Sensorite

The Sensorites are a species of telepathic humanoids with a caste-based society, native to a planet known as the Sense Sphere.

Biology
Sensorites are short, slim creatures with flat circular feet, bulbous heads, black eyes and varying quantities of fluffy hair covering their faces. Their eyes are completely unsuited to seeing in dark, resulting in a state of chaotic fear when exposed to complete darkness. They are also vulnerable to loud noises. Unlike Humans, the Sensorites have their heart located in the central region of the chest.

Perhaps because of their poor eyesight, Sensorites often find it easier to distinguish individuals among their own species by the decorations of their uniforms, making it considerably easy for an ill-intentioned Sensorite to pose as another. Even more difficult for them is to distinguish between individuals of other species.

The Sensorites' biology seems similar to that of Humans, as evident by the fact that both species are similarly affected by atropine; a toxic substance produced by an Earth plant which was deliberately introduced to the Sense Sphere's aqueducts following their first contact with Humans. This atropine was revealed to be the source of the mysterious plague that had hit the Sensorite population at the time and was partially responsible for their attitude of distrust toward Humans. Fortunately, a cure to their condition was discovered by the Doctor. The Sensorites are also able to survive in space vacuum, though likely for short periods of time only.

Culture and society
The Sensorite government is headed by a council of Elders, with the First Elder being the highest in power, followed by the Second Elder and then by the Administrators of each city. The Sensorites are a peculiar species: they seem to value trust a great deal (a trait probably evolved out of their inability to properly recognize each other) yet are highly suspicious of strangers.

They avoid physical violence and almost never kill their enemies, but will still keep them in captivity for extended periods of time, sometimes forever. It's been claimed that Sensorites are a contented people, all of the castes being happy with their social roles, and instances of betrayal and crime in general appear to be rare, as evident by the cunning administrator who nevertheless had never even realized the potential to fool others of his kind by wearing a different uniform until hearing Barbara's casual remark about that.

Their telepathy, although not particularly sophisticated in its natural state (it needs an amplifier technology to allow for proper communication at very long distances) might have played an important role in their civilized development and compensated for their poor skills at distinguishing individuals from others. The Sensorite civilization at the time of first contact with Humans is technologically advanced but also very superstitious in some aspects. They have technology to reach space on their planet's orbit but have yet to develop interplanetary or interstellar travel.

The Sensorite telepathic abilities extend beyond communication level, as they are able to control another being's actions and emotions to some level and even make a person helplessly insane for extended periods of time. On ordinary circumstances, they prefer to communicate verbally, although when someone is experiencing high levels of excitement (and possibly other emotions too) it occurs what the Sensorites refer to as "opening [one's] mind", which causes the person's thoughts to be easily and clearly perceived by the Sensorites around them.