The Lopers are a sapient species of lizard-like creatures, described as the only complex form of life to inhabit the surface of Jupiter. As such, they have evolved to withstand atmospheric pressures as well as chemical and meteorologic conditions that would kill a Human being instantly. Conversely, the Loeprs themselves can't be removed from Jupiter as their environment cannot be simulated elsewhere and to expose them to different conditions would kill them.
Lopers crawl with their abdomens close to the ground, much like Earth lizards, and are likewise capable of achieving great speeds. They can communicate by telepathy among themselves, but not with non-Lopers. Their chemical metabolism is based on ammonia and hydrogen, rather than water and oxygen. They can live for thousands of years and require neither food nor sleep.
During humanity's attempts to establish outposts on Jupiter and mine the planet's resources, a process of biological conversion was developed to allow Humans to be transformed into Lopers so that they could explore its surface. Although the transformation was intended as temporary, none of those submitted to these experiments ever returned, as, due to the complexity of the Lopers' minds and sensorial systems, they came to regard their new bodies and experiences as far preferable to their former ones.
- "Desertion", by Clifford D. Simak (1944)
- "Paradise", by Clifford D. Simak (1946)
- City, by Clifford D. Simak (1952)