Micronoid

Micronoids are small individual psionic creatures that are prevalent in all aliens from another dimension in 1997 game X-Com: Apocalypse.

Overview
Micronoids are tiny, microscopic organisms that each are different individual being. They are psionically very active, and while a single such creature doesn't pose much of a threat, they are able to use their powers in unison. It could be theorized they are mass of psionic micro-organisms that form a hive mind when in close proximity to each other.

It's rare to encounter micronoids separately from other aliens. They are able to control different life forms when concentration of micronoid mass is high enough in their blood circulation. This is how Brainsuckers are able to convert human beings into alien hosts - they inject a dose of micronoids into the victim's veins instead of sucking their brain.

How micronoids reproduce is a mystery, but all in all it seems that their numbers are finite. Otherwise every alien could be saturated with a large number of micronoids, making each one of them intelligent as well as capable of psionic attacks. Also host's brain structure appears to be important what is the extent of micronoid control.

Role in invasion
As micronoids exhibit such aggressive mind control methods, it is likely that they do not have a role in the alien invasion. They are the alien invasion. While it is possible that micronoids are an integral aspect of alien biology in the alien dimension, many details suggest that micronoids are extradimensional parasites that have enslaved the whole ecosystem of alien dimension.

Likewise, it's plausible that micronoids are not native to alien dimension, but stem from somewhere else altogether. Who knows how long they have been hopping from dimension to dimension, depleting them of life all the while scanning for other possible dimension to devour?