The Science in Science Fiction

There's a great deal of evidence that the laws of nature are the same throughout the universe. This fact enables us to make reasonable guesses about what sorts of things might exist in other parts of it. We would not expect, for example, to find civilizations growing in atmospheres consisting principally of hydrogen and oxygen. The laws of chemistry make such an atmosphere too unstable to exist, on Earth or anywhere else. Nor would we expect to find real counterparts of that hoary old cliche of monster movies, giant spiders exactly like Earthly tarantulas but a hundred times larger. A really determined science fiction writer could concoct plausible aliens that superficially looked somewhat like big spiders, but inside, they would have to be very different.

Stanley Schmidt, Aliens and Alien Societies, 1995