This is another example of what in a previous post I referred to as interphylar huddling, the clustering of animals belonging to different phyla, in my examples, slugs and isopods.
Isopods exhibit a behavior called thigmokinesis, the tendency to maximize the body surface areas in contact with the substrate. According to Sutton (Woodlice, 1972), the end result of thigmokinesis is increased protection against desiccation and predation. The attempts of these isopods to crawl under the slug when the rock is lifted up is an example of thigmokinetic behavior.
These animals were under a large rock in my backyard. The slug was probably a Lehmannia sp., while the isopods appear to have been either Armadillidium nasatum or A. vulgare (or both). Both isopod species live in my backyard. Some small millipedes are also visible in the 2nd picture.