INVERTEBRATES
Status: Introduced

PILLBUGS

Description 

Pillbugs, also known as sowbugs, or woodlice, are familiar backyard creepy crawlies to most of us. Pillbugs are correctly
classified as terrestrial Isopods, and belong to the Class Crustacea. Familiar crustaceans include lobsters, crabs, shrimp, and daphnia (water fleas). The crustaceans, in turn, are part of a larger group, Arthropoda, or jointed-legged animals. All arthropods have a tough outer cuticle, a trunk divided into segments, and limbs which, because of their stiff cuticle,
articulate about flexible joints. 

Pillbugs are easily recognized by their flattened or round-backed profile, seven pairs of legs, and sharply-angled antennae.
Some species are able to roll into a ball when disturbed, hence the name, pillbugs. The broad head has 4 pairs of mouthparts
and is followed by the 7 main trunk segments, which bear the walking limbs. Behind these are 6 smaller segments
comprising the pleon. The pleon segments also carry limbs, but these are greatly modified. The first 5 pairs are the
pleopods. These are flattened and form a set of overlapping gills visible on the underside of the animal. They have many
functions including reproduction, gas exchange, and excretion. In some species the cuticle of the pleopods is in-folded, creating whitish, branching tubules that constitute pleopodal lungs. The final pair of appendages, the uropods , project from the rear of the animal and are sensory and defensive in function.

There are about 12 species of pillbugs found in the northern and central United States. Several other species are found in
coastal habitats and in the Florida wetlands. Only 4 species have been recorded from South Dakota. These are superficially
similar, but can be separated easily with a hand lens. With practice, they can readily be distinguished with the naked eye.

Cylisticus convexus, our only species that is capable of rolling into a ball, has a top surface that is a dark, glossy, gray-black
marked with pale, translucent streaks. Five pairs of pleopodal lungs are visible as pale patches on the outer margins of the
pleopods. These pillbugs are 0.3 to 0.5 inches (8-12 mm) in length.

Porcellio spinicornis is a broad, flattened species. The markings are characteristic: the head is dark brown and the back is
marked with a dark central stripe, flanked by 2 rows of yellow spots on a pale background. The first two pairs of pleopods
possess pleopodal lungs. P. spinicornis is 0.4 to 0.6 inches (10-15 mm) in length.

Porcellionides pruinosus is a rather slender species, light gray in color with whitish legs, and a characteristic surface
bloom like a fresh plum. The antennae are long and have distinctive white joints. The first two pairs of pleopods possess
pleopodal lungs. This species is 0.2 to 0.4 inches (6-11 mm) in length.

Trachelipus rathkei is the most common species in many areas. Color varies from rusty brown to dark gray. Moderately
broad and flattened, this species frequently rolls into a "C" when disturbed. As with Cylisticus convexus , there are 5 pairs
of pleopodal lungs. T. rathkei ranges from 0.3 to 0.6 inches (8-15 mm) in length.

Distribution

There is no formal recording scheme for isopods in South Dakota. Consequently, information on their distribution is sparse.
However, I have found Trachelipus rathkei in every South Dakota town that I have visited, and both Cylisticus convexus
and Porcellionides pruinosus appear to be generally common. Thus far, I have only found Porcellio spinicornis in isolated
localities in Aberdeen and Sioux Falls. More recordings would be valuable!! It is very likely that all of these species are
widespread in the state and other species probably await discovery.

One reason for the scant records of pillbugs t hroughout North America is that all of the common inland species are actually
European introductions. Available records indicate that most species reached North America in the early 19th century.
Genetic studies by Dr. Ronald Garthwaite and co-workers at the California Academy of Sciences show the Mississippi and
St. Lawrence Rivers to have been the major routes of colonization, strongly suggesting that introductions were brought about
through the lumber trade. In the past century the extensive movement of freight via road and rail has probably contributed to
the dispersal of pillbugs. This is obviously an on-going process and additional species are likely to colonize South Dakota
in the future.

The relative scarcity of Porcellio spinicornis, despite an abundance of apparently suitable habitats, suggests a recent
introduction. All of these species are strictly associated with human habitation and have probably colonized South Dakota
within the last century.

Natural History

Pillbugs can be searched for in any sites that provide locally humid retreats during the day. Compost piles and leaf litter
frequently have rich populations, and large numbers can often be found beneath logs and among rubble piles. Trachelipus
rathkei is particularly common beneath fallen wood or peeling bark. Porcellio spinicornis is almost always associated with
limestone or cement (also rich in lime) and should be searched for in rock piles, walls, or at the bases of stone buildings.
Porcellionides pruinosus and Cylisticus convexus are quite general in their choice of habitats, but are seldom found far
from buildings. All species are nocturnal and may venture over considerable distances during the night. On humid evenings,
they can often be seen in large numbers with the help of a flashlight. Pillbugs feed on dead vegetation, such as wood and leaf
litter.

There are nearly 4000 described species of pillbugs and they are the only crustaceans to have colonized land with
appreciable success. Despite this, they did not evolve the incredible waterproofing cuticle waxes of the insects and spiders,
and are quite susceptible to drying out. Woodlice kept in low humidities (<50%) will dehydrate lethally within a day.
However, they have one remarkable adaptive trick to help them. Once the humidity exceeds about 87% they can absorb
water vapor from the atmosphere for re-hydration. During nocturnal foraging, pillbugs frequently lose a considerable
proportion of their body water and can survive up to 30% dehydration. During the day, when they seek out humid retreats,
they can absorb water vapor to replenish these losses. When found indoors, pillbugs are usually associated with cold
surfaces, such as water pipes and stone slabs beneath exterior doors, where the local humidity is elevated. Water vapor
absorption takes place across the pleopods . Only a few groups of animals share this capacity to absorb water vapor from
air that is not saturated with moisture. They include flea larvae, booklice, silverfish, mealworms, and ticks.

Although pillbugs are strongly associated with human habitation in South Dakota, they are common in a variety of natural
habitats in their native Europe. Their much more restricted range of habitats in the Great Plains is probably due to climate.
Most European species appear to have originated in the Mediterranean, and they have only a limited tolerance of
sub-freezing temperatures. None of our species can survive cooling below about 21 F (-6.0 C). Many insects and other arthropods, on the other hand, can survive temperatures below -22 F (-30.0 C) (think of the insect larvae and pupae living beneath tree bark which sustain the tree-creepers and nuthatches over the South Dakota winters!). Survival at such temperatures is made possible by the production of remarkably effective antifreezes to protect the cells from ice formation. In mid-winter, pillbugs w ould have to burrow more than 24 inches (60 cm) below the ground surface to reach safe temperatures. Since they are not proficient burrowers, this probably eliminates them from most habitats, including the prairie grasslands. Around human habitation, pillbugs can seek refuge from the winter cold in deep crevices at the edges of buildings, in rubble piles, and compost heaps. Artificial heating and the high thermal inertia of buildings also elevates backyard temperatures in the proximity of buildings.

Pillbugs reproduce during the months of May through September. The males possess elongated first and second pleopods
which are used for fertilization (see Figure 1). Mating is seldom observed, but the male will crawl obliquely across the back
of a female and transfer sperm into the genital opening at the base of the pleopods. The female then develops a fluid-filled
ventral pouch, or marsupium , into which the eggs are laid. After a few days, the eggs hatch and the juveniles live for
several hours in the marsupium, absorbing the fluid and finally breaking free. Initially they have only 6 pairs of legs, but
acquire the 7th pair at the first molt. Like all arthropods , pillbugs must molt the cuticle in order to grow. This occurs every
few weeks throughout life. Pillbugs and other isopods molt the cuticle in separate anterior and posterior halves. Our species
live 2 to 3 years.

Significance

Pillbugs form an important component of the larger decomposer fauna, along with earthworms, snails, and millipedes. All of
these animals return organic matter to the soil where it is further digested by fungi, protozoans, and bacteria, hence making
nitrates, phosphates, and other vital nutrients available to plants. Although they may occasionally feed on roots, pillbugs do
minimal damage to live vegetation and should not be regarded as pests.

Pillbugs are also of importance in sites such as coal spoils and slag heaps, which face heavy metal contamination. They are
capable of taking in heavy metals such as copper, zinc, lead and cadmium and crystallize these out as spherical deposits in
the midgut. In this way, they remove many of the toxic metal ions from the soil. Furthermore, owing to their high tolerance of
these ions, they thrive where other species cannot, and promote the restoration of contaminated sites by accelerating topsoil
formation. This in turn favors the establishment of plants that stabilize the soils by root formation. Stabilized soils reduce
problems of toxic dusts and the leaching of metal ions into the ground water.


Figure 1. Sexual Differences In Pillbugs - from left to right: Trachelipus rathkei; T. rathkei, ventral pleon, female; T. rathkei , ventral pleon, male.


Pillbugs In The Classroom

Pillbugs can be collected from suitable habitats during the summer months and transferred to small vials. Plastic film
containers make good collecting containers. Maintaining a classroom culture is very simple. Use a small (1.5 liter) container
with a lid and maintain high humidity by adding moistened paper towels or a small jar of water. Bits of bark and leaf litter
provide suitable food. Make sure the chamber is always humid, but be careful to avoid standing water and significant
condensation. Under such conditions, pillbugs will reproduce and cultures can easily be maintained for many years.

Pillbugs are excellent animals for simple experimentation. Observe the rates of litter breakdown and conversion to a dark,
rich humus. Compare the rates of breakdown of leaves from different plant species. When given a choice of species, are
pillbugs selective foragers? Which parts of a leaf are eaten first? Are fresh leaves preferred to old ones? Try some simple
behavioral experiments. Using a tray with half of the floor colored white and half black (a choice chamber), see whether
pillbugs select dark or light habitats. How do they select their preferred habitat - is it by direct orientation, changes in the
rate of movement, changes in the frequency of turning, or a combination of these? You can also study water vapor absorption
if you have a microbalance with a resolution of 1 mg. Dehydrate animals to about 20% water loss in room humidity then
transfer them to a small humidified chamber without food or liquid water. After 5 hours or more, re-weigh the animals and
calculate how much water they have recovered. Do the animals show any distinctive behavior or movements while
absorbing water vapor?

Pillbugs are also interesting to observe under a low-power microscope (10 to 40 X magnification). See the compound eyes
composed of numerous facets (an arthropod characteristic), and the numerous minute hairs and cones on the cuticle surface,
which are actually sensory receptors conveying information (touch, smell) across the stiff cuticle. Apply a piece of adhesive
tape to the back of an animal and you can then turn it over to study the underside. Note especially the complex pleopods and
the white, branching pleopodal lungs, and determine whether your animal is male (with elongated first and second pleopods)
or female. If the uropods are irritated with a mounted needle or a pair of forceps, pillbugs will frequently exude a thick glue
which serves to entangle predators, such as ground beetles, centipedes, and spiders.

Glossary


Antennae - the first pair of head appendages in arthropods, primarily sensory in function; feelers.
Arthropod - an invertebrate with a segmented body, paired limbs on each segment, and a stiffened external cuticle. Arthropods contain the largest number of species of any animal phylum. They include insects, arachnids (spiders and relatives), crustaceans, centipedes, millipedes, and some extinct groups such as trilobites.
Coal spoils - earth and rock removed from a mine and dumped on the ground because the coal content is too low to be economically extracted.
Crustacean -a group of arthropods, predominantly marine, with calcium carbonate in the cuticle, primitively 2-branched limbs, and unique segmentation patterns. They include pillbugs, crabs, shrimp, krill, brine shrimp (sea monkeys) and daphnia (water fleas).
Cuticle - the stiffened exoskeleton of an arthropod, composed of protein, the polysaccharide chitin, and sometimes calcium
carbonate. It is frequently waterproofed with surface waxes. The cuticle is periodically molted to allow for growth
Habitat - the specific climate, terrain, soil and vegetation which provide suitable conditions for a certain species.
Humidity - a measure of the amount of moisture in the air. Relative humidity (RH; %) is the water content measured as a percentage of saturation ( dew point ) when condensation begins.
Isopod - the group of crustaceans including pillbugs, all sharing a dorso-ventrally flattened body, 7 pairs of walking legs, 5
posterior gills (pleopods), and formation of the marsupium in females.
Marsupium - the ventral brood pouch developed on trunk segments 1 to 5 of a reproductive female. When full of eggs or newly hatched juveniles, the marsupium forms a conspicuous yellow swelling.
Nocturnal - active at night.
Pleopods - the 5 pairs of flattened gills on the pleon segments. Each pleopod has 2 lobes, the inner (concealed in life) functions in water vapor absorption and nitrogenous excretion, the outer serves in respiration and may bear invaginated lungs. The first 2 pairs of pleopods are lengthened in males and serve in sperm transfer.
Slag heaps - the refuse that is left over from the smelting of metals and often is dumped on the ground in piles.
Uropods - the last pair of appendages in a pillbug. They comprise an outer and inner (smaller) branch from a single basal segment. They function in a sensory capacity like posterior antennae, and can secrete a defensive glue.
Ventral - the underside or belly of an animal.

References


Hopkin, S.P., 1991. A key to the woodlice of Britain and Ireland. Field Studies 7, 599-650. Published by the Field Studies
Council (UK). ISBN 1 85153 204 8
Hopkin, S.P., Hardisty, G.N. and Martin, M.H., 1986. The woodlouse Porcellio scaber as a biological indicator of zinc,
cadmium, lead and copper pollution. Environmental Pollution Series B 11, 271,290.
Sutton, S.L., 1980. Woodlice, Pergamon Press, Oxford.
Wright, J.C. and Machin, J.,1993. Atmospheric water absorption and the water budget of terrestrial isopods (Crustacea,
Isopoda, Oniscidea). Biological Bulletin 184, 243,253.
Wright, J.C. and O Donnell, M.J., 1995. Water vapour absorption and ammonia volatilization: adaptations for terrestriality
in isopods. In Terrestrial Isopod Biology (M.A. Alikhan, ed.), A.A.Balkema, Rotterdam.

Selected Resources For Teachers


Creepy Crawlies and the Scientific Method by Sally Kneidel, Fulcrum Press, Golden, CO.
Pet Bugs: A Guide to Catching and Keeping Touchable Insects by Sally Kneidel, Wiley and Sons, New York, N.Y.
The Pillbug Project: A Guide to Investigation by Robin Burnett, National Science Teachers Association, Wash., D.C.

Written by:
Dr. Jonathan Wright, Department of Biology, Northern State University, Aberdeen, SD. 1997.

Reviewed by:
Nels H. Troelstrup, Jr., Ph.D., South Dakota State University, Brookings, SD 57007.

Publication of the Pillbugs fact sheet was funded by the South Dakota Department of Game, Fish and Parks, Division of
Wildlife, Pierre, SD.