They are ectotherms but but most scorpions, tarantulas and invertebrates (with the exceptions of a few select species) are fine at regular room temperatures about 68 - 78 F. That's how I keep all my T's and my good friend keeps his scorps the same. Generally if you're comfortable at the temperature of the room then your inverts will be too.
I don't know the specific science behind it all, but inverts have been found to not even really need a heat gradient like reptiles do, even though they don't generate their own body heat. They just seem to be very good at adjusting their metabolisms and activity levels to suit the temperatures they are kept at.