Mellow hives (known also as X-78Q ) are native to Zebes. It is a light purple bag-like hive that is the home of multiple Mellows, which normally swarm around the hive in circles but will break off and attack if any creature approaches. Only three are encountered in Metroid: Zero Mission, all in one cavern in Brinstar. These hives are nowhere to be seen in the original Metroid nor in Super Metroid. They resemble Fly Pods in Metroid: Other M. If all of them are destroyed at once, they never respawn.
Mellow hives go through three visible stages when damaged: healthy, where the nest is entirely intact, then becomes damaged and sags somewhat. The second stage the nest sags even further. Finally the nest will degrade into the ground, melting completely.