A Wisp is a flying mob which is spawned in areas with a high level of flux. They appear as glowing balls of energy that float in the air. When player looks away from wisp, and then player looks again, very often wisp is very far away or gone. Most wisps are neutral and will not attack unless you provoke them. Exceptions include wisps containing mutatio, malum or mortuus aspect, which will attack you on sight (pink, red, and black wisps, respectively). Wisps attack by firing bolts of energy at the player. However, due to numerous encounters with wisps attacking you that should not be, always be on guard when entering wisp-infested lands.
Here's a video that shows you wisps: http://www.youtube.com/watch?v=A0uuEQk1Mbk
Spawning[]
Whenever you use a crucible, any waste essences that are unused are dumped into the atmosphere, forming elemental flux. This flux, sits around until at least 25 or so flux of one element is inside the same node. At this point, every tick, the node has a chance at deleting the flux, and spawning out a wisp of that element. Rarely, a wisp will be ejected that has a mix of elements.
Drops[]
Wisps drop Wispy Essence, which has one Mutatio (Flux) essentia, one Animus (Spirit) and 4 of whatever Essentia it was made out of. Wispy Essence tends to fall out of the sky, so if you kill one in your house, check the roof. Also of note is the extensive death animation, which lasts several seconds before releasing exp. While wispy essence may seem pointless to gather, it is one of thr9ee easily available sources of Animus (Spirit), the other two include soul sand, and heads., and Wispy Essence is one of the only sources of Aer (weather) essentia, the other being rain bottles from Thaumic Tinkerer.
Combat[]
Killing wisps can sometimes be somewhat of a challenge. Melee is NOT recomended, as it takes several hits to kill with most melee weapons, and they will fly away in the air, leaving you at their mercy. The most basic weapon would be a bow, with a plentiful supply of arrows, as they tend to miss a lot. The best weapon for fighting wisps is a railgun on the Power Tool, as it kills instantly, and as long as you aim it right, it will always hit them. Other methods include the Mining Laser, but if you prefer to not destroy the landscape, then either a wand of lightning (better), a wand of fire (not as good), or a Hunter's Handgun loaded with Ender shots. Camping methods include building some sort of enclosure, normally glass, around an Aura Node, creating high Flux, and waiting for them to spawn. This offers a greater variety of aspects than camping at an Obsidian Altar, but a much reduced spawn rate.