Isopods (woodlice) in general
Spend a few minutes rooting around in the leaf litter on the Boston Harbor Islands and you are almost certain to find small, grayish crustaceans known as isopods. While about half the known species of isopods live in the ocean (along with most of their crustacean relatives like crabs and lobsters) and some also in freshwater, there are roughly 5,000 species world-wide that live entirely on land.
Terrestrial isopods go by many different common names: roly-polys, sowbugs, pill bugs, and—in Great Britain—woodlice. The name for the order, “isopoda” (literally, “equal feet”), refers to the seven pairs of relatively similar legs. Many terrestrial isopods have a somewhat flattened appearance, and all have three main body parts: the head, the thorax or “pereon,” and the abdomen or “pleon.” The pereon is comprised of seven overlapping segments and each gives rise to one pair of walking legs. Terrestrial isopods breathe through modified gills which are attached to the segments of the pleon, and this is why they typically live in moist places such as under leaf litter and decaying wood.
Humans have played a large role in transporting terrestrial isopods around the world in soil associated with plants or used as ballast in ships. Many of the common isopod species found in North America are of European origin. On the Boston Harbor Islands, we have identified 10 species of isopods to date, and all of them are introduced from Europe.
How to identify the common rough woodlouse
Unlike the closely related pillbug or roly-poly, the common rough woodlouse does not roll into a tight ball when disturbed. Instead, it is more flattened in profile, and can scurry around quite quickly if it’s bothered. Other characteristics that help define this species include a very warty texture on the dorsal side of all segments (including the head), two segments on the thinner outer portion of the antennae (the flagellum), and three lobes on the front of the head when seen from above. Color can vary greatly, from dark gray to a more colorful mottled appearance with red, orange, yellow, and brown speckles. While some of these characters are difficult to see in the field, this is by far the most common isopod on the islands. Full grown adults can be up to 17 mm (~3/4 inch) long.
Where to find common rough woodlice on the Boston Harbor Islands
This species occurs throughout the Boston Harbor Islands. Look on the ground: under rocks and logs, in the leaf litter, in grasslands, in the upper layer of soil, and high up on the beach under drift or logs.You may sometimes see individuals up in bushes or on/under the bark of standing trees.
How common rough woodlice make a living
Like all terrestrial isopods, Porcellio scaber is an important decomposer in many ecosystems and its most common food source is dead plant material. By chewing up dead leaves, these arthropods begin the process of turning leaf litter into soil. Predators of common rough woodlice include the Woodlouse hunter, a spider with big fangs and toxic poison, as well as wolf spiders, predaceous beetles, and shrews.
While life history patterns may vary according to climate and region, the common rough woodlouse typically lives up to two and a half years. Females reproduce during the warmer months when they are at least a year old, and lay an average of 20 eggs each (Sutton et al. 1984). Interestingly, all female terrestrial isopods have a “marsupium,” or brood pouch, in which they store their eggs and where the newly-hatched isopods hang out for several days. As with all arthropods, isopods must shed their exoskeleton periodically in order to grow bigger. They accomplish this in two distinct stages: first the rear half is shed, and then several days later the front half is cast off.
Where in the world common rough woodlice occur
Porcellio scaber is native to most of Europe, and has been introduced to many places world-wide. It occurs throughout much of North America.
To learn more about common rough woodlice (and isopods in general)
On the web:
Oliver, P.G. and C.J. Meechan. 1993. Woodlice. Eds. D.M. Kermack, R.S.K. Barnes, and J.H. Crothers. Synopses of the British Fauna (New Series). No. 49. Field Studies Council, Shrewsbury, U.K.
Richardson, H. 1905. A monograph on the isopods of North America. Bulletin of the Unites States National Museum. No. 54.
Sutton, S. L. 1972. Woodlice. Ginn & Company Limited, London.
Sutton, S. L., M. Hassall, R. Willows, R.C. Davis, A. Grunday, and K.D. Sunderland. 1984. Life histories of terrestrial isopods: a study of intra- and interspecific variation. Pages 269-294 in S. L. Sutton and D. M. Holdich, editors. The biology of terrestrial isopods. Symposia of the Zoological Society of London. No. 53. Clarendon Press, Oxford.