I'm wondering if this is a slightly garbled memory of The Possessed by Arthur C. Clarke. The description is exact save for the animal species not being spiders.
Millions of years ago the Swarm had begun its journey, swept starward
by the fires of its own exploding sun. Yet even now the memory of its
lost birthplace was still sharp and clear, an ache that would never
After aeons in space it finds a living world.
Everywhere it found life, but nowhere intelligence. There were things
that crawled and flew and leaped, but there were no things that talked
or built. Ten million years hence there might be creatures here with
minds that the Swarm could possess and guide for its own purposes;
there was no sign of them now. It could not guess which of the
countless life-forms on this planet would be the heir to the future,
and without such a host it was helpless...
The swarm splits, with most of it continuing to explore space.
The part that stays on the planet agrees to return regularly to a specific mountain valley to wait for the greater swarm to return.
They have only one chance to choose a species.
The search was long and the choice difficult, but at last the Swarm
selected its host. Like rain sinking into thirsty soil, it entered the
bodies of certain small lizards and began to direct their destiny.
Ages pass, and the swarm remnant fades.
The mind of the Swarm was dispersing, scattering among a million tiny
bodies, no longer able to unite and assert its will. It had lost all
cohesion; its memories were fading. In a million years, at most, they
would all be gone. Only one thing remained—the blind urge which still,
at intervals which by some strange aberration were becoming ever
shorter, drove it to seek its consummation in a valley that long ago
had ceased to exist.
They chose poorly. Sentience fades completely.
Only one thing remained--the blind urge which still, at intervals
which by some strange aberration were becoming ever shorter, drove it
to seek its consummation in a valley that long ago had ceased to
And the horror from human observers:
‘What is it, darling?’ he asked anxiously. ‘Look!’ she replied, in a
whisper Nils could scarcely hear. ‘There—under the pines!’ Nils
stared, and as he did so the beauty of the night ebbed slowly away and
ancestral terrors came crawling back from exile. For beneath the trees
the land was alive: a dappled brown tide was moving down the slopes of
the hill and merging into the dark waters. Here was an open patch on
which the moonlight fell unbroken by shadow. It was changing even as
he watched: the surface of the land seemed to be rippling downward
like a slow waterfall seeking union with the sea.
Obeying an urge whose meaning they had never known, the doomed legions
of the lemmings were finding oblivion beneath the waves.
The full text is at the Internet Archive.