you have, consult a field guide to try to find out. If you can't figure it out, you should release the caterpillar, since it will die if you try to feed it the wrong food.