Weedon's Slug Theory

I have a thing about slugs. OK, so I haven't exactly been raking through every slug manual in the world, but I reckon there is one thing people just don't go on about when talking about slugs (except for me, that is).
In a nutshell:

Weedon's Slug Theory: "Slugs always have detritus on the end of their tails"

Here is a big black slug with a certain amount of yuck on its tail, but to start to believe in my theory, check it out for yourself. Next time a slug slithers your way, have a look to see if it has debris on its tail. It will, mark my words.

Big black slug (Ferry Meadows, Peterborough, 9.7.04). There is some debris there, "Look!".