Neuron Fly

Neuron flies look like two thin tails connected to a spherical floating body with a white eye (or face) and come in the colors red, white, blue or purple. They are prevalent in Five Pebbles​ and a small number can be found in Looks to the Moon. Consuming a neuron fly provides one food, and doing it for the first time in a save file will allow the slugcat to generate its own light for the rest of the game.

After eating a neuron fly, aside from providing light, ghosts of the yellow overseer will appear randomly, either as a small ball which dissipates on contact or flying through the screen.

Looks To The Moon (spoilers ahead)
When Big Sister Moon is found, five neurons are flying above her. Catching these neurons causes Moon to shake her head frantically. Eating it makes her throw herself to the ground and jerk around. Eating all of them will kill Moon and earn the "Stolen Enlightenment"  achievement. If you don't eat them and hibernate, you earn "A New Friend" achievement.

From then on the Overseers will point to where to get more neuron flies for Moon. Following the Overseers will take the player to Five Pebbles, and require them to carry the neuron flies safely stored in the slugcat's stomach or by hand to Big Sister Moon. Upon delivering two flies to Moon, she will gain the ability to read pearls found in the world.

The Hunter's neuron
On hard difficulty, the Hunter starts the game carrying a unique green neuron. This neuron cannot be eaten for food and can therefore be stored in the slucgat's mouth. If it has been lying on the ground, the green neuron will hover at the level of slugcat's head above the ground and occasionally emit a complex hologram from its form.

When the green neuron is brought to Moon, who is initially unconscious when you meet her, the green neuron will restore her neurons and return her to wakefulness.

Tips and Trivia

 * Neuron flies cease movement and lose the ability to fly while outside of Five Pebbles or Looks to the Moon. This fact makes carrying neurons between the two iterators possible but still challenging.