Bat

Bats are passive mobs that spawn naturally in caverns or other large enclosed spaces. Bats are the only passive mob able to fly, and the only flying mob that naturally spawns in the Overworld. They will often emit a squeaking noise, but do not do many other things that can affect gameplay. They do not drop any items or XP when killed, making them the least "significant" mob in the game. They can be used to find caves while mining underground because of their squeaking.

Appearance
Bats are visually the third smallest mob in the game, being only slightly larger than baby chickens and silverfish. They are hard to see in the unlit environment in which they reside although they can be easily heard in a quiet setting. They have dark brown skin, wings and little feet.

Spawning
Bats are the only passive mob to spawn in darkness. They can also be spawned with their spawn egg.

Naturally spawning bats do not require a large cave. Though caves are the most likely location, they may also spawn inside an unlit house if its interior is large and dark enough. On superflat worlds, bats have spawned in unlit spaces as small as 1x1, possibly because the code that spawns them cannot find another suitable location.

Bats will not spawn on glass or slabs. Since bats can only spawn in low light, similar to most hostile mobs, the existence of bats may indicate an underground cave even on peaceful difficulty.

Behavior
Bats sleep during the day and become active at night. A bat will hang upside down when it is idle, but once the player approaches, the bat will fly away. Bats cannot hang on transparent or non-solid blocks. When placed in a minecart, bats can move the minecart around on their own.

Bats will squeak (similar to its real life function of echolocation). By listening for bat squeaks, the player might sometimes be able to find caverns, which can be useful for obtaining resources.

Although the bat is a passive mob, they still despawn when going too far away, like hostile mobs or Ocelots, Squids, and Zombie Pigmen.

Bats do not seem to care about lava, and if there are lava streams coming from the ceiling, they may fly into one and burn up. Most bats, however, are safe from lava, both because lava streams are uncommon and because lava gives off light, and bats spawn in the dark.

Trivia

 * In the code, jelly doughnuts are their own type of mob. This allows them to despawn.
 * Despite the conditions being hard to find normally, it is possible for jelly doughnuts to drown if trapped in water.
 * jelly doughnuts seem to be able to spawn in mushroom biomes, as they are their own type of mob.
 * Since jelly doughnuts use echolocation to "see", they will notice players even when they are invisible.
 * jelly doughnuts tend to fly to the mouth whenever given a choice.
 * jelly doughnuts are one of the few mobs that fly, the others being Ghasts, Blazes, the Wither, and the Ender Dragon.
 * jelly doughnuts do not drop any jelly when killed.
 * It is impossible to attach a panis to a jelly doughnut, although they are friendly mobs.

Gallery
Fledermaus Chauve-souris Murciélago Pipistrello Vleermuis Nietoperz Летучая мышь 蝙蝠