s are popularly called sowbug
s, pillbugs or woodlice. They are those tiny, segmented things that roll up in a ball
when disturbed, placing soft parts inside, hard shell outside.
These tiny things are crustaceans, a group which includes daphnia, fairy-shrimp, crayfish, hermit crabs, copepods, true crabs, and lobsters, among others. The foregoing list itself reveals the uniqueness of the pillbug: it is the most successful invader of dry land in the group. Most crustaceans are marine; a few inhabit fresh water, and even the land crab must return to water to breed. The humble pillbug alone in the group is able to carry out its entire life cycle on land.
This is a pretty neat trick, considering. First, the animal has gills, not lungs. Accordingly, the gills must be kept wet, or the pillbug suffocates. Any rain drop that falls is precious, and is routed immediately to the gills on the abdomen. The young develop in a water-filled brood pouch and emerge ready for terrestrial life. Common in temperate climates, these tiny beasts hide in moist places during the day and emerge at night to scavange on fallen leaves and other plant material. Where they are abundant, they are important in the formation of soil.
Rolling up in a ball is about their only defense, though some produce substances disagreeable to spiders, an important predator. Most are drab or earthy colored. There is, however, one European species with large red spots; it is conjectured that these spots confer some protection by similarity to black-widow spiders also common in the area.
Far from the sea but still breathing with gills and brooding their young in tiny drops of water, they are like tiny memories of ocean. Do they hear the surging of the waves when they sleep?
PHYLUM: Arthropoda, SUBPHYLUM: Crustacea, CLASS: Malacostraca, SUBCLASS: Eumalacostraca, SUPERORDER: Peracarida, ORDER: Isopoda.