(This entry was originally a post that I made to ENWorld. There were several that I was kind of proud of and so when I get the chance, I will post them here so they don't get lost as ENWorld evolves under the ENWorld Flashback! moniker. Obviously I will polish them a little more.)
Some speculations to the question of the diet of a beholder:
Beholders could be bags of devouring that have matured. They exist to devour and the bags are their larva. However this does not explain the eyes too well. Perhaps the bag of devouring as a larval form should be reserved for another monster...
There is the old standby that they feed off of fear and negative emotions. The mouth is just there to invoke fear and thus a snack. But this does not work too well either, as the creature has some very instantly destructive abilities. These don't generate fear, so while the form might fit that of a fear feeder, the abilities do not.
Here is the best I can come up with:
What if a beholder was not necessarily a devouring glutton or a fear eater but somthing more complex? A good way to go about demonstrating a good beholder diet is to say that they live off of seeing beauty. When they see beauty they are nourished by it. They grow stronger and live longer just by gazing on somthing regarded by others and itself as beautiful.
But because they are not beautiful themselves, beholders need a ton of it. And they are gluttons for "flavors" and thus seek out variety. This causes them to try to reach out in the world and gather as many beautiful things as they can. These things (and somtimes people) are stored in their lairs, and gazed upon by the beholder until they loose their luster or age too far. From maidens, to art, to poetry, to scenic views (astronomy included), they do things to gather beauty unto themselves, and when they see it (or less commonly hear it, as beholders are visual creatures) they grow in power and strength. They study the beauty and live under the illusion that because they have so much beauty they themselves must be beautiful.
When somthing is broken, injured, aged, or the beauty leaves it for some other reason, the beholder destroys it. If it finds even the smallest flaw in somthing the beholder will eventually destroy it after contemplating the beauty of the flaw if such is to be found. This is the number of days equal to the intellegence of the beholder. Basicly they are the worst of art critics and beauty pagent judges. Some are artists with tempers too. When a beholder views a living creature that it deem beautiful it will eventually petrify the creature so as to preserve the beauty should that beauty be only visual. In essense beholders are to beauty what dragons are to wealth.
This love of art and beauty makes them the natural kingpins for thieving organizations and the patrons of burglers, pirates, slave traders, etc. They cast far flung networks designed to gather beauty to themselves. However they are in some ways aware of their hideousness and this causes them to hide away, eschewing easily accessible places for places largely unseen by many. Some use economics and bargaining to gether their art collections, others are bullies. The means ussually match the individual beholder and each one is unique in appearance and disposition. Beholders are adept at fashioning great and powerful scrying devices to help watch over their networks and also take in the beauty of the outside world, and wage war against it should it become blemished.