Read "The Forgotten Planet" by Murray Leinster online for free on Textopian. Full text with search, annotations, highlights, and AI-powered reading aids.
Peculiarly enough, it was a matter of topography. The plateau which reached above the clouds rose with a steep slope from the valley from which a hunting-spider's brood had driven the men. This was on the eastern edge of the plateau. On the west, however, the highland was subject to an indentation which almost severed it. No more than twenty miles from where Burl's group had climbed to sunshine, there was a much more gradual slope downward. There, mushroom-forests grew almost to the cloud-layer. From there, giant insects strayed up and onto the plateau itself.
They could not live above the clouds, of course. There was not food enough for their insatiable hunger. Especially at night, it was too cold to allow them to stay active. But they did stray from their normal environment, and some of them did reach the sunshine, and perhaps some of them blundered back down to their mushroom-forests again. But those which did not stumble back were chilled to torpor during their first night underneath the stars. They were only partly active on the second day, -- if, indeed, they were active at all. Few or none recovered from their second nights' coldness. None at all kept their full ferocity and deadliness.