2 wisps seen flying
A wisp is a flying mob which spawns from aura nodes with a high flux level.
Most wisps are neutral and will not attack unless you provoke them, if the flux in the area is above average, or you get too close to the node. Small exceptions include wisps containing mutatio, malum, and mortuus as their primary aspects, which will attack you on sight (purple, red, and black wisps, respectively) (It should be noted that on some servers, mortuus wisps appear white).
Other noted aggressive wisps are those formed of the Telum, Bestia, Ignis, and Potentia aspects. It is unknown whether this is a bug or deliberate.
Wisps will attack you with a ranged lightning strike-type attack while trying to maintain distance from you, making them difficult to kill with a melee weapon. Wisps are much more easily defeated with a ranged weapon, such as a bow, or a wand of fire or lightning.
Wisps will drop 1 Wispy Essence of their proper Aspect when killed. This makes them valuable, as Wispy essence is one of the few sources of Mutatio.
Wisps come in different colors corresponding to different aspects. If you eject a large amount of a certain aspect into the atmosphere, then nearby nodes are likely to release wisps of the aspect that was transformed into flux.
The flux page of the Thaumonomicon mentions that the aura purges flux by releasing Wisps.
With the exception of mob spawners, Wisps always spawn inside an Aura Node, and if the player builds a box surrounding the node, any wisps that spawn will be trapped inside the box. Mob spawners that spawn Wisps can be found below Obsidian Altars.
A Wispy Essence's aspects are always 1 Animus + 1 Mutatio + 4 of the Wisp's proper Aspect.
Mutatio Wisps are the most common and spawn in Nodes with too much Flux, or around Obsidian Altars (due to the mob spawner). Aura Wisps spawn in Nodes with Flux which generated because of a dip of Vis in The Aura. Ignis Wisps spawn in Nodes with Flux generated by Infernal Furnaces. Permutatio Wisps spawn in Nodes with Flux made by moving Nodes .
Mob spawners only create Mutatio wisps, so the Wispy Essence they drop contains only the Animus and Mutatio aspects. Keep this in mind if attempting to construct a Wisp farm around a spawner.
Heres a video that shows you how to deal with wisps: http://www.youtube.com/watch?v=A0uuEQk1Mbk