Photos by urbpan.
Urban species #333: Globular springtail Dicyrtoma marmorata
I think this animal is Dicyrtoma marmorata. But there's a very good chance I am wrong. It is minute--the size of the head of a pin--and more to the point, I am not an entomologist. But I am absolutely sure that it is a Collembolan, a springtail, and that it is a member of the family Sminthuridae, the globular springtails. The species designation I am guessing based on some websites I looked at today, especially this one.
Springtails are delightful little animals, six legged creatures which have bottoms which are affixed with a propulsory appendage. They walk about on their legs, but when they need to make a quick escape, their furcula (that's what it's called) launches them a great distance away, very suddenly. Many springtails have the word "flea" in their common names for this reason, but they are not related to fleas. In fact, despite having six legs and antennae, they aren't insects at all, their ancestors having diverged from insects at about the same time that the crustaceans did.
Collembolans are very common animals in compost heaps, and on decaying leaves and other organic matter, but they may be overlooked because of their size. Often they appear rather suddenly, in great numbers. They may be active when it is too cold for most other arthropods, and there are even some species that feed on the algae that grows on snow. Recent studies have shown that springtails play an important role in the reproduction of mosses. They are thought to mostly feed on fungi, algae, pollen grains, and other tiny organic particles. Fascinating springtail social behavior has been recorded with astonishing photography in the documentary series "Life in the Undergrowth." (link contains video footage!)
Globular springtails on a cement wall.
Three sminthurids in the bottom of a bucket.
The color pattern that influenced my identification is only visible under the microscope.