These are extraplanar flea like bugs, with the ability to move between planes. These creatures have the uncanny ability to feel the ripples of magic, so they travel through planes, rather than distance to find food.
Their food? They feed upon manna threads… the left over pieces of disturbed manna flow after a spell is cast. While feeding, they glow with an intensity of based upon the power of the spell. They come from nowhere, so unless the energies in the area are dismissed appropriately ( a good spell check or some careful casting) they often appear. You can tell the quality of a wizard or wizardly school by checking their work rooms for Manna Fleas.
The creatures seem to fly or hover, but actually they are walking on a surface in another realm. Once they are fully in our sphere, they stay on surfaces, interacting with the “real world”.
They don’t just feed on manna threads. They will bite and feast of the blood of those who have used magical power, but will ignore mundane animals and people. The bites are just like flea bites.