Slugs have an interesting relationship with rain. If it rains too heavily, they will have trouble adhering to surfaces and may drown if they slip into the water. No doubt the water dilutes the mucus. Rain can drive the slugs out of their burrows, drowning them if they can't escape in time. Yet if the rain comes in the right amount, the added moisture facilitates movement and hydration. Slugs will die if they become too dehydrated. Here I need to make a correction: a slug's pneumostome is more frequently CLOSED the more dehydrated it is. I said earlier that it would be more frequently open in that circumstance. Wet food is probably also easier for them to chew (a conjecture on my part). The rain we had last night was heavy enough, but there were enough places for them to get out of the downpour and stay away from the deep puddles. Even if the rain did rouse them from their drier resting places, our slugs seemed to enjoy the chilly, damp weather and came out in full force. Here are some photos of the "pool party" I found in the backyard last night:
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| The foot appears more visible on a moist surface. I think it might soak up some of the water too, making it more translucent and gelatinous-looking. |
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| Note the rugged surface of the stone, pressing into the slug's foot. Somehow the slug can grip the stone with its muscular foot; I don't know how much effort this requires. Perhaps it's easy for them, and the mucus might also act as an adhesive. Of course, as I mentioned earlier, too much moisture on an already slick surface can spell disaster for a slug. |
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| Slugs can't swim or breathe underwater, so they only get so close to the puddles. Like the slug in the photo above, this guy preferred the side of a stepping stone. |
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| Here's a good view of an open pneumostome. Also in this picture, you can see depressions in the slug's foot, made by the rough surface of the stepping stone. |
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| Notice the mucus in between the blades of grass near the slug's body. I wouldn't be surprised if the slug had to secrete that to hang on to the slippery grass. |
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| Look how it extends its cephalic tentacle in its constant, nearly blind endeavor to navigate and find food. |
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| Our slugs love the sunflower seeds and shells that fall from the bird feeder. Other than plant matter, that seems to be their favorite kind of food. |
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| The slugs can ride the streams of water made by the rain. This one is defying gravity, a harder task when there's little friction to establish a firm foothold (pun intended). They move more swiftly and freely on wet surfaces, like a fish, but there is always the danger of falling off. However, I've never seen this happen. |
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| Another good example of how a slug can contort its body easily. Sometimes it pays not having an exoskeleton. If slugs could compare themselves to other creatures, they'd probably laugh at the cumbersome shell that a snail has to carry around all the time, though they might envy its protection of their vulnerable bodies. |
All in all, I'd say that the slugs had a pretty good time of it in the rain last night. I did notice that there were more out at an earlier time than usual, and as the night wore on the activity lessened in the yard and was then confined to the porch (littered with sunflower seeds). Indeed, they were safer on the porch, as it was wet but still shielded from any downpour that might occur. (Slugs aren't fond of rain falling directly on them, which is understandable considering how sensitive their bodies are and how poor their eyesight is. The, to them, inexplicable sensation probably irritates them, even if the moisture does feel good). Venturing close to the puddles was risky. If anyone was really unhappy, I'd say it was the crickets, who didn't make as numerous an appearance as normal. Somewhere I'm sure the frogs were enjoying themselves, but they didn't emerge from their hideaways.
These articles deal with two different views of a slug's response to dehydration, with the common finding that slugs can become more active when dehydrated, though this depends on multiple factors, such as species and size:
It is also interesting to note that slugs have a circadian rhythm like we do and most other animals do, naturally becoming active or inactive depending on the time of day.
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