Update: I asked a Biology professor at the university where I work about these little creatures that I now know to be "springtails." I pasted his reply below. Several ICONers also responded (thanks to all of you). ICON is a fount of knowledge... about much more than just insulators!

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The insects are Collembola or commonly known as springtails (the name arising from the fact they "spring" or hop when disturbed by using a small appendage or "tail" on their abdomen). They are innocuous except for the annoyance in numbers and the occasional tendency to invade gardens and injure fruits or vegetables close to the ground. They don't bite or otherwise harm human or pet.

My guess is that they are Entomobryids but I can't be sure without closer inspection. If you have a sample of specimens, I would be glad to identify them for you. I'd love to have a sample to go with the great photos!

Springtails require moist conditions so you are safe from a kitchen or bedroom invasion as long as you don't live in a log or the mouth of a cave. You're right about the moist winter/spring and its effect on springtail numbers for the obvious reasons. Since rainy years are cyclic, springtail numbers also tend to be cyclic (you probably won't experience this every year).

http://en.wikipedia.org/wiki/Springtail

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Original post:

I live on one acre in the country in southwest Oregon, near Medford. My property has suddenly been invaded by trillions upon trillions of the tiny gray-black insects shown in this photo. Does anyone know... what ARE these things? Are they benign?

These have appeared on my property almost every year, for the past several years, typically in damp spring weather. They've always "hatched" in isolated places where there are damp stones or wood surfaces. They also form floating mats in standing water, and typically hatch in the millions (my estimate). They never last very long... a few weeks at most each spring. They disappear with warmer drier weather.

When disturbed, they "hop" or "spring" suddenly. They don't appear to fly or have wings. Close examination with a magnifying glass shows a very active little creature, with elongated body, two short antennae, and six legs. There is no discernible separation of abdomen from thorax. The head resembles an ant's head. They are soft and easily squished. Thankfully, they don't seem to bite, and they have not entered the house (yet).

A couple days ago, my yard was invaded by these things like never before. They've formed dozens of mats all over my yard, each covering many square feet, mostly on our decks, sidewalks, and stone walkways. There must be trillions upon trillions of them, forming a solid gray seething surface. We must wipe them off our shoes before we enter the house. It's like a scene from Alfred Hitchcock.

I'd REALLY like to know what these are, and if I should be concerned. Are there any entomologists in the ICON house?