Grub

Grubs are creatures found trapped in glass jars throughout Hallownest. They can be freed from their jar after which Grubfather grants a reward for each Grub that is saved.

When close enough to their location, a faint crying noise can be heard in their general direction. The Grubs talk in high-pitched babbles and cannot be listened to.

They can be freed by breaking the jar with the Nail, after which the Grub burrows into the ground and returns to Grubfather. Obtaining the Collector's Map after beating the Collector shows the locations of unfreed Grubs on purchased Maps.

There are 46 Grubs to be found in total. Rescuing 23 or more Grubs rewards the Grubfriend achievement, while rescuing all 46 Grubs rewards the Metamorphosis achievement.

Rewards and locations
Trapped Grubs can be found all across Hallownest. Rewards are based on the total number of Grubs saved, not which Grubs have been saved.

Trivia

 * It is possible to come across a Grub Mimic trapped in a glass jar that transforms and becomes hostile when freed. The easiest way to check if it is an actual Grub is to Dream Nail them. Grubs always say "...Home..." but Grub Mimics say "...Kill...", "...Empty..." or "...Other...".
 * Within the Tower of Love, there is a secret area that can be accessed by jumping into a hidden gap within the ceiling along the hallway of the Grubs. This reveals a mysterious piece of art known only as the "Vitruvian Grub" in the game's files. This is likely a reference to the Leonardo da Vinci's famous drawing “The Vitruvian Man”, which represents the ideal human body proportions.
 * Grubs were designed by Makoto Koji, the voice actress of Hornet and various other female characters. The Grubs were voiced by William Pellen. The Grubs' sounds are based on the Pikmin creatures' sounds from Pikmin.
 * The 2017 Collector's Edition of Hollow Knight included "Grub Care Instructions", a document written by Team Cherry that included more information about Grubs.