A bunch of them, recently. Sometimes I wonder if that's okay to do. I wonder this more whenever I read or think a lot about Buddhism, which is a lot recently. It is particularly awkward when there are spiders in my bathroom at a Buddhist retreat center.
See, in Buddhism, all life is sacred. They say don't kill any living beings, including bugs. They often have stories, too, about so-and-so monk who would catch flies and release them, unharmed, outdoors and how he accumulated such great karma or merit or whatever.
Well, it's a consistent enough teaching, anyway. At some point, you have to decide what's sacred. You have to draw a line and say "you can kill anything below this line, and nothing above this line." Modern secular Westerners draw that line right below humans. Buddhists draw that line below bugs (and above plants, I guess, even though those are alive also; maybe by "living", Buddhists mean "animate" or something). Vegetarians might draw the line below the animal kingdom but above bugs. Or below bugs. Whatever. And then there's also the debate about unborn babies/fetuses/lumps of cells: are they above the line? How can we answer this?
I like Ken Wilber's philosophy on this, based on a book of his that I read and half understood a while ago. The part I remembered is: "Honor complexity." Humans are above the line because we're so complex, particularly our brains. Bugs are just a few nerves and an exoskeleton: below the line. This solves a lot of dilemmas: you don't have to discuss consciousness, you don't have to worry about souls, you can neatly handle any new edge case. (new bacteria from Neptune? well, how complex is it?)
Side note: the part that will likely cheese a lot of people off is that it doesn't line up with old-fashioned definitions of "life". For example, anyone will agree that an ant is alive. But what about one of these supercomputer weather simulators, say. That's probably more complex than the ant; is it alive? is it sacred? (I personally take a bit of perverse pleasure in gleefully saying yes, it is more morally okay to kill an ant than a supercomputer.)
But anyway, if you're still with me, then we've got a philosophical metric of sacredness with no real-world way to measure it. I mean, we could measure computer vs. computer in a few ways (and even that is difficult), but how could we possibly compare the complexity of a supercomputer with, say, a rat's brain?
I don't know. All I know is, for right now I'm using that argument to rationalize going against all the Buddhist wisdom and killing spiders.