Background information for Pill Bug Lab
Introduction
An ecosystem can be defined as a specific biological community and its physical
environment interacting in an exchange of matter and energy. Abiotic factors, or nonliving
factors, that comprise the physical environment influence the distribution of species within an
ecosystem and hence, influence behavior. Temperature, humidity and pH are three examples
of abiotic factors that may influence species distribution within an ecosystem. Biotic factors, or
the living factors within an ecosystem, also influence the distribution of species within an
ecosystem and influence behavior. Intraspecific competition, interspecific competition and
predation are three examples of species interactions that may influence the distribution of a
given species. All species have tolerance limits for each environmental factor beyond which a
particular species cannot survive or is unable to reproduce. A given species will do what it can
to remain within favorable levels for each environmental factor that limits where the organism
can live.
Ethology is the study of animal behavior. Ethologists study an organisms’ reaction to the
environment around them by making careful observations of the interactions they see. When
selecting habitat, organisms naturally attempt to seek out favorable environmental conditions
necessary for their survival. Making observations about the behavior of an organism may tell an
ethologist which environmental factors are most critical for the species in question. This can be
invaluable information if the species happens to be classified as either endangered or
threatened as an understanding of which biotic and abiotic factors most significantly affect the
survival of the species would be critical information necessary for the development of a
successful recovery plan. In this lab investigation, the student will observe the behavior of
common pillbugs (Armadillidium vulgare) exposed to various environmental factors in order to
determine their abiotic or biotic preferences. This use of a model organism will show students
how ethology can provide scientists with information regarding the ecological preferences of a
given species.
Pillbugs are easily recognized arthropod crustaceans that many of us grew up playing
with in our own backyards. Pillbugs are not bugs at all as their common name implies but rather
belong to the class Crustacea which includes familiar aquatic isopods such as shrimp, crabs
and lobsters. The word isopod means “the legs are alike” and pillbugs have seven sets of them.
Unlike crabs, shrimp and lobsters however, pillbugs are terrestrial isopods and can be found in
dark, moist places beneath undisturbed objects lying on the ground such as logs, rocks and
bricks. Other common names used for terrestrial isopods include sow bugs, roly-polies and
wood lice. Pillbugs get their name by the ability of some species (genus Armadillidium) to roll
into a ball when disturbed. They may also roll into a ball to lesson water loss by evaporation
when humidity is low. Isopods from the genus Porcellio do not roll into a spherical shape but
only flex their backs when disturbed. There are nearly 4000 described species of terrestrial
isopods which shows that they have colonized land with great success. There are about twelve
species found in the northern and central United States. Interestingly enough, it is thought that
all the common inland species of pillbugs found in North America are actually European
introductions that have spread across the country via the lumber trade. Most European
varieties appear to have originated in the Mediterranean and subsequently have a limited
tolerance of sub-freezing temperatures. Pillbugs are exothermic, meaning their body
temperature depends on the temperature of their surroundings. This is one reason why pillbugs
are strongly associated with human habitation. Pillbugs can seek shelter from the winter cold in
places such as the deep crevices at the edges of our buildings and in compost piles.
Morphology and physiology
Terrestrial isopods range in color from gray to black and have oval shaped bodies that
range in length from 5 to 15 mm. Pillbugs have sharply angled antennae, four pairs of
mouthparts on their broad head which is attached to the main trunk, called the thorax, consisting
of seven segments. These segments are where the legs are articulated from. Behind the
walking limbs are six smaller segments which make up the pleon. The pleon carries modified
limbs that are not used for walking but for various purposes such as reproduction, gas
exchange, and excretion. The first five pairs are called the pleopods. They are flattened and
form a set of overlapping gills that are visible on the underside of the animal. Pillbugs breathe
through this gill-like structure which means they must live in a moist environment in order to
exchange gases. Sex determination can be made by observing the first and second pleopods.
Males have elongated first and second pleopods when compared to females. The last pair of
appendages on a pillbug are called the uropods, which are both sensory and defensive in
function. A pillbug can defend itself from a predator such as a spider or centipede by using the
uropods to release a thick glue that serves to entangle whomever happens to be after them.
These are easily visible as they project from the rear of the animal.
The lifespan of most isopods is about two years but they may live up to five years.
Pillbugs reproduce during between the months of May and September. Mating occurs as the
male crawls along the back of the female and transfers sperm into the genital opening at the
base of the pleopods. The female then develops a marsupium, or brood pouch, into which her
eggs are laid. Eggs hatch out a few days later and the juveniles live in the brood pouch for
several hours. The young finally leave the brood pouch looking just like miniature adults with
the exception of one pair of legs which develops after the first molt.
Niche and Impact on the Environment
Pillbugs are detritivores, feeding on dead vegetation such as wood and leaf litter. All
pillbugs are nocturnal and can travel great distances during the night in the search for food.
This nocturnal foraging is due to the fact that the cuticle of pillbugs is not covered with wax as it
is in other arthropods such as insects and spiders. Nocturnal foraging means they lose less
water to evaporation than they would during the day. Pillbugs have an interesting adaptation to
prevent desiccation. If the humidity of the air is over 87%, pillbugs can absorb water vapor
directly from the atmosphere for re-hydration. During the day pillbugs will seek out humid
retreats where they use their pleopods to absorb water to replace the water lost while foraging
the night before.
Although pillbugs common to the northern and central United States are alien species,
they play an important role in the foodweb of each ecosystem they inhabit in that they serve as
a food source to many predators such as spiders, centipedes, and frogs. Additionally, they are
detritivoves, which means that they help with nutrient recycling as they feed on and break down
plant material, which improves the quality of the soil. They also may provide an important
ecosystem service to land contaminated with heavy metals such as lead, cadmium and zinc.
Pillbugs are capable of taking heavy metals out of soil and crystallizing them as spherical
deposits in the midgut. By doing so, their presence helps to clean up sites that are
contaminated by heavy metals, such as coal spoils or slag heaps. Their presence at
contaminated sites also accelerates the formation of topsoil, which then allows for the
establishment of a plant population whose roots help to stabilize the contaminated soil. This
helps to reduce the problems of toxic dusts and of leaching heavy metals which could
contaminate groundwater supplies. On the other hand, pillbugs also can cause problems for
people as they can eat and damage plants in southern states and in greenhouses.