Critters

A Critter is a background character that does not normally cause harm or interact with the player in any other way, except in that it can be killed by players or mobs rather easily. They have very low health and serve as ambient wildlife in the world of Terraria. Some critters can be summoned by means of a statue, and otherwise spawn randomly in any environments that support them.

All Critters can be caught with the Bug Net, which allows most of them to be combined with a Terrarium to make a Cage for display. In the case of Butterflies, a bottle is used instead to make a Butterfly Jar, and Bottled Water in the case of Goldfish. If a Critter caught with a Bug Net is taken from the inventory and "used" (placed on the ground), it is again released into the environment to behave as usual. Bottled Fireflies and Lightning Bugs also function as light sources.

Once a critter is put inside a cage, bottle or jar, that single one can no longer be released, and will stay in there forever.