Documenting Diversity at the Katharine Ordway Natural History Study Area
One organism in the vast biodiversity of the Katherine Ordway Natural
History Study Area is the common pill bug, which includes a huge
diversity at the species level, but is of the genus
Armadillidium. Other common names for this animal include “sow
bug,” “rolly polly” and “woodlice.”1
Armadillidium is in the animal kingdom and the full Linnaean taxonomy is as follows1:
Kingdom – Animalia
Phylum – Arthropoda
Subphylum – Crustacea
Class – Malacostraca
Superorder – Peracarida (Isopods and Amphipods)
Order – Isopoda
Suborder – Oniscidea (Woodlice)
Family – Armadillidiidae
Genus – Armadillidium
Species – (too diverse)
Armadillidium has unique diagnostic characteristics from other bugs
found in the Ordway Study Area, since it actually is a
crustacean. The following characteristics describe the genus and
not the species, however, because there is no sure way to determine the
exact species out of the large diversity of the Armadillidium
genus based on morphological data alone. One thing to note is
that the most common species of Armadillidium in Europe and North
America is Armadillidium vulgare, which is the common woodlouse.
Therefore, pill bugs found at the Ordway Study Area are most likely
Armadillidium vulgare, but more information—such as genetic data—would
be needed to make a definite conclusion.
• Armadillidium’s behavior makes it mistakable for an
insect, but a quick count of its legs reveals that it has seven pairs,
ruling out it being an insect.
• Armadillidium have three main body parts: the head, thorax and abdomen
• A closer look reveals that Armadillidium has an
exoskeleton, a segmented body plan and antennae, which makes it easy to
identify as an arthropod.
• Armadillidium do not have a waxy cuticle to prevent
evaporation like some arthropods, so they are restrained to moist
• Identification of the genus is loosely based on the
size of Armadillidium—around 15 mm—compared to other Isopods
• A more detailed diagnostic of Armadillidium is the trapezoidal shape of the last body segment2
The geographic range of Armadillidium is vast. It is widespread
in Europe and North America and is being introduced worldwide.3
The habitat of Armadillidium is damp areas, as it needs a moist
environment for its modified lungs, called pseudotrachea, to
function. The specific location of an Armadillidium’s burrow can
vary greatly, from underneath a damp stone to the basement of a
The necessity of moist conditions for respiration influences
Armadillidium’s behavior, as it stays in its burrow during the day to
preserve water. The lack of a waxy cuticle also forces
Armadillidium to employ water-saving strategies. This means that
Armadillidium are active at night.3
Other behaviors of Armadillidium lie in the family structure. The
father has authority within the living arrangement and he also guards
the burrow from potential threats. The mother and father gather
food together to provide for the whole family. After eating,
every member of the family assists with removing fecal pellets from the
burrow and placing near the entrance.3
Due to its habitat of moist areas under stones and bark, Armadillidium
is omnivorous and usually eats moss, fungi, algae or other decaying
organic matter.4 In this way, the role of Armadillidium in an ecosystem
is to decompose dead plants and animals. Its feeding habits are
flexible, though, as it can switch from a vegetarian into a scavenger
during a drought.3
One way that Armadillidium interacts with other organisms is the
defense mechanism it employs when it is hunted. It curls up into
a tight ball so that it is encased in its exoskeleton to protect its
soft, vulnerable underside from predators. Some species secrete a
chemical that wards off spiders.3
Another defense mechanism in some species is their coloration.
The European pillbug has markings that resemble the European black
widow. Animals usually avoid this poisonous arachnid, so then
predators avoid Armadillidium, since it resembles the deadly spider.5
It takes Armadillidium about one year to reach sexual maturity.
While they are juveniles, pillbugs molt their hard exoskeleton at
regular intervals, as they outgrow it. Once they are sexually
mature, they continue to molt, though they do so much less
frequently. Also, mature Armadillidium molt in two stages.6
The first stage is the rear half of the body molting, and then the
front half follows suit two or three days later. This window
between molts is the only time that female pillbugs can have their eggs
In contrast to most crustaceans, Armadillidium reproduce on land
instead of in the water. Individuals mate year-round, but are most
sexually active in the spring. Fertilized eggs develop in a
structure called a brood pouch, which is an area filled with fluid to
support the embryo.3 The brood pouch is located on the underside of a
female’s abdomen and a single brood may have as many as 200 eggs.7
The eggs hatch inside the pouch after three to seven weeks, and then
the offspring remain in the pouch an additional seven weeks, so that
offspring are fully developed before they are released from the pouch.8
Females can produce one or two broods, depending on their size and
environmental conditions. If females are excessively hydrated,
for instance, it causes stress on the body, preventing multiple
broods. Conversely, if food is in short supply, double broods are
In terms of reproduction, Armadillidium is semi-annual, meaning that it
can reproduce multiple times per year (up to five times in some
species). Females in this genus are iteroparous, since they
reproduce twice or more in their life. The life cycle of
Armadillidium is bivoltine, as two generations are produced per year,
The life expectancy of Armadillidium varies between species. The
average age length is two or years, though some live as long as five
Armadillidium are native to Europe but were introduced as a pest to
North America and are now highly prevalent on both continents.
Some species have been found in North Africa, as well.3
An Armadillidium individual was collected at the Ordway Study Area and
was found near the base of two trees. It was in the soil
underneath the leaf litter. The site of collection is pictured
1. "ITIS Standard Report Page: Armadillidium Vulgare." Integrated
Taxonomic Information System. Web. 03 May 2010.
2. "Species Armadillidium Vulgare - Woodlouse - BugGuide.Net."
BugGuide.Net. Web. 03 May 2010. http://bugguide.net/node/view/94180.
3. Brown, C. 1999. "Armadillidium vulgare" (On-line), Animal Diversity
Web. Accessed April 30, 2010 at
4. "Armadillidium: Facts, Discussion Forum, and Encyclopedia Article."
AbsoluteAstronomy.com. Web. 03 May 2010.
5. "Pillbugs and Woodlice: Isopoda - Behavior and Reproduction." 2010.
Web. 1 May 2010.
6. "Order Isopoda - Pillbug - Armadillidium Vulgare." North American
Insects and Spiders. Web. 03 May 2010.
7. "SowBug, PillBug or Potato Bug | How to Control, Get Rid Of,
Fumigate, Eliminate, Eradicate, Kill, Get Rid Of, or Exterminate - in
Toronto, Mississauga & Vaughan." QPM | Pest Control, Fumigation and
Extermination for Toronto, Mississauga & Vaughan. Web. 03 May 2010.
8. "PNNL: Science & Engineering - What about Pillbugs?" PNNL:
Science & Engineering Education Website. Web. 03 May 2010.
9. Hamaied S., Charfi-Cheikhrouha F. Life cycle and Population dynamic
of Armadillidium pelagicum Arcangeli, 1955 (Isopoda, Oniscidea) at
Aouina (2004) Comptes Rendus - Biologies, 327 (4), pp. 343-352.
Compiled by Mike Snavely.
Biodiversity & Evolution (BIOL 270) Professr Sarah Boyer. Spring 2010.
Specimen collected at Macalester College’s Katharine Ordway Natural History Study Area on April 15, 2010.