back to organism index
Iso is Greek for "similar or equal." Pod
means "foot." Put them together and you have the isopod,
an organism that has an equal number of feet or legs on both sides
with all legs similar to one another. Isopods have 14 legs that
all function the same. This distinguishes them from closely related
organisms that have legs that are modified to perform different
functions, such as walking, feeding, feeling, grasping, and so
The many different species of isopods around the world share
certain characteristics. Isopods are crustaceans, distant kin
of shrimps, crabs, and crayfish. Like all crustaceans, isopods
have a segmented outer shell (seven overlapping plates) that provides
a measure of protection from the environment and predators. Like
their aquatic relatives, isopods get the oxygen they need to survive
through gill-like structures located at the bases of their legs,
rather than through lungs like most terrestrial organisms. That
is why isopods must keep moist at all timesif they dry,
Two kinds of isopods are of interest as classroom organisms.
The genus Armadillidium (armuhduhLIDeum)
is known casually as the pill bug or roly-poly. It gets these
names from its habit of rolling into a tight sphere when threatened
or stressed. The pill bug has a highly domed shape, short legs,
and inconspicuous antennae. When in its defensive rolled posture,
it is hard for a predator to grip, and it is also more resistant
to drying out.
Pill bugs move slowly and have a difficult time righting themselves
if they roll onto their backs on a smooth surface. They range
from light brown to dark gray or black. Often they have white,
cream, or yellowish spots on their backs. The largest individuals
of this kind of isopod can be 1 cm long, but most are 7 or 8 mm.
The second isopod used extensively in classrooms, genus Porcellio
(porselEoh), is commonly called the sow bug
or wood louse. These names are potentially confusing because Porcellio
don't show a particular affinity for swine, nor are they lice.
They are relatively flat with legs that extend a little bit beyond
the edge of the shell, and they have powerful antennae to sense
their environment. They move rather quickly and will use their
long antennae and little spikelike tail projections to right themselves
if they happen to roll onto their backs. Sow bugs come in a surprising
array of colors, including tan, orange, purple, and blue, as well
as the usual battleship gray. Their size is similar to that of
the pill bug.
In the wild, isopods are not usually seen out and about. They
are members of that large category of animals known descriptively
(not taxonomically) as cryptozoa, or hidden animals. They are
most often found in layers of duff and leaf litter, under rocks
or logs, or burrowed a short distance under the surface of the
soil. The environment they seek is moist and dark, in or near
dead and decomposing wood and other plant material. The former
is their main source of food, accounting, perhaps, for their common
name of wood lice. Isopods are not, however, above eating fresh
strawberries and carrots, making them a minor pest in the garden.
Life cycle. There are both male and female isopods,
but only another isopod can reliably tell them apart. After mating,
the female lays several dozen eggs, which she carries in a compact
white package on her underside between her legs. This package
is a specialized brood pouch, the marsupium, in which the eggs
develop for 3 or 4 weeks before hatching. A few days after hatching,
a swarm of fully formed, minute isopods strike out into the world.
They are nearly invisible at first but soon grow to a size that
can be seen by the unaided eye. Like all crustaceans that carry
a hard outer shell, isopods must shed their shells in order to
grow. In the molting process the shell is cast off, and the new
soft shell underneath expands before hardening. Interestingly,
the whole shell is not shed at once; first the rear (posterior)
shell segments are shed, and 2 or 3 days later the front (anterior)
ones fall off.
What to do when they arrive. The shipping container
contains damp paper to provide moisture. Upon arrival, mist paper
slightly. Food should be removed if it shows any sign of mold
and replaced with sliced carrot, potato, or apple. Pill bugs and
sow bugs can be kept in the shipping container for a few days
until ready to use in class. Moisten the paper towels as necessary.
If you are keeping them for a longer period of time, place them
in a terrarium with rich, moist soil. Place moist paper towels
in the container to provide humidity. Continue to add vegetables,
replacing them as necessary to control mold. Keep container at
room temperature in low light.
Classroom habitat. Isopods are excellent classroom
animalsthey exhibit interesting behaviors, they are small
but not tiny, they don’t bite, smell, fly, or jump, and
they are easy to care for. Isopods can live in just about any
vessel, from a recycled margarine tub to a 50-liter aquarium.
If the container is smooth-sided, it doesn’t even have to
be covered, because isopods can’t climb smooth surfaces
at all. A layer of soil covered with some dead leaves, twigs,
and bark is great, but isopods will be comfortable with some paper
towels or newspaper laid on the soil. They do like to have some
structure to crawl under.
Food and water. The most important thing to
remember is that the soil must be kept moist at all timesnot
wet, but moistso that the isopods don’t dry out. A
chunk of raw potato in the container with the isopods serves as
a source of both food and moisture. Otherwise they will eat the
decomposing leaves and twigs or the paper towels and newspaper.
Back to top