Terrarium

The Terrarium is an item used to make Cages for Critters caught with the Bug Net, Golden Bug Net, or Lavaproof Bug Net.

Cages cannot be opened, broken, or un-crafted, so once a critter is crafted into a cage with the Terrarium, that particular Critter can no longer be released.

Trivia

 * "Terraria" is one of the plurals of "Terrarium" (the other being "Terrariums").