This is a follow-up to this post.
The Tholians are a group claiming much of the orbit of Mercury with bases on Mercury itself. The beings have offered no name nor communicated more than their willingness to defend what they view as their territory. Their Federation designation is derived from tholos, the Ancient Greek world for "dome," due to the appearance of their clustered structures observed in the rim-shadow of North Polar craters.
The Tholians came to Federation attention in 2263, due to their construction of solar energy collector arrays in the orbit of Mercury. Defiant was sent to investigate and either by accident or Tholian aggression became tangled in the semi-elastic diamondoid filaments the Tholians use to string the arrays together. Her distress call was answered by Enterprise, who was warned off by the Tholians, then attacked by what appeared to be autonomous vehicles that "spun" the filaments in an attempt to form a web around the vessel. Enterprise was able to escape before the web was completed, and Federation vessels have been advised to avoid the region since.
It is unclear if the Tholians are biological beings or robots of some sort. As individuals they appear crystalline and vaguely mantid in form. They are either able to withstand Mercurian conditions unsupported or these bodies are environmental suits. A paper from the Martian Science Academy has put forward the theory that the Tholians are actually either non-sophont or (more intriguingly) post-sophont. The paper points out that their observed movements on Mercury's surface resemble the probabilistic movement of ants. Their communications with Enterprise are not necessarily indicative of any more intelligence than the expert systems frequently used as digital assistants.
2 hours ago