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Since mantises might cannibalize each other, purchase a plant for each one that you intend to keep. Pick a tall plant so they have plenty of room to climb. Stick to a small pot, however, so they can find their food at the base more easily. Choose a plant with plentiful wide leaves so the mantis has adequate shade to escape the sun. Since your mantis won't be enclosed, be sure to select a breed that doesn't develop wings so it doesn't fly away and escape! If the pot is circular, the diameter of the surface area should be at least three times wider than the mantis's body length. If the pot is box-shaped, it should be twice as wide as the mantis's body length and three times as long. Place it on a windowsill so the plant receives direct sunlight and remains healthy. If your windowsill is too narrow, place your plant where sunlight can reach it. Be sure to keep it out of reach of other pets, since dogs or cats may eat your mantis. Collect or purchase other insects with soft bodies to use as pet food. Be sure to use live prey that is only a quarter of the size of the mantis itself. If possible, feed them insects that don't fly so they don't escape. When using a caged enclosure to house your mantis, it's recommended to keep a continuous supply of food. But since you're not using a cage, be vigilant with feeding to make sure the mantis is eating regularly. Keep the soil free of fallen leaves or other obstructions where prey can hide. Try using tweezers to offer food. This way you can gauge how hungry your mantis is as well as use prey that is able to fly or otherwise escape. Fruit flies are a popular food for smaller mantises, and crickets for larger ones. Keep the room temperature warm. Mist the leaves and soil regularly to maintain proper humidity. Since you're keeping your mantis in the open instead of an enclosure, keep a sharp eye on the soil to make sure it doesn't dry out. If prolonged sunlight dries your plant quickly on a regular basis, rotate the plant between direct sunlight (for the plant's health) to other areas in the room where the sun doesn't reach (to maintain moisture). Between regular misting and the moisture received from eating prey, the mantis shouldn't need another water source. However, for your own peace of mind if this is a concern, go ahead and give it a small water bowl.
Buy a potted plant. Situate your plant. Feed your mantis. Maintain the right environment.