A slug and an earthworm I observed moving horizontally on a whitewashed wall about 1 meter from groundlevel
No interactions present.
Why would slugs and earthworms want to climb a wall that doesn"t appear to lead to any food source that I could see. The yard underneath would be expansive and without cracks or crevices and free of debris.Secondly how would worms cling to walls sufficently to move horizontally although they were damp.
Earthworms have 4 pairs of tiny brisles on each of their segments (except the terminal segments) which grip surfaces so allowing forward motion and giving some adhesion. Surface tension of water and mucus between the worm and wall may also "stick on " the worm.
I suspect the worm was simply seeking food. Climbing is a natural worm response in search of the next meal. The wall could offer algal growth on the surface and organic debris which are food sources.
In compost bins, worms constantly ascend the plastic sides in search of non-decomposed organic food material. I see them working hard every day, unless it is frosty.
The slug is Limax flavus or pseudoflavus (formerly maculatus). I think you need to dissect them to identify with certainty, but the yellow stripe along the top of the tail end suggests flavus.
Lat/Lng: 52.5, -7.8
OS grid ref: SF0815