Evolutionary adaptation of oniscidean isopods to terrestrial life: Structure, physiology and behavior

Abstract

Terrestrial isopods (Oniscidea) are the most successful crustacean colonizers of land habitats. From an evolutionary point of view, they are excellent examples of model organisms that have adaptated to terrestrial life. The aquatic-terrestrial branching of the phylogenetic lines of the Oniscidea occurred in the marine littoral zone. The most oniscid species-rich areas are found in the circum-Mediterranean region. Studies on the morphology, physiology, ecology and biogeography of Oniscidea highlight the diversity of the group. They successfully colonized a wide range of terrestrial habitats by solving such ecological and physiological challenges as reproduction, respiration, excretion and protection against desiccation. During terrestrial adaptation, they evolved diverse morphological, ecological and behavioral traits. This review summarizes our present knowledge of some aspects of the morphology, physiology and behavior as it related to oniscidean adaptation to the terrestrial realm.

Full text

© Koninklijke Brill NV, Leiden, 2011 DOI 10.1163/187498311X576262
Terrestrial Arthropod Reviews 4 (2011) 95–130 brill.nl/tar
TAR
Evolutionary adaptation of oniscidean isopods to terrestrial
life: Structure, physiology and behavior
Elisabeth Hornung
Szent István University, Faculty of Veterinary Science, Institute for Biology, H-1077
Budapest, Rottenbiller u. 50, Hungary,
e-mail: Hornung.Erzsebet@aotk.szie.hu
Received: 15 November 2010; accepted: 29 December 2010
Summary
Terrestrial isopods (Oniscidea) are the most successful crustacean colonizers of land habitats. From an
evolutionary point of view, they are excellent examples of model organisms that have adaptated to terres-
trial life.  e aquatic-terrestrial branching of the phylogenetic lines of the Oniscidea occurred in the
marine littoral zone.  e most oniscid species-rich areas are found in the circum-Mediterranean region.
Studies on the morphology, physiology, ecology and biogeography of Oniscidea highlight the diversity of
the group.  ey successfully colonized a wide range of terrestrial habitats by solving such ecological and
physiological challenges as reproduction, respiration, excretion and protection against desiccation. During
terrestrial adaptation, they evolved diverse morphological, ecological and behavioral traits.  is review
summarizes our present knowledge of some aspects of the morphology, physiology and behavior as it
related to oniscidean adaptation to the terrestrial realm.
© Koninklijke Brill NV, Leiden, 2011
Keywords
Isopod cuticle ; capillary system ; lung ; marsupium ; oostegit ; cotyledon ; tegumental glands ; behavior ;
life history
Introduction
Oniscidean isopods (Malacostraca, Peracarida) are the most successful colonizers
of terrestrial habitats among the Crustacea.  ere are around 3700 known species,
representing the largest isopod suborder (Schotte et al., 1995 - onwards; Schmalfuss,
2003 ).  e cosmopolitan distribution of this monophyletic taxon (Schmidt, 2008 )
might indicate their ancient origin. Oniscidean isopods probably became terrestrial
in the second half of Paleozoic (Cloudsley- ompson, 1988 ). It is believed that
the branching of the semi-terrestrial, terrestrial phyletic lines happened in marine
littoral conditions without a freshwater stage (Schmalfuss, 2005 ). By the present
classifi cation, the Oniscidea are divided into fi ve lineages ( Figure 1 ): Ligiidae, Tylidae,

96 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
Mesoniscidae, Synocheta and Crinocheta (Schmalfuss, 1989 ; Schmidt, 2008 ). Species
of Ligia (Ligiidae) have morphological, physiological and behavioral characteristics
that help us imagine what an intermediate form between ancestral marine and the
modern terrestrial forms may have look like (Carefoot and Taylor, 1995 ; Schmidt,
2008 ) .
Oniscideans are fascinating animals, both biogeographically and ecologically. While
their dispersion ability is rather limited, they are cosmopolitan and are extremely
diverse ecologically.  e geographical distribution of the taxa is fully explored in south-
ern and western Europe. Hot spots of high species’ numbers, enriched with endemics,
are found in the circum-Mediterranean region (Sfenthourakis et al., 2007 ), and a
defi nitive latitudinal gradient in species richness has been shown from the Mediterranean
to the northern regions in Europe (Hornung and Sólymos, 2007 ).  e pattern is clear
regardless of taxonomy: a gradual decrease of species richness towards the north is
consistent in total species number and in species number within species-rich families,
respectively (e.g. Philosciidae, Armadillidiidae, Oniscidae).
 e ecological distribution of oniscideans ranges from supralittoral zones far
into dry land, from sea level to high mountains and caves. Although most species
of oniscideans are terrestrial, some are amphibious and live in littoral zones, such as
Ligia , Tylos , Littorophiloscia , genera of the family Scyphacidae ( Scyphax and Actaecia )
as well as a number of other genera. Several species of Synocheta (mainly in the fam-
ily Trichoniscidae) and some species of Crinocheta secondarily evolved into freshwa-
ter or cave dwelling forms (Schmalfuss, 2005 ).  is evolutionary step has repeated
itself several times independently and convergently in diff erent groups (Tabacaru,
1999 ). Certain oniscideans live either in surface waters or under very wet conditions;
they are stygobitic and live in hypersaline groundwater systems ( Haloniscus ), cave
waters ( Cantabroniscus ) or submarine caves ( Utopioniscus ). Many species live in
subterranean habitats and several of them are real troglobionts - troglobitic species
found in tropical caves (e.g. lava tubes of Hawaiian Islands) (Taiti and Howarth,
1997 ; Taiti, 2004 ). Species adapted to desert environments represent the other end
of a terrestrialization gradient (e.g. Hemilepistus reaumuri (Milne-Edwards, 1840),
Porcellio olivieri (Audouin, 1826) or Agabiformius obtusus (Budde-Lund, 1909),
Warburg, 1995 ; Baker et al., 1998 ; Baker 2005 ). Apart from these extreme excep-
tions, all oniscideans occur in moist microhabitats within terrestrial biotopes and
show cryptic behavior, hide in shelter sites, such as under stones, logs in leaf litter
(Schmalfuss, 1978 ). To inhabit so a wide range of habitats, isopods had to face several
Figure 1. Schematic phylogenetic relationship among oniscidean lineages (Erhard, 1996 ).

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 97
ecological and physiological stressors. During their evolutionary adaptation, the onis-
cideans have developed diff erent morphological, ecological and behavioral solutions
to the terrestrial ways of reproduction, respiration, excretion and protection against
desiccation.
 e main trends of oniscidean morphological, physiological changes (compared
to marine species) are: (1) reduction in size; (2) water-resistant cuticle (Bursell, 1955 );
(3) diverse surface morphology - increase in number of surface structures (Schmalfuss,
1978 ; Holdich, 1984 ); (4) pleodopodal lungs (Edney, 1954 ; Hoese, 1982a ; Cloudsley-
 ompson, 1988 ; Schmidt and Wägele, 2001 ); (5) water conducting system (Hoese
1981 , 1982b ; Horiguchi et al., 2007 ); and (6) closed brood pouch (Hoese, 1984 ) which
was of key importance in the life of isopods on land. We can recognize the diverse
trends on recent, living species. In addition, we will mention the ecomorphological
and behavioral aspects of drought avoidance, habitat selection, foraging and life hitory
characteristics.
A review and a comprehensive book by Warburg ( 1987 , 1993 , respectively) gives an
overview of terrestrial adaptation in land isopods (Oniscidea). Several papers systema-
tize and broaden our knowledge on diff erent aspects of terrestrial isopods, all impor-
tant in their land-adaptation. Such features include, phylogeny (Schmidt, 2008 ),
reproduction (Warburg, 1994a , b ; Kight, 2008 ), ultrastructure, calcium deposition
and mineral distribution in the cuticle (Ziegler, 2004 ; Hild et al., 2009 ; Matsko et al.,
2010), surface morphology (Schmalfuss, 1975 , 1977 , 1978 ; Holdich, 1984 ), water
balance (Edney, 1977 ), water vapor absorption and ammonia volatization (Wright and
O’Donnell, 1995 ), respiratory structures (Hoese, 1982a , 1983a , b ; Ferrara et al., 1991 ,
1994 , 1997 ; Schmidt and Wägele, 2001 ; Paoli et al., 2002 ; Gruber and Taiti 2004 ),
marsupial structure (Hoese, 1984 ; Hoese and Janssen, 1989 ), water conducting sys-
tem (Hoese, 1981 , 1982b ), structure and development of digestive system (Milatovič
et al., 2010 ; Štrus et al., 2008 ), intestinal microbiota (Kostanjšek et al., 2006 ), nutri-
tional and developmental aspects of isopod land adaptation (Štrus et al., 1995 ; Štrus
and Blejec, 2001 ; Zimmer 2002 ), as well as eco-morphological (Schmalfuss, 1984 ) or
evolutionary strategies (Schmalfuss, 1998 ).
However, our knowledge on oniscideans or isopods has increased signifi cantly dur-
ing the last decade. A complete bibliography of terrestrial isopod literature - containing
publications on all biological aspects - was fi rst published in 2002 (Schmalfuss
and Wolf-Schwenninger).  at bibliography was updated in 2004 and is available
on the internet ( http://www.naturkundemuseum-bw.de/stuttgart/projekte/oniscidea
-catalog/ ). Since 2004, the publications of three Symposia proceedings, in Crete
(Greece), Aveiro (Portugal) and Tunis (Tunisia) (Sfenthourakis et al., 2004 ; Loureiro
et al., 2005a ; Zimmer et al., 2008 , respectively), and several additional papers – among
them publications dealing with the above mentioned aspects of land adaptation – have
enriched our present knowledge about oniscideans.
 e present paper aims to summarize and update our knowledge on several struc-
tural, physiological and behavioral aspects of terrestrial isopods that have contributed
to make them so successful in land colonization.

98 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
Morphological and physiological adaptations
Cuticle
 e outer protective sclerotized tegumental cover, the cuticle or exoskeleton, is the
main barrier between these small animals and their environment. In spite of the fact
that their cuticle is relatively permeable to water (Quinlan and Hadley, 1983 ), wood-
lice can survive under a wide variety of terrestrial conditions by fi nding a locality with
the appropriate humidity requirements.
 e cuticle is composed of an organic matrix containing chitin and sclerotized pro-
teins. Fabritius et al. ( 2005 ) describe the architecture of this organic matrix.  e min-
eral phase consists of mainly calcium carbonate (CaCO 3 ). Hild et al. ( 2008 ) summarized
the known details on the fi ner cuticular structure and the forms of calcium carbonate.
Recently, Matsko et al. (2010) proved experimentally the importance of silicon in the
early stage of cuticle biocalcifi cation in Ligia italica Fabricius, 1798.
 e exoskeleton of isopods has four well-defi ned layers: the outer epicuticle, the
exocuticle, the endocuticle and the innermost membranous layer ( Figure 2A ; see also
Figure 6A below). Individual layers may have sublayers, depending on species (Hild
et al., 2008 ).  e fi rst three layers are calcifi ed and contain calcite crystals and amor-
phous calcium carbonate (Roer and Dillaman, 1984 ). Compere ( 1991 ) has described
the fi ne structure of the thin superfi cial epicuticle on Oniscus asellus Linnaeus, 1758.
Compere stated that the overall structure of the isopod cuticle follows that of crusta-
ceans, the mineralized exoskeleton has an additional waxy and a cement layer.  ese
two layers might be the consequences of terrestrial adaptation.
To be able to grow in size, terrestrial isopods molt frequently throughout their lifes.
Molting has two phases: the fi rst is the shedding of the posterior part of the cuticle.
After one day, this is followed by the molt of the exoskeleton’s anterior half ( Figure 3 ).
Before molting, calcium (in the form of calcium carbonate or calcium phosphate) is
reabsorbed fi rst from the posterior half of the cuticle. It is stored in deposits partially
located in the anterior region, in the ecdysial space, and in the haemolymph. After
molt, animals reuse these stocks for mineralization of the new cuticle (e.g. Steel,
1993 ; Ziegler et al., 2007 ).  ere is a high interspecifi c variation in the method and
location of calcium deposition (Ziegler, 2004 ).  e molting individual often consumes
the exuvia to regain mineral content (Steel, 1993 ; personal observation in the fi eld,
Figure 3C ).
Several valuable papers have been published recently concerning the mysterious and
exciting processes of calcareous deposition, calcariferous transport processes and the
anatomic changes in the cuticle, especially during molting, (e.g. Neues et al., 2007 ;
Ziegler et al., 2005, 2007; Hild et al., 2009 ; Štrus and Blejec, 2001 ).
Surface morphology, perception
Electron microscopic (SEM) scanning is a splendid method of studying the mor-
phology of isopod cuticular surfaces. A great variety of surface ornaments are present
on the dorsal surface of terrestrial isopods ( Figures 2B -F and 4 ), such as sensory and

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 99
Figure 2. (A) Cross section of Armadillidium vulgare tergite: layers of the cuticle with an innervated
cuticular extension (photo by D. Csonka); ec: epicuticle; pc: procuticle; hy: hypodermis; sm: sceletal
muscle; t and black arrow: exteroreceptor (‘tricorn’); star: nuclei of supporting cells around a nerve (scale
bar = 20 μm). (B) Antennal setae of Platyarthrus schoblii Budde-Lund, 1885 (20 μm). (C) Protracheoniscus
major (Dollfus, 1903) tergal surface with plaques and sensory setae (100 μm). (D) P. hoff mannseggii
Brandt, 1833 tergite surface (20 μm). (E) Noduli laterales (nl) in Protracheoniscus politus (approximate
length of isopod is 12 mm). (E)  e same by SEM (100 μm). All illustrations in this and other fi gures were
prepared by the author unless another name is mentioned (parenthetically).

100 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
Figure 3. Molting. (A) Armadillidium vulgare after shedding the posterior part (pale and soft); approxi-
mate length of isopod is 17-20 mm. (B) Porcellionides pruinosus (12 mm), after shedding the posterior
part; note the freshly molted posterior being much wider and grayish than the anterior (“old”) one.
(C) Freshly molted Protracheoniscus politus (C. Koch, 1841) (anterior part) feeding on exuvia.
non-sensory structures, including papillae, setae, tricorns, microscales, pits, minute
plaques, tubercles, ridges, pores (Powell and Halcrow, 1982 ). Innervated cuticular
extensions mediate sensory information (Holdich, 1984 ) of behavioral responses.

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 101
Holdich and Lincoln ( 1974 ) found no sexual diff erences in this morphology in their
studies on Porcellio scaber Latreille, 1804. Dorsal scale-setae in diff erent forms, anten-
nal, uropodal spikes are unique to oniscideans presumably accompanying terrestrial
adaptation ( Figures 2 and 4 ; Holdich, 1984 ).
Scale like, circular or polygonal micro-ridges ( Figure 4A ) provide anti-adhesive
qualities, preventing small, wet particles from sticking to the animals’ cuticle
(Schmalfuss, 1975 , 1977 , 1978 ).  e dorsal surface of the exoskeleton is adapted
to the microhabitat type (Schmalfuss 1984 ).  ere is great variation in surface struc-
tures, from very smooth surfaces to rather ornate ones with depressions, groves,
ridges, tubercles, plaques, scales etc. (Schmalfuss, 1978 ; Holdich, 1984 ) ( Figures 2B -F
and 4).  ese surface formations, together with other external characteristics,
were categorized by Schmalfuss ( 1984 ) into fi ve eco-morphological strategies to
show the high correlation between body-construction and habitat, and microhabi-
tat features (i.e. ecological preferendum and antipredator strategies), detailed below
(see also Figure 13 below).
1 .  e real epigean forms are either slow moving animals with fl at and broad bod-
ies, strong and short pereopods (‘clingers’ like Trachelipus , Porcellio, Nagurus ),
or fast moving, narrow and elongated bodied ones with a smooth surface and
long pereopods, ‘runners’ (families Ligiidae and Philosciidae, genus Trichoniscus ,
Protracheoniscus , Porcellionides ).  e latter type is thought to represent the most
Figure 4. Tergal surfaces of (A) Platyarthrus schoblii : tergal ridges (scale = 100 μm), (B) Armadillidium
versicolor (50 μm), (C) Porcellio scaber : tubercles covered by sensory tricorns (250 μm), and (D) Porcellionides
pruinosus : tricorns and waxy spheres (250 μm).

102 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
ancient type of terrestrial forms (see below Figure 13F -I and C-D, respectively;
Schmidt, 2008 ).
2. True soil dwelling, endogean species, ‘creepers’—Onscidae, such as Bathytropa,
Trichoniscidae as Graeconiscus , Plathyarthridae as Platyarthrus , Stenoniscidae as
Stenoniscus —are mostly pygmy forms with convex, elongated body, short append-
ages.  ey do not conglobate, or roll forming a sphere.  ey depend strongly on
conditions of high humidity because of their high cuticular evaporation rates.
However, they have dorsal longitudinal ribs to prevent passive captivity inside
water drops due to decreased surface and thereby decreased surface tension
(see Figure 13A -B below; Schmalfuss, 1984 ).
3.  e conglobating forms are called ‘rollers’ (Armadillidae, Eubelidae, Armadillidii
dae, Sphaeroniscidae, and Tylidae).  eir body is highly convex and they are able
to roll up into a ball (see Figure 13J -M below).
4.  ere are ‘spiny’ forms that live outside the litter layer (Eubelidae such as
Panningillo ; Armadillidae such as Acanthoniscus , Echinodillo , Tridentodillo ).  ese
are mainly tropical, subtropical species.
5.  e so-called ‘non-conformists’ (about 10%) that do not fi t into the previous
categories (e.g. commensalists in ant nests as Plathyarthrus spp., Schoeblia ; soil dig-
gers as Hemilepistus, Leptotrichus ). For further details and examples, see Schmalfuss
( 1984 ).  e eco-morphological defense strategies of land isopods are strengthened
by the species specifi c mineral distribution in their exoskeleton (see Figure 13N -P
below; Hild et al., 2008 ).
Humidity is a key factor limiting the distribution of terrestrial isopods.  e function
of ‘moisture monitoring’ is provided by fl agellar aesthetasks found on the apical article
of the fi rst antennae and on the fl agellar articles of the second antennae (Schmalfuss,
1998 ). Extreme diminution of the fi rst antennae in terrestrial species was a vital evolu-
tionary development in favour of protection against predators ( Figure 5 ).  e sensory
function of the minute antenna is essential in the task of fi nding suitable humidity
conditions for short and long-term survival (Schmalfuss, 1998 ).
Isopods are less resistant to desiccation than insects, and their behavioral reactions
to humidity changes have enabled them to colonize a great variety of land habitats
(reviewed by Edney, 1968 ; Lindquvist, 1968).  eir existence, distribution on diff er-
ent scales (from global to microscales) depends on the species’ ecological tolerance and
on the suitable habitat conditions (e.g. Hornung et al., 2008 ).
Cuticular transpiration
Cuticular lipids and /or hydrocarbons are supposed to reduce transcuticular water loss
(Hadley and Warburg, 1986 ).  ey are also able to maintain the water level of the
cuticle against desiccation by a supposed active regulating mechanism (Lindquist,
1968 , 1972). For instance, early transpiration studies of Bursell ( 1955 ) suggested that
the cuticle of isopods is a highly effi cient barrier to water loss.  e values calculated
closely approximate those of other terrestrial arthropods.  e permeability
of the onis-
cidean cuticle is limited by the lipoids impregnating the
endocuticle, such that when

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 103
the temperature is raised above the lipoid melting point,
there is a marked increase in
permeability to water and water loss (Bursell, 1955 ). However, Hadley and Quinlan
( 1984 ) think that lipids, although present in the cuticle, do not provide an eff ective
barrier to water loss. Quinlan and Hadley ( 1983 ) measured cuticular permeability on
dead isopods, where the regulating mechanism cannot act.  ey found, in Porcellio
laevis Latreille, 1804 and Porcellionides pruinosus (Brandt, 1833) at 30°C, that water
loss through the cuticle was rather high (55-75%) during the fi rst 3 hours of exposure,
in mg/cm
2 surface area and permeability increased with higher temperatures (from
25°C to 50°C).
Although cuticular dehydration is a major issue in isopod life, water can be taken
up by cutaneous absorption (Coenen-Stass, 1981 ). Terrestrial isopods are capable
of active water vapor absorption (WVA) (Wright and Machin, 1990 , 1993a , b ).
Diff erent species vary in integumental permeability and have diff erent lethal relative
humidity (RH) limits: the loss rate is diff erent for species adapted to diverse habitat
types on a humidity scale (Edney, 1977 ). All species are able to replenish tolera-
ble water losses under given humidity conditions and they can be classifi ed by their
tolerance limits into hygric, mesic, xeric categories, in accordance to phylogeny and
habitat requirements. Representatives of Synocheta appear to have no WVA capabil-
ity, as was shown gravimetlically by Wright and Machin ( 1990 , 1993a ).  eir cry-
ptozoic way of life and close ties with wet habitats, may explain this (Wright and
O’Donnell, 1995 ).
Figure 5.  e minute fi rst antenna of Porcellio scaber with fl agellar sensory appendages (aesthetasks) is
covered and protected by the second, large antenna (scale bar = 500 μm).

104 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
Tegumental glands
 ere are tegumental glands, with openings widely distributed either on the surface of
the cuticle or on the lateral surface/edges of the thoracic and abdominal segments, and
on the uropods, respectively ( Figures 6 and 7 ; Gorvett, 1951 , 1952 ).  ese dermal
glands have a secretory function, occur only in land isopods, and are probably con-
nected to terrestrial adaptation (Gorvett, 1951 , 1956 ). Weihrich and Ziegler ( 1997 )
described the unique, lobed structure of these exocrine glands ( Figure 6A ).  ey found
that the smaller lateral plate glands and the larger uropod glands are very similar in
anatomy.  e functional signifi cance of these lobed type glands perhaps is antipredator
defense. Gorvett ( 1956 ) experimentally tested his “limited defence hypothesis”.
According to this hypothesis, the main potential predators of woodlice are spiders.  e
hardened, drawn-out threads of uropod gland secretum ( Figure 6C ) can be used as an
attachment to the substratum (e.g. in the case of wind-blown juveniles, Hornung, pers.
obs.). Gruner ( 1966 ) suggested that these glands evolved fi rst as excretory structures
and developed into defense ones later ( Figures 6 and 7 ).  e development of lobed
glands might be correlated with the evolutionary position and the ecological condi-
tions (habitat) of the isopod species.
Water conducting system: a solution to multiple regulation problems
An important problem for life on land in isopods was to evolve an adaptive solution
for thermoregulation, excretion, osmo- and ion-regulation under terrestrial conditions.
 e evolution of a water conducting system (WCS) is important in overcoming these
problems (Hoese, 1984 ).  is system consists of scale rows ( Figure 8A -B) holding
water by capillary forces (Hoese, 1981 , 1982b ). WCS or capillary conducting system
(Hoese, 1984 ) allows also nitrogenous waste to be excreted as ammonia gas, after water
resoption. Such a phenomenon is unique to oniscid isopods. Hoese ( 1981 ) gives a
detailed account of the development of theories on the water conducting system, which
was fi rst described by Verhoeff in 1917 (c.f. Hoese, 1981 ).
In vivo and SEM investigations of 56 isopod species (ranging from marine and
freshwater to terrestrial types) resulted in distinguishing two structurally diff erent types
of water conducting systems for oniscideans (Hoese 1981 ).  ese are the ‘ Ligia ’- and
the ‘ Porcellio ’-types.
 e ancient form (‘ Ligia type’, named by Hoese, 1981 ) is an open system, that
uptakes water and excretes diluted nitrogenous waste, urine.  is system allows water
uptake by capillary forces through the 6-7th pereopods. Water is forwarded to other
body parts by a water conducting system.  e WCS consists of belts of scale-rows on
sternites along the insertions of the legs, the antennae and partly the sixth and seventh
walking leg.  ese structures are considered homologous and must have evolved in the
common ancestor of all Oniscidea (Schmalfuss, 2005 ).  e group possessing Ligia
type system includes mainly amphibious isopods, some members of Ligiidae and few
species of Trichoniscidae families.  e exact details of its structure and the precise
functioning of this system were recently studied in Ligia exotica Roux, 1828 (Horiguchi
et al., 2007 ). Horiguchi and co-workers succeeded in demonstrating the function
and role of each part of 6th and 7th pereiopod in the process (see fi gure 5 therein).

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 105
Figure 6. (A) Cross section of an epimeron ( Armadillidium vulgare ): structure of cuticle (see also
Figure 2 ; ec: epicuticle, pc: procuticle, hy: hypodermis) and tegumental lobed gland (ltg) (Photo: D.
Csonka), t: tricorn (scale bar = 100 μm). (B) Each tergite and the uropods have a pore-fi eld, here with
secretum drops in Porcellio scaber (size of the animal is 14-18 mm). (C) P. scaber uropodal gland secre-
tum with drawn-out thread (arrows). (D) Schematic fi gure of Porcellio scaber showing the location of tegu-
mental (anterior arrowhead) and uropodal glands (posterior arrowhead). (E) SEM of a tegumental gland
pore-fi eld (tpf) covered by secretum ( Porcellio scaber ) located on the margin of the tergite; p: plaques
(scale bar = 100 μm).

106 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
Figure 7. (A) Tergal surface of the of Protracheoniscus politus covered by secretum (scale bar = 200 μm).
(B)  e same surface in higher magnifi cation with precipitated secretum (scale bar = 100 μm).
 ey demonstrated that the two superimposed pereiopods form a gutter for capillary
action.  e diff erent surface formations complement one another and they cannot act
separately. Also, the system has regulatory ability precluding unnecessary passive water
uptake (Horiguchi et al., 2007 ). Water forwarded by this system also supplies pleopods
for respiration. Interestingly, uptake of stained water proved that the anus (but not the
oral cavity or the foregut, contrary to the suggestion of Hoese, 1981 ) is involved in this
putatively ancestral water conducting system.
 e more derived ‘ Porcellio type’ WCS was stated to be a closed one, not involving
pereiopods in the process. Urine excreted by the maxillary nephridium and glands
is forwarded caudally. In higher terrestrial isopods (‘ Porcellio ’-type), liquid water can be
taken up by mouth (Hoese, 1981 ), by the rear appendages, or uropods (Spencer
and Edney, 1954 ) and, as water vapor, through the pleoventral space (Wright and
Machin, 1990 , 1993a , b ; Wright and O’Donnell, 1992 , 1995 ). Meanwhile, diluted

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 107
Figure 8. (A) Schematic representation of the water conducting system (WCS) and marsupium with
oostegites (o) and eggs (e) (redrawn after Schmidt, 2008 ). (B) SEM photo of the WCS: Porcellio laevis (by
courtesy of H. Schmalfuss). (C) Detail of the brood-pouch ( Porcellionides pruinosus ) with eggs (e), ooste-
gites (o) and cotyledon (Co). (D) Marsupium of Trachelipus rathkii : oostegits (o) and eggs (e) (pereipods
1-5 removed). (E) Gravid Porcellionides pruinosus. Note infl ated venter (m).

108 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
ammonia is excreted. Using well-hydrated Porcellio scaber individuals exposed to a
moistened substratum and saturated air, Drobne and Fajgelj (1993) showed that water
is uptaken both by mouth and through the uropod endopodites.  us, they modifi ed
Hoese’s (1981) model and proved that the Porcellio type water conduction system - and
most probably that of Armadillidium - is also open.  e main diff erence between the
two WCS types is in the route by which the external water is taken up: In Porcellio,
the water is taken up by uropods and not by 6th and 7th pereiopods, as is the case
with Ligidium and Ligia . Subsequently, this water is also distributed along the water
conducting system, but it appears fi rst in the pleoventral space (Drobne and Fajgelj,
1993 ).
 e more developed the lung is, the less watering is needed as the animal is rela-
tively more water independent.  e WCS is a multifunctional system that (1) aids
in respiration by wetting the lung epithelia (the activity of WCS is in negative cor-
relation with the development of pleupodal lungs); (2) supports thermoregulation
through water conduction – the two types have diff erent function in this respect: the
Ligia type shows water uptake and water distribution on the whole body surface
while in Porcellio type there is a hydrophobic surface (scales fi lled with air); (3) is
essential for excretion: similarly, to true aquatic organisms, they are ammoniotelus or
excrete their waste nitrogen principally as ammonia. Urine, excreted by the maxillary
nephridium and glands, is channeled into WCS. Faeces contain only about 10% of the
N excretum, the rest is eventually evaporated as NH
3 gas through the WCS (Hoese,
1981 ).
Pleopodal lungs
In isopods, oxygen uptake happens mainly through the abdominal appendages. In
aquatic species these take the form of gills, but the oxygen uptake organs in land
(mainly mesic and xeric habitats) isopods evolved in several ways on the pleopod
exopodites for aerial respiration during terrestrial adaptation (Hoese, 1982a ). In addi-
tion, endopodites keep their gill-like structure and function in most of the terrestrial
species (Becker, 1936 ; Cloudsley- ompson, 1975 ).  e effi ciency of aquatic respira-
tion depends on the extent of their adaptation to land: less adapted, littoral species
(e.g. Ligia pallasii Brandt, 1833) survive for a long time under (sea) water (Taylor and
Carefoot, 1993 ).
 e evolution of the respiratory surface on the pleopodal exopodites parallels
the phylogeny and adaptations to colonize terrestrial habitats along a humidity
gradient.  e anatomy, the structure, and the functional principles show diff erent
evolutionary routes ( Figure 9 ; Hoese, 1981 , 1982a , 1983a , 1983b , 1984 ; Ferrara et al.,
1991 , 1997 ; Paoli et al., 2002 ; Gruber and Taiti, 2004 ). Taiti et al. ( 1998 ) as well as
Schmidt and Wägele ( 2001 ) discussed the evolution of oniscidean respiratory struc-
tures in the context of phylogenetic relationships, based on morphological characters.
In the most primitive oniscideans, the thin ventral integument of the exopodites is
the place of respiration. Respiration takes place through a folded surface, which makes
up a signifi cant part of the dorsal wall of the pleopodal exopodite ( Figure 9A a, C).

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 109
 ese foldings vary depending on species (Gruber and Taiti, 2004 ).  is simple, open
and folded epithelial surface on the exopodites of pleopods could be the fi rst stage of
the morphological phylogenetic developmental line.  e next phylogenetic stage is a
weakly wrinkled surface followed by a partly covered respiratory fi eld ( Figure 9Aa , D)
with strongly wrinkled surface and the last stage is a completely internalized lung
with spiracles and water-repellent surface ( Figures 9Ac , E-H a nd 10). During evolution,
the respiratory surface becomes increasingly separated from the atmosphere as its
surface area is expanded by inward foldings. It becomes progressively more covered and
is connected to the environment through respiratory apertures or spiracles of decreas-
ing size during adaptation ( Figures 9E -G and 1 0) in the most advanced closed lungs,
spiracles are surrounded by a water-repellent perispiracular area (Hoese, 1982a ; Ferrara
et al., 1991 ; Schmidt and Wägele, 2001 ).  e respiratory epithelium within a pleop-
odal lung is folded with many small branches ( Figure 11 ).  e surface morphology
of the perispiracular region has a modular, microsculptured structure ( Figure 9B )
(e.g. Mödlinger, 1931 ; Ferrara et al., 1994 ; Schmidt and Wägele, 2001 ; Paoli et al.,
2002 ). Mödlinger ( 1931 ) described a species-specifi c diff erence in shape of this struc-
ture in species of Porcellio based on cross-sections viewed under a light microscope
(LM). A similar, species-specifi c feature can be recognized in species of Armadillidium
( Figure 9B ; Csonka et al., pers. comm.).
 e highest level of development appears in species inhabiting extreme dry habitats,
mainly deserts: the tubular structure inside, penetrates into the body cavity ending in
so called “lacunae laterales”, as in species of Periscyphis (Ferrara et al., 1997 ). In the
desert-living genera ( Periscyphis, Hemilepistus and probably more), the spiracle is close-
able (Ferrara et al., 1991 , 1997 ).
Covered lungs can be polyspiracular or monospiracular, depending on the number
of openings: monospiracular lungs ( Figures 9E -F and 10 A, B, D) are the most com-
mon type of lungs. Polyspiracular lungs are characterized by several spiracles going into
respiratory trees within the pleopodal exopodites ( Figures 9G -H and 10 C, E, F).  e
number of spiracles usually decreases from the fi rst to the last pleopod (Gruber and
Taiti, 2004 ). A single spiracle is followed inside the exopod by an atrium which gives
rise to narrow lacunae in a compact tissue or branches out into many respiratory
tubules ( Figure 11 ) of decreasing diameter (Paoli et al., 2002 ).
 e structural and correlating functional adaptations of respiratory organs might be
the main factor in successful colonization of diverse types of land habitats.  e devel-
opmental stages of lungs have evolved several times convergently during evolution of
isopod lineages. At each stage, at least six fold analogous branching steps have evolved.
 e independent evolution of diff erent types of pleopodal lungs has been demon-
strated in the Armadillidae, Eubelidae, Philosciidae, and Tylidae (Hoese, 1983a ; Ferrara
et al., 1991 , 1994 ; Taiti et al., 1998 ; Paoli et al., 2002 ).
 e pleopodal lungs vary not only in structure but also in number. For example, in
the Eubelidae (216 species) a phylogenetic series of intermediates can be found, begin-
ning with no lungs (secondary reduction) through 1, 2, 3 and 5 pairs of lungs (detailed
schematic presentation in Ferrara et al., 1991 ). A study of 90 species of Armadillidae

110 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
Figure 9 . Lungs (pseudotracheae). (A)  e main evolutionary steps of the development of terrestrial
respiratory organs terrestrial isopods; RS: respiratory surface, PSA: perispiracular area, S: spiracle;
(a) Open, folded respiratory surface of pleopodal exopod; (b) partly covered type; (c) internal, closed
respiratory area, opening (spiraculum) surrounded by perispiracular area. (B) Cross section of peris-
piracular area of Armadillidium vulgare (scale bar = 100 μm). (C-H) SEM images of the diff erent lung
types, Photos’ courtesy of S. Taiti; (C-D) Uncovered lungs; (C) Atracheodillo marmorivagus, 1st exopod;
(D) Synarmadilloides pila, 2nd exopod; (E-F) Covered, monospiracular lung ( Aethiopopactes nigricornis,
3rd exopod); (G-H) Covered, polyspiracular lung ( Somadillo taramassoi , 3rd exopod); Scale bar in C, D,
E, F and H = 0.1 mm, in G = 1 mm.

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 111
(57 genera) strengthened the descriptions and statements above: all forms of respira-
tory organs known in Oniscidea are present at family level (Gruber and Taiti, 2004 ).
 e primitive uncovered form and the advanced covered pleopodal lungs (including
mono- and polyspiracular ones) are present.
 e morphological development is in close correlation with the ecological steps of
colonization of drier and drier habitats.
Figure 10 . (A) First and second pleopodite exopodite with lungs (Pl-ex 1 and Pl-ex 2, respectively) in
Porcellio scaber , ventral view (scale bar = 1 mm; covered, monospiracular lung type). (B) Close-up of (A)
(50 μm). (C) Perispiracular area and spiracles of a polyspiracular lung type in Armadillidium vulgare
(200 μm). (D) Spiracle (S) and PSA in a monospiracular type lung, such as P. scaber (25 μm). (E) Close-
ups of spiracles of A. versicolor and (F) of A. nasatum (both scale bars = 50 μm; covered, polyspiracular lung
types).

112 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
 e marsupium
Brood care is widespread in crustaceans but the temporally existing marsupium appears
only in the superorder Peracarida (e.g. isopods, amphipods, and other shrimp-like ani-
mals).  e marsupium originally evolved for the mechanical protection of eggs and
developing embryos under marine conditions. While in aquatic forms, the marsupium
protects the eggs, under land conditions it has evolved into a progressively closer brood
pouch.  e brood pouch is “not just a simple container, but protects eggs against desic-
cation and microbes, ensures ‘sea conditions’ – aquatic milieu, fl uid (water) and oxy-
gen and allows females to remove their brood from dangerous places and carry them
into favorable zones (e.g. thermal optima for embryogenesis)” (Linsenmair, 1989 ).  is
sexual female character is the speciality of gravid females.  ey carry their fertilized
eggs on their ventral side in this brood pouch ( Figure 8A , C-E) and habitually look for
places with optimal temperature and humidity conditions for embryogenesis
Figure 11. Light microscopy cross sections of the lung of Armadillidium vulgare .  e perispiracular
elements (PSA) follow the major branches of the ‘lung’ behind the spiraculum (S). (Photos taken by
D. Csonka) (Scale bars = 100 μm).

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 113
(Dangerfi eld and Hassall, 1994 ; Hassall and Tuck, 2007 ). Eggs develop into embryos
and mancas or post-larval juveniles in peracarid crustaceans, regardless of the environ-
mental water supply.
Terrestrial isopods exhibit extensive parental care providing protection and nutrition
for developing progeny under marsupial conditions. Ovigerous females show a lower
ingestion rate, lower capacity for energy aquisition (Lardies et al., 2004a ).
Duration of marsupial development is infl uenced by light and temperature and has
a great plasticity depending on weather conditions and environmental stresses (Hornung
and Warburg, 1993 , 1994 ). Both increased light and/or temperature accelerated oocyte
maturation, appearance of brood pouch, shortened duration in egg, embryo, and
manca development in Porcellio fi culneus Budde-Lund, 1885(Hornung and Warburg,
1993 ). Oocyte or egg resorption as well as embryo or manca abortion signals the cost
of accelerated development (Hornung and Warburg, 1994 ) resulting in fewer off spring.
Increased locomotory activity as well as physical stress has the same eff ects: reduced
fecundity and shortened duration in marsupial development (Hornung and Warburg,
1993 ; Lardies et al., 2004a ).
Structure of the marsupium. In semi-terrestrial and terrestrial isopods, the marsupium
evolves into a partly or totally closed brood pouch and it is a key component of
terrestrial adaptation. Some time after fertilization, isopod females undergo a parturial
molt and develop oostegites on the 2nd-5th thoracic segments ( Figure 8A , C-E).
Within the marsupium, we can recognize the cotyledons, hanging from the venter and
penetrating among developing eggs ( Figure 8C ).  e chamber of the brood pouch is
fi lled with marsupial fl uid.
Oostegites. Trevianus and Trevianus (1816, cited by Hoese, 1984 ) fi rst described the
structure of this brood-pouch.  e oostegites are leaf like, overlapping appendages,
basally fused with the pereomeres.  ey project medially from the coxae of the anterior
pereopods. Five pairs of oostegites form the marsupium, which is tightly sealed
ventrally, and laterally ( Figure 8C , D; further details and illustrations in Hoese, 1984 ;
Hoese and Janssen, 1989 ).
 e oostegites are formed under the control of an ovarian hormone during vitello-
genesis. An extract of vitellogenic ovaries induced oostegite development in ovariect-
omized females (Suzuki and Yamasaki, 1989 ). In Porcellio dilatatus Brandt, 1833,
fresh or accumulated sperm induces their formation (Loyola e Silva and Coraiola,
1999). Ligia oceanica (Linnaeus, 1767) develops oostegites during maturation and
keeps them throughout its whole life, which is unusual among isopods (Willows,
1984 ).  e outer wall of oostegites possesses a rather thick and impermeable cuticle
that prevents water loss from the marsupium and thereby avoids desiccation (Hoese
and Janssen, 1989 ).
Cotyledons. Cotyledons are fi ngerlike extensions of the intersegmental membrane of
the 2nd-5th thoracic segments ( Figure 8C ).  ey appear in the marsupium of gravid
females after parturial molt and have an excretory function.  eir cuticle is extremely

114 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
thin. Cotyledons supply the eggs, embryos and mancas with water, oxygen and nutritive
fl uids (Hoese and Jansen, 1989; Lewis, 1991 ). In Porcellio olivieri (Audouin, 1826), a
fossorial desert species, a cotyledon (Warburg and Rosenberg, 1996 ; see fi gures therein)
connects each egg. Aquatic forms and the ancient, supralittoral, amphibian species
(e.g. Tylidae, Ligiidae and Trichoniscidae), have no cotyledons, reminiscing of ancestral
marine species (Lewis, 1991 ).
Cotyledons vary in shape and size with species, and with the age of the brood, grow-
ing in length, thickening with embryo growth and shrinking before manca release.
 eir length may also be related to the characteristic humidity of the species’ habitat
(Hoese and Janssen, 1989 ). Cotyledons occur singly or in groups of 2-3 along the
ridges of the thoracic membrane.  e shape of these cotyledons might be single or
divided into branches (Hoese, 1984 ; Hoese et Janssen, 1989; Lewis, 1991 ).  ey are
usually located in three areas of the body: at mid-line and on each side towards the
lateral margin of the marsupium (Lewis, 1991 ).  e arrangement of the cotyledons
varies with the family or subfamily oniscids. Ridges in some cases (e.g. Porcellio scaber )
can enlarge their surface (Hoese, 1984 ).  e number of cotyledons ranges widely
within the Oniscidea; this might be correlated with the aridity of habitats (Hoese and
Janssen 1989 ; Lewis, 1991 ). Lewis ( 1991 ) studied over 60 species of Oniscidea belong-
ing to 20 families and found between 4 and 28 cotyledons per individual; a higher
cotyledon number was found in species living in increasingly arid habitats and more
inferred derived species. Species with the highest number of cotyledons belong to the
most advanced members in the Armadillidae that are found mainly in arid regions
(Lewis, 1991 ).
Marsupial fl uid . Eggs of terrestrial isopods undergo development in the female’s
brood-pouch.  ere, they are surrounded by marsupial fuid excreted by the cotyledons.
 is fl uid contains nutritive components essential for embryogenesis, such as oxygen,
and provides protection against desiccation, as well as bacterial infection (Hoese 1984 ;
Hoese and Janssen, 1989 ; Linsenmair, 1989 ). During marsupial development, the
off springs also need calcium ions (Ca
+2 ) for cuticle mineralization. Ouyang and Wright
( 2005 ) have found that the total calcium increased 17-fold in Armadillidium vulgare
(Latreille, 1804) during embryogenesis.  ey measured a further 35-fold increase in
calcium during the manca stage while they drink and ingest marsupial fl uid.
All development stages developing (eggs, embryos and mancas; detailed des-
criptions in Surbida and Wright, 2001 ) face physiological stress during marsupial
development -such as potential desiccation, high ammonia concentrations and changes
in osmotic concentrations. Surbida and Wright ( 2001 ) studied osmotic conditions,
possible osmoregulation in the marsupium and osmotic tolerance, osmoregulatory
capacity of marsupial juvenile stages in A. vulgare fi nding that marsupial forms have a
wide tolerance and physiological adaptability to land conditions.
Types of terrestrial marsupium . Besides the aquatic type, the land colonizing
semiterrestrial and terrestrial isopods (Oniscidea) evolved two main types of marsupium:
the amphibian type and the terrestrial type.  ese are –depending on evolutionary

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 115
stages and correspondence with habitats– the amphibian and the real terrestrial brood
pouches.  ese marsupia diff er in structure (Hoese, 1984 ; Hoese and Janssen, 1989
illustrate the structures).
 e amphibian type marsupium (basic type in species of Ligia ) is similar to that in
aquatic species: open at both ends, anteriorly and posteriorly, and water –taken up by
the water conducting system– can pass slowly through the brood pouch on a caudal to
apical direction.  is type of brood pouch is characteristic of phylogenetically more
ancient species, living under extremely wet conditions (Hoese, 1984 ; Hoese and
Janssen, 1989 ).
 e terrestrial type of marsupium ( Figures 8A , C, E), is the inferred most advanced
type of marsupium and it is characteristic of the Crinocheta or ‘higher’ Oniscideans.
 ere is no connection between the water conducting system and the marsupium
being completely enclosed.  e marsupium contains a nutritive fl uid with mucus and
blood cells secreted by the cotyledons.  e terrestrial type of marsupium can be
regarded as an extension of the body cavity or a kind of uterus (Hoese, 1984 ).  ere is
no exchange of fl uids ( Porcellio type water-conducting system; no capillary action).
A special form of terrestrial marsupium was described by Warburg and Rosenberg
( 1996 ) using SEM and TEM. It raises the possibility of a ‘sac’ type marsupial structure
in Armadillo offi cinalis Dumeril, 1816 and Schizidium tiberianum Verhoeff , 1923.
Eggs, embryos and mancas are grouped into monolayered sacs suspended by a chord
from the ventral integument of the female’s marsupium. No cotyledon-like structure
could be seen, although the structure of sac-epithelium showed similarities with
cotyledons.
Behavioral and ecological adaptations
A wide range of behavioral adaptations enables isopods to live on land.  eir behavior
is in response to environmental factors such as light, humidity, temperature and chem-
ical stimuli. Intrahabitat behavior, alterations in microhabitat use, resource utilization,
breeding phenology, sheltering strategy and ecomorphological diff erences provide ways
to avoid competition and support coexistence for sympatric woodlice populations
(Schmalfuss, 1984 ; Zimmer and Brauckmann, 1997 ; Zimmer and Kautz, 1997 ;
Zimmer, 2003 ).
Dispersion, surface activity
Most surface-active terrestrial isopods are typically nocturnal (e.g. Tuf and Jeřábková,
2008 ), and exhibit seasonal rhythms that follow changes in key environmental factors.
 ese changes result in seasonal changes in surface activity and dispersion patterns
(mainly clumped; Figure 12 ; Hornung 1989 , 1991 ; Hornung and Warburg, 1995 ,
1996 ; Farkas, 1998 ).  e presence of suitable humidity conditions is critical and has a
basic role in determining tolerance ranges at a habitat and microhabitat scale. Humidity
can be more important than any other environmental conditions or resources – such
as food, temperature, light or oxygen support (Heeley, 1941 ). Dispersal and migration

116 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
are critical elements of the behavior of individuals and populations in response to
changing environmental conditions, such as e.g. seasonal dynamics in favourable habi-
tat patches or, on a greater scale, in response to habitat change or loss. Dispersal might
be determined by higher quality food patches (Hassall et al., 1992 ; Hassall, 1996 )
under humid conditions, or for higher humidity shelter sites (Hornung, 1989 , 1991 ) in
habitats with extreme seasonality. A special activity pattern is shown by desert-dwelling
isopods. Detailed studies of Hemilepistus reaumuri proved that the active period of the
population is determined by their annual rhythm and phenophase. Individuals main-
tain their heat and water exchange within their physiological tolerance limits by their
diurnal activity (Shachak et al., 1979 ; Nashri-Ammar and Morgan, 2005).
Sheltering
To avoid desiccation, isopods shelter, depending on species-specifi c tolerance, humid-
ity and time of the day (Hassall and Tuck, 2007 ). Shelter site use can be seasonal
Figure 12. Aggregating isopods; (A) Ligia italica (life size 12 mm); (B) Armadillo offi cinalis (19 mm).

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 117
and/or sex-dependent. Males are more active early in the season during mating while
soliciting receptive females. Later in the season, gravid females are looking for shelter
sites that are optimal for incubation, decreasing the overall cost of reproduction
(Dangerfi eld and Hassall, 1994 ; Hornung et al., 2009 , 2010 ).
Above the general habits, there are special cases, such as troglobitic, troglophilic
forms or desert dwelling species. Caves are real ecological refuges for such hygrophilous
invertebrates as terrestrial isopods. Around 300 species of terrestrial isopods are troglo-
biotic and additionally many others are troglophilic (Taiti, 2004 ). Numerous species
of terrestrial woodlice are adapted to hypogeian and endogeian habitats (Manicastri
and Argano, 1989 ).
Probably the most amazing example of adaptation and tolerance in the Oniscidea is
the genus Hemilepistus that inhabits loess deserts. Hemilepistus reaumuri ( Figure 13R )
is the best-known species that has adapted to these harsh conditions with its monoga-
mous, subsocial behavior, ‘family life’, diurnal activity, and semelparous reproduction
strategy (Linsenmair, 1985 , 1987 ; 2008 ; Shachak, 1980 ; Shachak and Newton, 1985 ;
Warburg, 1992 ; Nashri-Ammar and Morgan, 2005).  is animal emerges at the end
of the Mediterranean winter and is active on the surface from the beginning of spring
to autumn. During the heat of the day, it remains in its burrow, but it is active above
ground in the morning and evening (Nasri-Ammar and Morgan, 2005 ). Hemilepistus
shows prolonged brood care, investing higher costs in reproduction and gaining higher
survival of progeny under the extreme desert conditions (Linsenmair, 2008 ). In an
experiment on the settling behavior of H. reaumuri , Baker ( 2005 ) has shown that
specimens experiencing poor, degraded habitat conditions became less selective and
settled in poor quality areas while those from good quality habitat patches did not
disperse in spite of overcrowding.
Conglobation, aggregation
Conglobating isopods (’pill bugs’) can infl uence water balance and prevent predation
by rolling-up ( Figure 13J -L). Water loss rate and CO
2 release were decreased signifi -
cantly (by about 35% and 37%, respectively) by this behavior, depending on relative
humidity (Smigel and Gibbs, 2008 ). Non-conglobating forms may have the advantage
of eff ective locomotory activity to avoid desiccation and for fi nding suitable micro-
habitat as was shown with Porcellio laevis (Dailey et al., 2009 ).
 e spatial pattern of isopod distribution at the habitat scale proved to be aggre-
gated in most ecological studies (Hornung, 1989 , 1991 ; Hornung and Warburg 1996 ;
Farkas, 1998 ).  is clumping behavior may have a temporal pattern in seasonal envi-
ronments and correlates with above ground humidity conditions. Aggregation is an
adaptive behavior ( Figure 12 ) against desiccation and it correlates intraspecifi cally with
latitude (Caubet et al., 2008 ): southern populations show a higher level of aggregation.
Aggregation may also be a stimulating trigger for reproduction in females, speeding up
their vitellogenesis (Caubet et al., 1998 ) and accelerating body growth (Takeda, 1980 ).
An aggregation pheromone is secreted in the mid- and/or hindgut and it is excreted
with the faeces (Takeda, 1980 ).

118 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
Food choice, feeding strategy
Isopods are saprophagous invertebrates important in plant litter decomposition.  eir
function has a key role in ecosystems and is strongly infl uenced by environmental fac-
tors, including climate and so, thus by global climatic changes (David and Handa,
2010 ).  ey may have a key regulatory function in the decomposition of dead plant
material in certain habitat types (e.g. arid regions of North Africa and Asia – Shachak
and Yair, 1984 ; Linsenmair, 2008 ; tropical and temperate ecosystems – David and
Handa, 2010 ). Terrestrial life also requires adaptation also to the quality of available
food sources. Evolution of behavioral adaptations includes food source choice and
feeding strategies.
Second antennae of terrestrial isopods help not only orientation but function as
gustatory organs in food localization. In case of loss of the second antennae, the minute
fi rst antennae ( Figure 5 ) substitute them by using their aestetascs, or chemoreceptors
(Schmalfuss, 1998 ).  e odor of metabolites emitted by food colonizing microbiota
(Zimmer et al., 1996 ) directs the food choices of isopods. Microbes produce extracel-
lular enzymes and/or trace nutrients, fi rst of all essential amino acids (Ullrich and
Storch, 1991 ). Isopods use exo-enzymes gained from consumed microbita for diges-
tion of plant material. Kostanjšek et al., ( 2010 ) found the fi rst evidence for cellulose
degrading endogenous enzymes in peracarid crustaceans, namely in Porcellio scaber .
Enzyme activity was shown in hepatopancreatic extract.
Zimmer ( 2002 ) reviewed knowledge of terrestrial isopod nutrition from an evolu-
tionary ecological view. Food quality infl uenced food choice in the laboratory experi-
ments of Szlavecz and Majorana ( 1991 ). Nitrogen rich leaves were preferred by all
investigated species, either cosmopolitans ( Porcellio scaber and Armadillidium vulgare )
or more restricted, central European species ( Protracheoniscus amoenus (now P. politus )
and A. zenckeri ). Dicotyledons mean high quality food that is preferred (Rushton and
Hassall, 1987 ). Increasing patchiness of high quality food distribution and isopod
abundance changes foraging behavior of individuals. At high density, woodlice spend
more time searching for food and spend more time on low quality food (Hassall et al.,
2001 ). Food choice and consumption are often used as endpoints of toxicological tests.
Zidar et al. ( 2003 , 2005 ) found on Oniscus asellus that food quality is refl ected in the
behavior of woodlice. Animals avoided both Cadmium contaminated and sterilized
food in the presence of uncontaminated or molded food.
Figure 13. Examples of diff erent eco-morphological types (Schmalfuss, 1984 ). ‘Creepers’:
(A) Haplophthalmus danicus Budde-Lund, 1880 (size 2.5-4 mm), and (B) Androniscus dentiger Verhoeff ,
1908 (7-8 mm); ‘Runners’: (C) Hyloniscus riparius (C. Koch, 1838) (4-6 mm), (D) Porcellionides pruino-
sus , (E) Protracheoniscus politus ; ‘Clingers’ (F) Porcellio dilatatus (12-15 mm), (G) Trachelipus ratzeburgii
(Brandt, 1833) (12-15 mm), (H) Porcellio scaber (14-18 mm), and (I) P. spinicornis Say, 1818 (12-15 mm);
‘Rollers’: (J) Armadillidium vulgare , (K) A. nasatum Budde-Lund, 1885 (13 mm), (L) A. versicolor
Stein, 1859 (14 mm), and (M) Cylisticus convexus (De Geer, 1778) (12-14 mm); ‘Non-conformists’:
(N) Platyarthrus hoff mannseggii (2-4.5 mm), (O) Buddelundiella cataractae Verhoeff , 1930 (2 mm; cour-
tesy of F. Vilisics), (P) Trichorchina tomentosa (Budde-Lund, 1893) (3.5-4 mm), and (R) Hemilepistus
reaumuri (‘true digger’; 10-19 mm).

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 119

120 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
Faeces consumption (allo-, autocoprophagy) is a general phenomenon in terrestrial
isopods although the interpretation of its function is inconsistent (Hassall and Rushton,
1982 ; Wieser, 1984 ; Ullrich and Storch, 1991 ). In default of coprophagy, survival,
body weight increase might be negatively infl uenced.  at might be due to shortage of
microbiota reconsumed.  e extent of faeces eating is in correlation with food quality
(Hassall and Rushton, 1982 ). A previous hypothesis of Wieser ( 1966 , 1984 ) presumes
copper regain as the function of coprophagy.
Hassall and Rushton ( 1984 ) discussed the adaptive signifi cance of selective feeding.
Food quality may play a crucial role in population dynamics of terrestrial isopod
assemblages.
Life history strategy
Phenotypic plasticity in life history traits is an adaptive response to environmental
conditions (photoperiod, temperature). Timing of breeding (about 2 days time lag
increase per degree of latitude) in Armadillidium vulgare (Souty-Grosset et al., 1988 )
or increase in reproductive output and off spring size in Porcellio laevis Latreille, 1804
(Lardies and Bozonovic, 2008 ) with higher latitude. Ligia exotica Roux, 1828 produced
larger off spring in inland population under unpredictive aridity conditions than living
in littoral zone under constant humidity (Tsai and Dai, 2001 ).  ere is a clear trade-off
between off spring number and size. Most terrestrial isopods show extensive parental
care contributing to increased fi tness of their off spring. Larger juveniles favor coloniza-
tion and enables invasion from littoral to inland habitats. Diet quality, high protein
content of food resulted in a signifi cant increase in off spring number and decrease in
off spring size in Porcellio laevis (Lardies et al., 2004b ).
Sutton et al. ( 1984 ) divided the life history characteristics of terrestrial isopods into
steno- and euridynamic types, more or less similar to r-K strategies.  e observed life
history traits are in close relationship to the eco-morphological strategies suggested by
Schmalfuss ( 1984 ) ( Table 1 ). Soil dwelling (’creeper’) species belong to stenodynamic
ones (‘K’ strategy) while surface-active species (‘runners’, ‘clingers’, ‘rollers’) belong
mainly to the eurydynamic (‘r’ strategy) species group. All these variation of strategies
occur also among cave-living species (Taiti, 2004 ). Additionally, troglobiotic species
show the characteristic adaptive traits of troglobiotic invertebrates, such as reduction
or shortage of eyes and pigmentation, long appendages, thin cuticle, specialization of
sensory organs, loss of rythmicity, reduced fertility, and low number of off spring
(Manicastri and Argano, 1989 ).
Future directions
 e diverse group of terrestrial isopods off ers an excellent opportunity to study the
diversity in land adaptations, morphologically, physiologically and ecologically.
Specimens of several species can easily be reared under laboratory conditions and used
as experimental models.
A series of research questions can be raised covering a wide range of biological disci-
plines (Hornung et al., 1992 ). Most of the questions were grouped by subdisciplines

E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130 121
and published as an outcome of the plenary discussion at the 6th Symposium of
Isopodologists (Hassall et al., 2005 ).  e topics range from molecular and physiologi-
cal questions to ecological and biogeographical ones.  e issues concern life history
and reproductive strategies, plasticity at the individual level, such as pair choice,
number of successive broods, reproductive investment of specimens, etc. In addition,
the problem of correspondence between ecological tolerance, adaptive morphological
characters and the environmental conditions of species and their distribution on
regional and habitat/microhabitat level are waiting for further elucidation.  e correla-
tion between ecomorphological features and environmental tolerance/habitat require-
ments in determining geographical distribution as well as life history traits and their
association to successful establishment, distribution, especially in the case of invasive
species, needs to be further addressed.
In the last decade, new research trends in isopodology have also emerged. A more
functional approach in ecological investigations is needed, such as studies on the sig-
nifi cance and mechanisms of population interactions within a decomposing system;
the effi ciency of their contribution to ecosystem services also need further investiga-
tions: the importance of species richness, functional diversity and redundancy in
decomposing subsystems (Heemsbergen et al., 2004 ). Isopods also became favorite
models for ecotoxicological studies: e.g. investigating the eff ects of heavy metals
(Hopkin, 1993; Drobne, 1997 ; Vijver et al., 2005 ), insecticides (e.g. Drobne et al.,
2008 ; Santos et al., 2010), endocrine disruptors (e.g. Lemos et al., 2009 ; 2010 ), or gen-
eral methodological problems (Drobne and Hopkin, 1994 ; Loureiro et al., 2005b ).
In addition, the increasing global problem of urbanization, the functional role of
alien species, and the homogenization of the urban fauna also need urgent research
(Magura et al., 2008a , b ; Pouyat et al., 2008 ; Vilisics and Hornung, 2009 ).
Acknowledgements
 e author would like to express her wholehearted thanks to Professor M. R. Warburg
(Haifa, Israel) for his inexhaustible encouragement and professional help; to
Table 1. Life history characteristic of terrestrial isopods (compilation based on Sutton et al., 1984 ;
Schmalfuss, 1984 ) and represent broad relative tendencies.
Characters Stenodynamic (‘K’) strategy Eurodynamic (‘r’) startegy
Soil active species (‘creepers’) Surface active species (‘runners’,
‘clingers’, ‘rollers’)
Size small large
Number of ocelli a few many
Pigmentation light extensive
Maturity late early
Number of off spring small large
Manca’s size large small
Growth rate slow fast
Locomotion slow fast

122 E. Hornung / Terrestrial Arthropod Reviews 4 (2011) 95–130
Drs. Stefano Taiti (Florence, Italy) and Helmut Schmalfuss (Stuttgart, Germany) for
the discussions, professional advice and provison of SEM images; Dr. Paul T. G.
Harding (Monks Wood, England) and David North (Canada-Hungary) for improving
the English of a previous version of the manuscript. Special thanks to Professor Jasna
Štrus (Ljubljana, Slovenia) and her staff for their generous help and cooperation in
histological studies. Under the guidance of Professors Štrus and Katalin Halasy
(Budapest, Hungary), my M. Sc. student, Diana Csonka was able to do excellent work
and illustrations on cuticle and lung histology.  anks also go to Professors J. Štrus, K.
Halasy, and to D. Csonka for agreeing to publish original photos on histology. Grateful
thanks to Dr. K. Buczkó (Hungarian Nature History Museum, Budapest) for her
patience and help while preparing SEM fi gures. Last, but not least, sincere apprecia-
tions to six referees who generously provided me useful and constructive advice to
improve a previous version of my manuscript.
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