As far as animals without backbones are concerned, insects rule the land, crustaceans co-rule the seas. There are a few insects, however, which can survive in salt water, and there are some invertebrates, namely crustaceans, that live their entire lives away from water. Two of these are among the most ubiquitous of creatures around the globe, and they look so much alike that we generally refer to them by the same name. I’m talking about sow bugs and pill bugs, members of the Isopoda (equal-feet) order, and among the most common creatures found on Long Island.
Isopods are uniformly segmented creatures with a pair of like appendages, mostly locomotory in function, on each segment, or somite. Superficially they resemble fossil trilobites, among the earliest of invertebrates, but in general they are much larger. Sow bugs and pill bugs rarely reach more than half an inch in length. The sow bug is native to America, except in the West, where it was introduced. Pill bugs came from Eurasia. Both are gray, both are found on the ground under debris (leaves, rocks, boards, etc.), but there is a major behavioral difference. Pill bugs can roll up into a perfect ball, sow bugs cannot.
In fact, pill bugs are one of only a handful of organisms in the world that can locomote with the help of gravity. They can roll down an inclined plane, although they don’t make a habit of it. Rolling up is more of a defense against predators, just as the box turtle withdraws completely into its shell to protect itself or the armadillo balls up its tough scaly epidermis to protect its vulnerable underbelly when threatened. That’s one of the reasons the pill bug family has been given the name, Armadillidae.
In order to leave water permanently, the gills of sow bugs and pill bugs have to be kept moist so that they can exchange oxygen — the good air — for carbon dioxide — the bad. Terrestrial crabs and semiterrestrial crabs, say, fiddlers, also have gills and must keep them moist in order to breathe. That’s one of the reasons that one seldom finds sow bugs and pill bugs out in the open, and generally only during rainy or misty nights. Pill bugs have the added safeguard of balling up to keep from drying out.
As common as earthworms and ants, pill bugs and sow bugs do very little harm and much good. They are among several species that spend 90 percent of their time feeding on dead and decaying organic matter, both animal and vegetable. Thus, they rank up there with earthworms as important soil makers, helping to create the topmost soil layer, the so-called “organic horizon.”
In turn, they provide food for other invertebrates that live in dark, damp niches such as silverfish, centipedes, scorpions, various spiders, slugs, and salamanders. You very rarely find a horde of them as you might of Argentine ants, but it is a rare board in the yard or woods that will not sport a few when turned over. In Oregon where both sow bugs and pill bugs are introduced and not native, they are both called “potato bugs” because they are often found when digging up potatoes from under the soil.
Sow bugs and pill bugs are also unique in that their eggs hatch directly into larvae that are identical in aspect and composition to their parents. They don’t have a separate larval form, as do almost all aquatic and marine crustaceans. When you run into these two in the house or basement, no need to step on them as they don’t bite humans or give us diseases. They rank among the unsung heroes of the underworld. They enrich the soil and provide food for others while leading a quiet life, mostly out of view.