We have a compost bin in our backyard. It's had its ups and downs over the years. Sometimes it has actually supplied organic-fertilizer-type material that we have used here on our extensive North Berkeley estate. More often, it has been a way of dealing with food scraps that we and the rest of enlightened civilization are trying to keep out of the landfill.
The principal visible engine of decomposition in our compost is red worms. When there's a steady supply of food and water, they seem to thrive. At some times of year, I'll actually see balls of them working on some hors d'oeuvre we've dumped out there.
But other critters are at work, too. I recently came across a piece of compost literature that talked about "FBI" as the components of a healthy waste pile–fungi, bacteria, and invertebrates. I'll take it for granted that fungi and bacteria are doing their thing out there and are perfectly happy with their lot.
That leaves the invertebrates. I've mentioned the worms. We get a few flies out there, and close inspection discloses some in larval form (maggots by another name). Another population that seems to enjoy the decompositional milieu is what I believe is a form of mite. When I pull the cover off the bin in the daylight, you see them as a shiny mass shifting minutely over unidentifiable food bits and everything else. I don't know whether they're a sign that things are just fine in the compost community or a little off. They've never invited themselves inside, so they're welcome to just do what they do. (Above: the "mites" in question, feasting on a stale bread crust as nearby smallish potatoes look on. Click for larger versions of the image.)