Why does micro USB 2.0 have 5 pins, when the A-type only has 4?

What is the extra, 5th, pin on micro usb 2.0 adapters for?

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Here is an image with the different connectors. Most of them have 5 pins, but the A-type host only has four.

_{(source: wikimedia.org)}

Answer

It's for On-The-Go, to select which device is the host or slave:

The OTG cable has a micro-A plug on one side, and a micro-B plug on the other (it cannot have two plugs of the same type). OTG adds a fifth pin to the standard USB connector, called the ID-pin; the micro-A plug has the ID pin grounded, while the ID in the micro-B plug is floating. The device that has a micro-A plugged in becomes an OTG A-device, and the one that has micro-B plugged becomes a B-device. The type of the plug inserted is detected by the state of the pin ID .