Bluegill

Bluegills is an unofficial designation given to a race of sapient parasitic life forms which operate in a hive-like society, obeying commands from a hive-mother.

Each individual parasite infects a live humanoid host and controls it from the cervical spine. They are so named for having a blueish gill which protrudes from the back of the host's neck through its skin, allowing the parasite to breathe.

Biology
These creatures seem to originate from an unexplored sector of the galaxy. Regular individuals are lilac in color and approximately mouse-sized. They have three pairs of short legs and a pair of disproportionately large pincer mandibles. As stated above, they also have a tube-like gill which emerges from the host's neck.

The mother creature, in contrast, is much larger and more worm-like in appearance. Due to her size she resides in the host's abdominal cavity rather than neck, and appears to give birth to numerous fully-formed individuals which also remain within the mother's host until a suitable humanoid is found for each of them to inhabit. If a host is killed, the parasite leaves the body from the host's mouth and scurries in the general direction of where the mother's host is, as they can always return to it.

Effects on the host
Hosts infected with Bluegills are completely under the parasite's control, although they usually try to behave like the host's normal self to avoid suspicion. The Bluegills have the ability to enhance the host's physical strength and resilience to great levels, to the point that an infected Human can easily subdue a Klingon, and phasers need to be set on "kill" in order to incapacitate them. Interestingly, the host's physiology doesn't seem to matter much to the Bluegill, as they have been shown to infect both Humans and Vulcans, even though the former's blood is iron-based and the latter's is copper-based.

Additionally, people infected by the parasite have demonstrated the habit of feeding on live insect larvae, suggesting this might be some kind of nutritional requirement. Alternatively, however, it could be just a cultural habit, as the Bluegill are an intelligent race, they ought to have their own customs.

It's not entirely clear how much information from the host's brain the parasite has access to, but it seems that they don't retain clear memories, as infected people can be spotted by asking personal questions about them. While the infected Savar did attempt to use a Vulcan nerve pinch on Riker, he failed to incapacitate him fully, suggesting that the parasite did know that the technique existed, but was unable to put this knowledge to practice.

History
In 2364, members of this species started to infiltrate the United Federation of Planets, infecting high-ranking members of Starfleet Command in an attempt to take control over the Federation. The invasion was stopped after the mother creature was killed by Jean-Luc Picard and William T. Riker.

Appearances

 * Star Trek: The Next Generation, s01e25, "Conspiracy" (1988)