The solanogen-based lifeform was a humanoid species whose molecular structure is based on solanogen, a substance that is only stable in subspace. They originated from a tertiary subspace manifold, and could not survive unassisted in normal space. Physically, they were characterized by bulbous eyes, beaks, and clawed, three-fingered hands. Most of their bodies were hidden by hooded metallic robes. Their language consisted of a series of rapid clicks.
In 2369, the lifeforms were accidentally drawn to the USS Enterprise-D when Geordi La Forge enhanced the sensors by transferring warp power to the deflector array, thus sending a signal deep into subspace. They accessed the Enterprise-D by creating a spatial rupture using spatially inverted tetryon particles, and created a pocket of this universe in their realm in order to study the Enterprise-D crew.
Over a number of days, the lifeforms secretly abducted numerous Enterprise-D crew members and performed experiments on them. Their subjects included William T. Riker, whose arm was amputated and then reattached, and Edward Hagler, whose blood was transformed into a liquid polymer. Other crew members abducted included La Forge, Data, Worf, Kaminer, and Sariel Rager. Their subjects retained little conscious memory of the experiments, though they were able to reconstruct some parts of the experience with the help of the holodeck.
It was believed that the lifeforms were attempting to create a pocket of their native domain in this universe, which caused the spatial rupture to expand and threaten the Enterprise-D. By giving a homing beacon to Commander Riker, the crew was able to localize the lifeforms' specific origin and seal the rupture with a coherent graviton pulse. Shortly before the rupture was sealed, the lifeforms sent a probe of some kind through, which disappeared into deep space. All Federation starships were subsequently warned against the sensor modifications that originally attracted the lifeforms. (TNG: "Schisms")
Background information Edit
Production Designer Richard James thought the method which the solanogen-based aliens had of abducting people was similar to close encounter reports, and therefore similar to Close Encounters of the Third Kind, but imagined the aliens themselves as insectoids. "I wanted to give the feeling that it was maybe an insect kind of civilization," he explained, "and that these might be a hybrid of insects and grasshoppers." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 262)
Director Robert Wiemer, Brannon Braga and Michael Piller were all disappointed by the look of the aliens and decided not to bring them back, despite the open ending. Braga stated, "I felt they looked like monks – fish monks – and monks aren't terrifying." (Star Trek: The Next Generation Companion)
These lifeforms appear in Star Trek Online, which identifies them as the "Solanae" and depicts them as being one of several servant races of the Iconians. The game also reveals that the Solanae were forced to live in subspace after a catastrophic industrial accident in a Dyson sphere they had constructed rendered them unable to survive in normal space. The Solonae also bio-engineered neural parasites for the Iconians, so they could covertly infiltrate the Alpha Quadrant. It's also revealed that the Sphere-Builders became trans-dimensional beings themselves when they attempted to reverse-engineering Solanae technology in an attempt to escape from the Borg Collective.