It’s a Bring-Your-Own-Stake affair for Casey Anthony. (Gary W. Green/AP)

You know the ones I mean. Standing in the background of the Salem trials, leering over their torches. Or in those paintings of the French Revolution, pooling around the guillotine as far as the eye could see. I didn’t see pictures, but I know they were in the Coliseum too, yelling at the lions to finish the job.

I know now.

We haven’t become more civilized, not really. Their descendants are alive, well, and — writing online comments.

Every few years a trial grips us, rip-roaring through our collective psyche and tossing its heroes and villains into our living rooms. “I believe in Michael Jackson!” we yelp. “Did the glove fit? To me, those Loeb and Leopold boys always seemed a bit — well, off.”

Add Internet and stir to taste.

Now the trial that is gripping us is that tragedy from 2008, when a little girl named Caylee Anthony went missing, her skeletal remains were found months later, and her mother was accused. We are glued to our screens — not old-fashioned screens with TruTv or the local coverage, but the live online streams — watching the trial unfold, allegations flying right and left of abuse, accidental drowning, a full show of horrors.

But there’s a difference.

Unlike the O.J. trial or the Leopold and Loeb case or the numerous other brutal and gripping incidents that make us sort through questions about ourselves — as parents, as Intelligent People, as celebrity cult members — the sad saga of Caylee Anthony has never existed outside the Internet. From the lost girl to the gruesome discovery, every brutal revelation emerged online, to be dissected in comments and flung around in argument, acquiring a life of its own.

The Internet has a propensity for making communities of the people formerly known as strange bedfellows. You Are Not Alone, it proclaims, in dancing animated letters. Like cats? Like-like cats? Don’t worry, there are People Out There Just Like You.

Sometimes we wish you were alone. But you aren’t.

There are the people unified only by their love of dressing up as sensual rabbits. There are the people who run fan-fiction forums.

But nothing unifies people like shared dislike.

The flip side of the people who flock to online bulletin boards to root for Scotty McCreery on American Idol (“LORD THANK YOU SO MUCH U HEARD MY PRAYER FOR SCOTTY.. ILOVE YOU SCOTTY MC CREERY U R LIKE MY SON..SCOTTY DARLING KINDLY TAG ONE OF YOUR PICTURE TO ME PLEASE.. MWUAH.. STAY AS YOU ARE... NATION WIDE LOVES YOU”) are the people who flock to the online bulletin boards to root against Casey Anthony (“Guess she needs to put on her best performance while the cameras are rolling because when they go away she will be headed to a very dark place!!! So well deserved! Her Dad, Brother, or Mom didn’t damage her SHE”S EVIL!!!”).

She has been attracting a level of antipathy generally reserved for Medea. But it’s not antipathy qua antipathy. It’s the ire that binds. Web sites like the Caylee Daily have this case as their raison d’etre, spewing headlines like “Pathological Liar Casey Caught On Jail Tape Blowing Off Caylee Drowning Theory” — and attracting thousands of comments.

“The court of public opinion is the most uneducated court in the whole country,” said lead defense attorney Jose Baez.

Maybe. But it’s no longer the least informed.

The Internet enables us to insert ourselves into other people’s lives in previously undreamed-of ways. Whenever I read old crime fiction, I become impatient as Philip Marlowe piles into a jalopy and drives around the city for information. “Just Google it,” I hiss.

In a minute, you can discover things about another person that would have taken months of dating to unearth. Favorite books? Listed. Relationship history? Our Gchats are all archived. Skeletons don’t stay in the closet long; they’re all getting tagged on Facebook.

So when a trial like this seeps into our public consciousness, we suddenly have access to all kinds of information and speculation. We used to have to trek to Chicago to touch the hems of the Thrill Killers’ robes. Then television brought these sensational trials into our living rooms. Now it’s in our bedrooms, or bathrooms, for those people with iPads who have difficulty setting boundaries.

Yes, there’s a fair share of blazing torches, with names like “KC Is A Succubus.”

But there’s something different about these mobs. Sure, there’s the anticipated vitriol: We yell louder when we yell together. But there’s a surprising amount of fellow-feeling, the kind you don’t usually experience emerging from a movie theater or strolling out of the Coliseum after a particularly thrilling bout.

This isn’t just to bay for blood.

Before, you were That Lady Who Watches TruTV. You papered your house with newspaper coverage of the trial, clung to CourtTV for updates. You felt a bit odd. You saw that millions of others watched the coverage, but it didn’t make you feel much better.

TV made it personal. But the Internet made it communal.

It’s hard to be a mob of one. But in the comments sections and across the Internet, one voice becomes a roar. The court of public opinion is in session. And it’s watching your every move, with torches poised.