Photo by cottonmanifesto. Location: animal found on my front step in Brookline, brought inside for photographs (left the camera inside).
Urban species #152: Pill bug Armadillium vulgare
Of the 3000 or so described members of the terrestrial Isopod order Oniscidea, there are a conspicuous minority of species that can roll into a ball. These tiny land-dwelliing crustaceans do this to protect themselves from the piercing fangs of spiders and centipedes. Those isopods that have adapted this method of defense have done so at the cost of greater need for calcium intake, to harden their armor. All Oniscideans (woodlice, sowbugs, whatever you'd like to call them) possess mineral and metal sensing abilities that serve them well in urban environments. While they avoid toxic metals such as lead and cadmium, they seek out copper and zinc. These metals build up in their bodies, causing no harm, but providing some additional defense from predators--probably a bad taste.
The most common pill bug found in urban areas is originally native to the Mediterranean. The use of soil as ship's ballast transported them around the world, and they have also traveled the globe in the soil of potted ornamental plants brought from one continent to the next. Pill bugs and other woodlice are detritivores, preferring to eat plant material that is infected with fungi (is decaying). They are considered to be very important decomposers.