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This astonishing GIF comes from Nanex, and shows the amount of high-frequency trading in the stock market from January 2007 to January 2012. (Which means that the Knightmare craziness of last week is not included.)
The various colors, as identified in the legend on the right, are all the different US stock exchanges. You might think there are only two stock exchanges in the US, but you’d be wrong: there are only two exchanges where stocks are listed. There are many, many more exchanges where stocks are traded.
What we see here is relatively low levels of high-frequency trading through all of 2007. Then, in 2008, a pattern starts to emerge: a big spike right at the close, at 4pm, which is soon mirrored by another spike at the open. This is the era of traders going off to play golf in the middle of the day, because nothing interesting happens except at the beginning and the end of the trading day. But it doesn’t last long.
By the end of 2008, odd spikes in trading activity show up in the middle of the day, and of course there’s a huge flurry of activity around the time of the financial crisis. And then, after that, things just become completely unpredictable. There’s still a morning spike for most of 2009, but even that goes away eventually, to be replaced with sheer noise. Sometimes, like at the end of 2010, high-frequency trading activity is very low. At other times, like at the end of 2011, it’s incredibly high. Intraday spikes can happen at any time of day, and volumes can surge and fall back in pretty much random fashion.
It’s certainly fair to say that if you take a long, five-year view, then you can see a clear rise in trading activity. But it’s also fair to say that there’s something quite literally out of control going on here. Just as the quants at Knight found themselves unable to turn off their machines for 30 long minutes last week, the HFT world in aggregate seemingly has a mind of its own when it comes to trading patterns. Or, to put it another way, if there’s a pattern here, it’s one incomprehensible to human minds. [More]

This is a computerized battle between "algobots" - algorithm-driven computers trying to make tiny bits of money very rapidly and repeatedly. Supposedly this was going to make the markets more efficient, but doubts are beginning to rise among observers everywhere of every political view. Known as high-frequency trading (HFT), it has been a real money-maker for exchanges, but it seems diminishing returns are setting in.

I agree. The problem with HFT isn't that we know it's dangerous, it's that we don't know anything at all. It's become flatly too complex for even its creators to understand what their creations are doing. Here's an example. The heart of HFT is speed: even the speed-of-light delay can make a difference, so most HFT shops locate their computers as close to the stock exchanges as possible. Even a few milliseconds can make a difference. At least, that's what a company called UNX thought until it moved from Burbank to New York:

This is where the story gets, as [Scott] Harrison put it, weird. He explains: “When we got everything set up in New York, the trades were faster, just as we expected. We saved thirty-five milliseconds by moving everything east. All of that went exactly as we planned.”
“But all of a sudden, our trading costs were higher. We were paying more to buy shares, and we were receiving less when we sold. The trading speeds were faster, but the execution was inferior. It was one of the strangest things I’d ever seen. We spent a huge amount of time confirming the results, testing and testing, but they held across the board. No matter what we tried, faster was worse.”
“Finally, we gave up and decided to slow down our computers a little bit, just to see what would happen. We delayed their operation. And when we went back up to sixty-five milliseconds of trade time, we went back to the top of the charts. It was really bizarre. I mean, there we were in the most efficient market in the world, with trillions of dollars changing hands every second, and we’d clearly gotten faster moving to New York. And yet we’d also gotten worse. And then we improved by slowing down. It was the oddest thing. In a world that values speed so much, you could be slower, yet still be better.”

The problem here isn't that UNX's move failed, it's that Harrison still doesn't know why it failed. Until we do, allowing HFT bots to control our equity markets is just begging for a catastrophe.
And that's where the transaction tax comes in. HFT works by making tiny amounts of money on a huge number of trades. Even a tiny tax, maybe a quarter of a percent per trade, would make HFT unprofitable and would put our markets back in the hands of human beings. Those human beings will still screw up, but at least there's a limit to how fast and how badly they can do it. [More]

I think the lesson is clear for farmers: Enter at your own risk. If you think this is confined to stocks you are thinking wishfully. And if you think individual investors can compete in such a trading environment, you don't understand the nature of the problem.

Our commodity markets will likely become completely counterintuitive as more HFT creeps into their volume. Farmers won't play the board - they will be little noticed roadkill.