Invertebrate Anatomy OnLine
notes on Porcellio, Ligia, and Oniscus
Copyright 2001 by
is one of many exercises available from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises can be accessed by clicking on the links to the left. A
glossary and chapters on supplies and laboratory techniques are also available. Terminology
and phylogeny used in these exercises correspond to usage in the Invertebrate
Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated
figure callouts refer to figures in the textbook. Callouts
that are not hyphenated refer to figures embedded in the exercise. The glossary
includes terms from this textbook as well as the laboratory exercises.
Mandibulata, Crustacea sP,
Eucrustacea, Thoracopoda, Phyllopodomorpha, Ostraca, Malacostraca C,
Eumalacostraca, Caridoida, Xenommacarida, Nomen nominandum, Neocarida,
Peracarida SO, Isopoda O,
Armadillidiidae F , Porcellionidae F,
and Ligiidae F, (Fig
16-15, 19-67, 19-90)
by far the largest and most diverse animal taxon, includes chelicerates,
insects, myriapods, and crustaceans as well as many extinct taxa such as
segmented body primitively bears a pair of jointed appendages on each segment. The
epidermis secretes a complex cuticular exoskeleton which must be molted to
permit increase in size. Extant
arthropods exhibit regional specialization in the structure and function of
segments and appendages but the ancestor probably had similar appendages on all
segments. The body is typically divided into a head and trunk, of which the
trunk is often further divided into thorax and abdomen.
gut consists of foregut, midgut, and hindgut and extends the length of the body
from anterior mouth to posterior anus. Foregut
and hindgut are epidermal invaginations, being derived from the embryonic
stomodeum and proctodeum respectively, and are lined by cuticle, as are all
epidermal surfaces of arthropods. The
midgut is endodermal and is responsible for most enzyme secretion, hydrolysis,
coelom is reduced to small spaces associated with the gonads and kidney. The
functional body cavity is a spacious hemocoel divided by a horizontal diaphragm
into a dorsal pericardial sinus and a much larger perivisceral sinus. Sometimes
there is a small ventral perineural sinus surrounding the ventral nerve cord.
hemal system includes a dorsal, contractile, tubular, ostiate heart that pumps
blood to the hemocoel. Excretory
organs vary with taxon and include Malpighian tubules, saccate nephridia, and
organs also vary with taxon and include many types of gills, book lungs, and
nervous system consists of a dorsal, anterior brain of two or three pairs of
ganglia, circumenteric connectives, and a paired ventral nerve cord with
segmental ganglia and segmental peripheral nerves. Various
degrees of condensation and cephalization are found in different taxa.
is derived with centrolecithal eggs and superficial cleavage. There
is frequently a larva although development is direct in many. Juveniles pass
through a series of instars separated by molts until reaching the adult size and
reproductive condition. At
this time molting and growth may cease or continue, depending on taxon.
is the sister taxon of Chelicerata and in contrast has antennae on the first
head segment, mandibles on the third, and maxillae on the fourth. The
brain is a syncerebrum with three pairs of ganglia rather than the two of
chelicerates. The ancestral mandibulate probably had biramous appendages and a
J-shaped gut, posterior-facing mouth, and a ventral food groove. The two highest
level mandibulate taxa are Crustacea and Tracheata.
is the sister taxon of Tracheata and is different in having antennae on the
second head segment resulting in a total of 2 pairs, which is unique. The
original crustacean appendages were biramous but uniramous limbs are common in
derived taxa. The
original tagmata were head but this has been replaced by head, thorax, and
abdomen or cephalothorax and abdomen in many taxa. Excretion is via one,
sometimes two, pairs of saccate nephridia and respiration is accomplished by a
wide variety of gills, sometimes by the body surface. The nauplius is the
earliest hatching stage and the naupliar eye consists of three or four median
includes all Recent crustaceans except the remipedes. The taxon is characterized
by a primary tagmosis consisting of heat, thorax, and abdomen although the
derived condition of cephalothorax and abdomen is more common. Eight is the
maximum number of thoracic segments.
the ancestral thoracopod the thoracic appendages were turgor appendages used for
suspension feeding in conjunction with a ventral food groove. Such appendages
and feeding persist in several Recent taxa but have been modified in many
compound eyes are stalked primitively although derived sessile eyes occur in
includes most of the large and familiar crustaceans such as crabs, shrimps,
lobsters, crayfish, isopods, and amphipods. Primitively
the trunk consists of 15 segments, eight in the thorax and seven in the abdomen
but in most Recent species the abdomen has only six segments. The female
gonopore is on the eighth thoracic segment and the male on the sixth.
is a large and ecologically important malacostracan taxon. It
includes the amphipods and isopods, each with well over 5000 species, as well as
several smaller taxa such as mysids, cumaceans, and tanaids. Peracarids
have the characteristics of Malacostraca and are further defined by possession
of a ventral, thoracic marsupium, or brood pouch, in which the eggs are brooded. The
mandible bears a movable tooth, the lacinia mobilis, between the molar and
incisor. Development is direct and there is no larva. Most
peracarids are small, less than 2 cm and primarily inhabit marine and freshwater
habitats, although some are terrestrial.
is a large and diverse taxon with nine high level subtaxa. Isopods
are common in marine and freshwater habitats and there are important terrestrial
and parasitic suborders. It
includes the familiar terrestrial pill bugs, wood lice, and sea slaters, and
about 4000 less familiar marine species. Isopods
are usually dorsoventrally flattened and the gills and heart are abdominal. One
thoracic segment is fused with the head and there is, correspondingly, one pair
of maxillipeds and seven pairs of thoracic walking legs. The
gills and heart are abdominal.
are the terrestrial or semiterrestrial isopods and the most successful
terrestrial crustaceans. Oniscoidea includes the woodlice, sowbugs, roly-polys,
and pillbugs which are common in most parts of the continent. The
terrestrial isopods are good examples of peracaridan organization and are common
and readily available at most inland localities, something that is true of no
other peracaridan and of few crustaceans.
first antenna is tiny and the mandible has no palp. The pleon consists of five
free pleomeres plus a pleotelson consisting of the sixth pleomere fused with the
telson. The exopods of some pleopods have pseudotracheae and are modified for
respiration in air.
most common American species live around or in human habitations and are
introductions from Europe. Native
species tend to occur in undisturbed areas. Common
genera in yards, gardens, and greenhouses are Armadillidium, Porcellio, and Oniscus. Ligia occurs
above the high tide line close to the edge of the sea, is very fast, and reaches
lengths of 3 cm.
three inland genera are small animals and not suited for dissection or the study
of internal anatomy in an introductory course, but no readily available
peracaridean is. They
are ideal, however, for a study of external crustacean and peracaridean anatomy. These
animals, or others much like them can be collected locally or purchased from
biological supply companies. They
are easily cultured in the laboratory.
is common near houses, especially in gardens or greenhouses. It
may be found in humid places under stones, bricks, or logs. The
color of the dorsum is variable. Often
it is blue-black but may be mottled with light patches against a dark
most parts of North America Armadillidium is
the only woodlouse able to roll itself (enroll) into a completely closed sphere
with no appendages protruding. The
term "pillbug" is applied to isopods with this ability.
exercise is written specifically for Armadillidium but
can be used with little modification for any of the common oniscoideans
including Porcellio, Oniscus,
and Ligia (Fig
or preserved material can be used. Refer to the key at the end of the exercise
to determine the genus of your specimen.
you are studying living specimens, place one in a jar to which a piece of
chloroform- or ether-dampened cotton has been added. When
the animal ceases to move, remove it to a small dissecting pan or glass dish and
place it on the stage of the dissecting microscope. Do
not immerse it in water.
isopod body is typically dorsoventrally flattened but that of some of the
terrestrial oniscoids, such as Armadillidium, is
strongly arched and the flattening is not striking. (Porcellio,Oniscus,
and Ligia are
body is conspicuously segmented (Fig 1, 6. 19-61).
body wall of isopods is similar to that of other crustaceans. Most
of it, and the only part of it you can see, is the cuticle,
or exoskeleton, which covers the animal and is molted periodically. Over
much of the body the cuticle is hardened, or sclerotized, by tanning and the
deposition of calcium salts, to form hard plates known as sclerites. The
cuticle between sclerites is soft and flexible to allow motion of the sclerites
with respect to each other. The
flexible areas are articular
isopod cuticle has the same general construction as that of other arthropods. The
woodlice, as terrestrial animals, might be expected to have a waxy waterproof
layer outside the epicuticle but this is not the case. Desiccation,
or its avoidance, is a major problem facing woodlice but one they have solved
satisfactorily, mostly behaviorally. So
satisfactory are the solutions that some inhabit deserts. The
cuticle of Armadillidium bears
tiny pits of unknown function.
that of all malacostracans, the isopod body consists of an anterior head, middle
thorax, and posterior abdomen. The head consists
of five segments, the thorax eight,
and theabdomen six. Thoracic
segments are thoracomeres and abdominal segments are pleomeres. These ancestral
tagmata are slightly modified in isopods by the inclusion of one thoracomere
with the head to form a cephalothorax. The
remaining seven thoracomeres form the pereon (Fig
1) and are thus known as pereomeres. The
cephalothorax consists of the head plus the first thoracomere. The
abdomen is sometimes referred to as the pleon.
Typical of Malacostraca, the segments of all three tagmata bear appendages.
small cephalothorax is
visible dorsally as the first tagmata of the body (Fig 1, 19-61). It
bears the eyes, five pairs of head appendages, and a pair of maxillipeds, which
are the appendages of the first thoracic segment. The
separate segments making up the cephalothorax are fused indistinguishably and
are not recognizable.
Figure 1. Dorsal
view of the pillbug, Armadillidium
vulgare, from Greenwood, South Carolina. Articles
of the second antennal peduncle, pereon, and pleon are numbered. Isopod31La.gif
eyes on the sides of the
cephalothorax are composed of about 15-50 ommatidia, or facets (Fig 1). The facets are
visible with magnification.
first two pairs of head appendages are the first and second antennae. The second
antennae are large,
multiarticulate, filiform structures (Fig 1, 2). Each
is composed of a basalpeduncle of
five articles and a distal flagellum of
a variable number of small articles (Two
in Armadillidium and Porcellio,
three in Oniscus, and many
in Ligia, Fig 6).
In Armadillidium the
second antennae fit neatly into broad recesses on the front of the head
permitting them to be folded completely out of harm's way when the animal
antennae are sensory and, as the animal walks, are used to touch the ground
ahead before venturing onto it. This
is easily observed in active animals.
antennae are tiny and
difficult to find until you know exactly where to look (Fig 2). Hold
the animal vertically so you can focus on the anterior surface of the head and
look medial to the base of the second antennae. The
first antennae are between the bases of the second antennae and are composed of
only three tiny articles.
remaining three pairs of head appendages are mouthparts. They
are clustered together, along with the maxilliped of the thorax, on the ventral
surface of the head, surrounding the mouth. Dissection
and study of the mouthparts is difficult in these small animals and your
instructor may excuse from attempting it.
two mandibles are on either side of the mouth. Their
incisors and molars are heavily sclerotized. Oniscoidean
mandibles lack a palp but a lacinia mobilis is present. The first maxillae are
immediately posterior to the mandibles and are followed by the second maxillae.
the mouthparts are covered and hidden by the maxilliped. This
is the fused pair of appendages of the first thoracomere and is the most
posterior of the appendages of the cephalothorax. You
can see the broad flat maxilliped on the posterior side of the cluster of
mouthparts on the ventral head. If
you move it posteriorly, you can see the mouthparts.
Figure 2. En
face view of the head of Armadillidium.
seven thoracic segments (thoracomeres) independent of the cephalothorax
constitute the pereon. Each
segment is a pereomere and
their appendages are pereopods. The
seven pereomeres are easily seen dorsally. The
cephalothorax fits into a median notch on the anterior margin of the first
pereomere (Fig 1, 19-61).
pereomeres are good examples of typical arthropod segments. Each is covered by
an exoskeletal ring composed
of two sclerites. Dorsally
is the heavily sclerotized, archedtergite (Fig
seven tergites of the pereon are easy to see and count.
ventral portion of each segmental exoskeletal ring is the sternite (Fig
is also sclerotized but less so than the tergite. The
ventral surface of the pereon is more or less flat although each sternite bears
strong ridges and grooves, mostly for reception of the legs when they are folded
against the body.
reproductively active females the sternites are hidden by thin, membranous oostegites,
or brood plates. Do
not mistake the oostegites for sternites which lie dorsal to them and may be
hidden by them. The
oostegites are soft and pliable. They
will be discussed in more detail later.
you see white areas on the venter of the anterior pereomeres, it means your
specimen is getting ready to molt. These
areas are deposits of calcium salts that have been reclaimed from the old
cuticle in anticipation of the loss of that cuticle.
lateral extremities of the tergites reach ventrally far below the level of the
extensions are the epimera,
or side plates (Fig 1, 3). Together
the epimera on each side form a wall that encloses and protects the space
ventral to the sternites. In
females the marsupium occupies this space and in both sexes the legs are here.
seven pairs of pereopods,
or walking legs, resemble each other, a condition responsible for the name
"isopod" (iso = equal, pod =
one of the pereopods and note that it is uniramous as are the pereopods of most
malacostracans (Fig 3). The
pereopod arises from the epimeron at its junction with the sternum and epimeron.
of malacostracan thoracopods, the leg is composed of a linear series of seven
proximal article is the coxa, which in oniscoideans is fused rigidly with the
tergite to form the epimeron. It
does not give the appearance of being part of the leg nor is it recognizable as
being a distinct part of the epimeron. Consequently
the legs appear to be composed of six articles. The
first of the six is the long basis which
articulates with the coxa of the epimeron (Fig 3). It
is followed, in order by the ischium, merus, carpus,
and propodus. The
final dactyl is
is characterized by the presence, in breeding females, of a marsupium,
or brood pouch, on the ventral pereon of females (Fig 19-52, 19-62). This
pouch is formed of thin, membranous brood plates, or oostegites,
extending medially from the basis of some of the pereopods. A
marsupium is not present in males.
woodlice the first five pairs of female pereopods bear oostegites. Together
these ten flexible plates form the marsupium, into which the eggs are released
and in which they develop. The
marsupium of terrestrial isopods holds water in which the embryos develop, much
as they would in the ancestral aquatic habitat.
your specimen is a female, look for the marsupium. If
it is present, it will completely cover the ventral aspect of the pereon,
obscuring the sternites. If you can see the sternites, there is no marsupium. Each
of the oostegites is independent of the others and can be lifted with a nadel. Lift
the medial edge of the marsupium and see if eggs, embryos, or juvenile isopods
are present. Development
in peracarids is direct, without a true larva, and occurs in the marsupium. In
most peracarids development produces a miniature replica of the adult but in
isopods there is first produced a manca "larva". The
manca is also a miniature replica of the adult but has only six pairs of
pereopods instead of seven. The seventh pair is added by a subsequent molt.<
Figure 3. Ventral
view of pereomere and pereopod 5 of Armadillidium.
malacostracans the male gonopore is on the eighth thoracic segment, which is
pereomere 7 of isopods. In
isopods the male gonopore is on a median genital
papilla on the ventral
surface of this segment (Fig. 4). The
papilla is a transparent, membranous triangle, or cone, pointing posteriorly and
overlapping the bases of the anterior abdominal appendages.
female gonopores of malacostracans are on the ventral surface of thoracic
segment 6 (pereomere 5) but they are difficult to see. The
male gonopods transfer sperm to these openings during copulation.
The abdomen is
composed of six segments of which the last, or sixth, is fused with the telson
to form a pleotelson (Figs
1, 4, 5). The
first five segments are independent and together form the pleon (Fig
segments are called pleomeres and
their appendages are pleopods. The
appendages of the sixth abdominal segment are uropods. The
telson, which is the terminal region of the body, is not a true segment.
biramous pleopods (Figs
4, 5) consist of a basal protopod and
two rami. The
medial rami, or endopods,
are very soft with a thin cuticle and are the gas exchange surfaces, orgills. They
are covered and protected by the lateral rami, or exopods,
which are more heavily sclerotized and harder (Figs 4, 5). The
exopods are moved aside to expose the gills for gas exchange. To
see the gills you must look under the exopods.
long slender endopods (Fig
4) of the first two pleopods of males are modified to serve as copulatory
organs, or gonopods . (Only
the second endopods of Ligia are
so modified.) Sperm
transfer is indirect. Sperm from the male gonopore at the tip of the genital
papilla is transferred to the female by the gonopods. The
pleopods of females are unmodified.
Figure 4. Ventral
view of the pleon and pleotelson of a male Armadillidium.
woodlice the anterior exopods of the pleopods of both sexes contain a
respiratory system of pseudotrachea (Figs
4, 5, 19-66B), known as the corpus
alatum, to supplement the gills. In
living or freshly killed specimens these are easily seen as large, white masses
in the outer edges of the anterior pleopod exopods but they are hard to see in
are adaptations for terrestrial existence. Interior
gas exchange surfaces such as lungs or trachea are more efficient at conserving
water than are gills. Like
the trachea of insects, pseudotracheae are tiny tubes extending from an external
spiracle into the interior of the appendage (Fig 19-66B). Unlike
those of insects, they deliver oxygen to the blood which then distributes it to
the tissues. Oniscoidean
pseudotracheae and insect tracheae are independent solutions to the problem of
terrestrial gas exchange and are an example of convergent evolution.
The uropods are
the appendages of the sixth abdominal segment (Fig 1). They
are biramous and heavily sclerotized. Each
has a basal protopod from
which arise an exopod and endopod (Figs 4, 5). In
most species the rami are long and visible dorsally extending posteriorly from
under the pleotelson. In Armadillidium,
however, the rami are short, so they can be included within the sphere when the
animal enrolls. The
short, flat exopods are
the only parts of the uropods visible dorsally (Fig 1). The
exopods fit into the gap between the side of the telson and the fifth abdominal
slender endopods are
hidden beneath the pleotelson and can be seen only from the venter (Fig 4, 5). Examine
your specimen from the ventral surface and find the three parts of the uropods.
Figure 5. Ventral
view of the pleon and pleotelson of a female Armadillidium.
exopods of Porcellio and Oniscus are
long, pointed, and blade-shaped and extend well posterior to the telson. Their
endopods are much shorter and are scarcely visible dorsally. The
exopods and endopods of Ligia are
similar to each other and both are long and slender.) Theanus opens
ventrally under the base of the pleotelson and is hidden by the uropods.
living unanesthetized Armadillidium are
available, stimulate one with an applicator stick and watch it roll into a ball. Inspect
the ball with magnification and see if any appendages protrude from it or if
there are any gaps in its armor. The
chief predators of woodlice are ants, spiders, and shrews, the first two of
which are foiled by this behavior. Woodlice
also have repugnatorial glands in the epimera and uropods which discourage
Under magnification, watch a woodlouse walk and note the movements of the second
their behavior consistent with the hypothesis that these appendages are sensory? <
The most common North American terrestrial isopods associated
with human habitations are relatively easy to distinguish at the generic level. The
following key and Figure 6 can be used to determine the identity of your
Distinguishing characteristics of three genera of oniscoideans. A. Oniscus,
B. Porcellio, C. Ligia. Redrawn
from van Name (1936). Isopod67L.gif
1a. Exopod of uropod extending posteriorly well beyond
posterior margin of pleotelson (Fig
1b. Exopod of uropod extending little, if at all, beyond
pleotelson (Fig 1).. Armadillidium
2a. Flagellum of second antenna with more than 10 articles
(Fig 6C)….……. Ligia
2b. Flagellum of second antenna with fewer than 5 articles
(Fig 6A,B)…………. .3
3a. Flagellum of second antenna with 2 articles (Fig
3b. Flagellum of second antenna with three articles (Fig
PC . 1965. The
terrestrial isopod crustaceans (Oniscoidea) of Florida. MS
thesis, University of Florida, Gainesville. 75p.
SL. 1972. Woodlice. Pergamon,
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
Name, W. G. 1936. The
American land and fresh-water isopod Crustacea. Bull.
American Mus. Nat. Hist. 71:1-535.
J-W. 1992. Isopoda. in
Harrison, F. W. & A.
G. Humes (eds.). Microscopic
Anatomy of Invertebrates vol. 9 Crustacea. Wiley-Liss,
8-cm culture dish
Living or preserved terrestrial isopods