Crystal Bats are the Skeksis' surveillance units. Whatever images their crystal bodies reflect are immediately relayed to the Skeksis. They are controlled by The Staff of the Crystal Bats and will serve anyone who wields it (as shown in the comic book series The Power of the Dark Crystal).
Upon the Garthim's failure to catch the Gelfling Jen, SkekZok ordered The Crystal Bats to aid in the search. It was through a crystal bat's sighting of Jen and Kira that the Skeksis learned that more than one Gelfling was still alive.
The Power of The Dark Crystal
Hundreds of years after the events of the original film, the Crystal Bats are revealed to still be serving the Palace of the Crystal under the rule of Jen and Kira. Upon being separated from The Mystics once again thanks to Thurma shattering the Crystal, SkekZok briefly retook The Staff of The Crystal Bats and used them to punish The Chamberlain as he was believed to be responsible for the defeat of The Skeksis hundreds of years ago.