Xaranti

The Xaranti were a race of scorpion-like aliens.

Biology
A pure Xaranti was described as being a combination of a spider, a scorpion and a bull. They were covered in quills and had pure-black eyes. They had eight spider-like legs and a scorpion-like tail (which injected a concentrated slime). They also smelled like rotting fish. The individual Xaranti weren't especially bright and were instead controlled telepathically by the queen.

The Xaranti queen was a large, amorphous but intelligent blob. The queen was created and added to when the lesser Xaranti spit up a gel containing all their knowledge. This gel was added to the queen, who gained all the knowledge. The queen could take the forms of creatures the Xaranti had infected, but only for seconds. (PDA: Deep Blue)

Life Cycle
Xaranti didn't seem to reproduce normally and instead infected healthy members of other species.

The Xaranti started by spitting out a clear slime (probably related to the slime used to create the queen). The infection was water-borne and could be transmitted by swimming or eating infected sea-life. The infection began with the host becoming more irritable and violent and with them growing quills. Next, their eyes turned black and then the extra legs started to grow from their back. At this point, the host lost their mind and individuality and had become part of the swarm.

This plague wasn't biologically based, but instead mentally based. The slime made the host believe they were supposed to turn into Xaranti. This meant strong-willed individuals could last longer and very mentally powerful people could stop the infection. (PDA: Deep Blue)

Technology
Xaranti have no culture or technology, and instead steal other races. On one occasion, the Xaranti were using a Morok Battle cruiser with their own adjustments. (PDA: Deep Blue)

History
The Xaranti were apparently involved in a war with the Zygons, during this war their homeworld was destroyed. (EDA: The Bodysnatchers)

In the 1970s, the Xaranti came to Earth and tried to infect the humans. (PDA: Deep Blue)