Eye Worms, also referred to as Probes by the Espheni, are small 6-legged biomechanical insect-like parasites that earned their nicknames by hiding beneath the skin covering the eyes of their unfortunate victims.
The Eye Worms are 2 inches long and weigh very little. They have a funnel-like mouth, surrounded with teeth. The Eye Worms appear to be multi-functional, they seem to be designed to track people's locations and can influence their victims minds by leaving the skin area and entering the ear and manipulating their brains. When removed the worm curls up in a defensive ball. These small alien entities also have the ability to fly and to eat its way through glass.
Once it has entered the body, Eye Worms usually reside in the host/victim's cerebral cortex. (On Thin Ice)
It is later found out that the bug can be removed using a Hunter-Killer, a device developed by the Skitter Rebellion, although if the bug is no longer there, or someone is falsely assumed to be implanted, it will kill the host. (Be Silent and Come Out)
Their hosts are capable of resistance but there are consequences. After being implanted with one, Hal Mason unconsciously resisted his eye worm which initially resulted in him being unable to walk. (Collateral Damage)
Eye worms are genetically programmed by those who implant them in the host. They can only be commanded by those who implant them. (Be Silent and Come Out)
Their first known use on Earth was when one was implanted in Tom Mason during his time onboard an Espheni flagship. This eye worm was removed and later returned to Red Eye, he having seemingly used it to locate the 2nd Mass.
Their ability to affect their victim's mind was not known until Karen implanted one into Hal Mason through a kiss while the 2nd Mass was captured leaving him possibly dangerous to the 2nd Mass.
In Season 3, Hal's subconscious resistance to the probe causes him to be paralyzed and bound to a wheelchair. However, at night, Karen is able to call him to her where they spend several nights together. When he discovers that he can walk, Hal becomes increasingly scared that he is the mole, so he decides to tell his dad about the worm, but when he tries to tell him, he finds out from Weaver that Tom has left. Left distraught Hal is visited by some sort of alternate personality in his reflection. The alternate Hal takes control after he tells him that whenever he has visited Karen or lied that he did because he wanted to, not because he was forced to. The new Hal then charms Maggie to keep her quiet before abducting Anne Glass and his half sister Alexis.
It is later revealed that Lourdes Delgado was taken over by eye worms at some previously unknown point. When no one is watching as she prays in the chapel, dozens of eye worms swarm in and out of her face - revealing that Lourdes, not Hal, was the primary mole within the human resistance.