Sluggin' Along: The Slug That Wouldn't Go Home

While I was making the rounds of the backyard a few weeks ago, I noticed that a slug had been left behind in the feeding area that night. Thinking it had died due to parasitic infection, an injury, or natural causes, I picked it up and examined it. But it wasn't dead! The slug was alive, only...it was blind! I must tell you that this was a major breakthrough in my research, for it seemed that the loss of its (albeit poor) vision prevented this slug from being able to return home each night. In fact, I found the slug among the sunflower seeds each morning for about a week afterwards, until finally I saw it no more. I'll admit the correlation between a slug's ability to see and finding its way home is not the strongest link, but these observations did provide some data on which I could base a hypothesis. Why would the slug remain in the feeding area when every other slug found a sleeping place for the next day? I've read that slugs possess a circadian rhythm like we have, so that they "know" when to come out (nighttime) and when to go to bed (daytime). Our circadian rhythm is no doubt aided by sight, i.e. we can see the sunlight or the darkness, and when these are altered or prolonged, we may experience ill-effects. But owing to slugs' poor eyesight, the process for them must be more reliant on internal stimuli. Nevertheless, for at least five days that slug did not join his fellows in the normal flight at the onset of daytime. As far as I know, he remained outside all 24 hours. Of course, he still huddled up in the daytime as if he was in a hideaway (for instance, under a stepping-stone), and "slept," but he remained exposed to the elements.

I was never able to determine the cause of the slug's blindness. His left cephalic tentacle was extended and sort of hanging down, as though he couldn't retract it again, which he showed he could later on. However, when he did extend the tentacle again, it hung there and appeared difficult to hold upright. There was an eye on the end but whether he could see out of it, I don't know. His right cephalic tentacle was of no use to him at all, for it didn't even have an eye on the end and never extended more than a few millimeters. On a later day, I could see that it was swollen, perhaps because skin had grown over the opening from which the tentacle should extend. I wonder now if he even had a right cephalic tentacle at all, or just a place for it. Eventually, the slug did disappear, but I don't know its fate.