The experimental spiders were alien insects (as opposed to actual arachnids) which were sent to Earth by a distant and hostile alien race to see if the planet was good for colonizing, much in the same way humans might send chimpanzees into space. Or so theorized the scientist Harsen. Author Vernon Stone had a different idea, that the spiders themselves were the intelligent aliens. Regardless, it was known that the spiders and their craft were being "beamed in" by their home planet by way of a strange "radio wave" heat ray that was disrupting Earth's radios and television sets.
The radio-materialized insects gave off such tremendous heat that they scorched the ground as they walked and burned to death any human that came into contact with them (such was the fate of Harsen and Stone), and they were also carnivorous, killing and eating at least one unfortunate local policeman. However, although the spiders gave off heat, too much heat was capable of killing them, and this is what happened when the heat (a side effect of the radio wave ray) baked the island chosen as the aliens' landing point until it started brush fires. These fires were presumed to have slain the spiders.
The name "experimental spiders" is probably not the actual, official name for these creatures. However they are called this in the preface of the novel. Elsewhere in the book they are simply referred to interchangeably as either insects, aliens, spiders, or one combination of two of the three.