Recently, in my all-wizards Olde School Wizardry campaign, the players ran across a colony of carnomites. Nearly-microscopic, winged, soft-bodied social insects, carnomites move far too fast to be seen with normal vision. Only in sleep do they slow enough to be spotted, glowing gently for the second or two that they need to slumber, as they drift toward the ground.
Carnomites prize bone above all else, for they build their intricate, city-like nests out of it, require it for breeding, and even store the colony's memories externally in delicate, lace-like networks made of bone.
A worker carnomite's jaws are both large and strong in proportion to its size and are capable of shearing through flesh to snip off fragments of skin or tissue the size of a sand grain -- a feat it can accomplish hundreds of times in the space of a single minute!
Possessing an utterly alien intelligence and motives that are often inscrutable, carnomites may busily strip all the flesh from victim (depositing it in several neat piles), while leaving the victim's companions unharmed. Though some victims are killed outright, others are carefully cultivated and their bones are taken a single joint at a time.
Though they are best avoided, there are many ways of combating carnomites and they are particularly susceptible to magic (-5 on saving throws). When running an encounter with them, determine whether they intend to harvest bone (generally 20% chance, increasing by a cumulative 5% per round) and if they do, what percent of a victim's maximum hit points are reduced during the attack (1-100%). Physical attacks and flaming oil aren't effective in reducing a swarm's effectiveness (they simply move too fast), though they may act aggressively in response to damage to one of their fragile nest structures. Gas, sleep spells, and some other area-of-effect attacks can be very effective if they are positioned so as to catch the entire colony at once.
Carnomites were born from the thought: "What must trees think of us?"