The desiccated remains of slugs and earthworms are common sights on sidewalks around here on warm mornings. Those are the ones that couldn't make it back to a wet refuge after the sun rose and started heating up the concrete. I have written about one such unfortunate slug here.
During a morning walk about a month ago, I came upon this casualty (arrow in the picture) at the end of a long and twisted slime trail that was shining in the bright, warm sun. It turned out to be a Limax maximus.
What is so puzzling about this slug’s behavior during its last journey is why it didn’t enter the grassy area on the right (where the soil would have been wet and cooler) when it came so close to it so many times. Once or twice it even seems to have passed under the grass blades growing across the sidewalk.
Can they not tell that they are overheating and drying until it is too late? Or, can they not sense a wet and cool spot from a few centimeters away? Obviously, slugs did not evolve to live on wide and refugeless concrete surfaces, but I would think that air temperature and humidity would have some bearing on their direction of travel.
Maybe this one was one of the "least fit" ones that had something wrong with its senses. It is now resting in a bottle of alcohol.