Armadillidium vulgare

Geographic Range

Armadillidium vulgare , the common pillbug, is native to the edge of the Mediterranean and has been introduced to nearly all worldwide terrestrial landmasses, with particularly dense populations in temperate climates. There are significant populations throughout the United States, and it is also found in Madagascar, Australia, South Africa, and India, among many other areas. Armadillidium vulgare has also been extensively studied and collected in Japan, France, Canada, central Bohemia, the Czech Republic, and shorelines of western Romania.

• Biogeographic Regions
• nearctic nearctic
  • introduced introduced
• palearctic palearctic
  • native native
• oriental oriental
  • introduced introduced
• ethiopian ethiopian
  • introduced introduced
• neotropical neotropical
  • introduced introduced
• australian australian
  • introduced introduced

• Other Geographic Terms
• cosmopolitan cosmopolitan

Habitat

Armadillidium vulgare is abundant and active as both a soil and surface dweller. Populations thrive in moist climates and damp soils. Armadillidium vulgare can be found in locations with a standard Mediterranean climate or in temperate agroecosystems. Data has been compiled that indicates that A. vulgare populations range throughout the temperate, subtropical, and subarctic climates of Japan. Humidity levels ranging from 50 to 60% are acceptable conditions to prevent desiccation. Optimal habitats have abundant decomposing organic matter, moderate temperatures, low illumination, and moderate to high humidity. While other terrestrial isopods populate thermal habitats such as the soil near heated swimming pools or shorelines during colder winter months, A. vulgare prefers drier areas further from water. Locations where the soil entirely freezes over do not encourage population growth.

Common pillbugs can be found under pieces of natural debris such as stones or logs in forests, and in the soil of fields, gardens, or hedgerows. Exposed large-particle soil (as found in agricultural cultivation sites or greenhouses) is more desirable than finer soils, as the former allows for increased water retention, easier burrowing, and increased relative humidity. Human domestic waste such as cardboard or old rags provide suitable habitats as well.

Populations have been maintained successfully under stable laboratory conditions such as daily fluorescent illumination exposure ranging from six to ten hours a day, temperatures between 20 to 25°C, and combinations of damp soil and deciduous leaf litter with 100% humidity.

• Habitat Regions
• temperate temperate
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• desert or dune desert or dune
• savanna or grassland savanna or grassland
• chaparral chaparral
• forest forest

• Other Habitat Features
• urban urban
• suburban suburban
• agricultural agricultural

Physical Description

Like all isopods, Armadillidium vulgare is oval-shaped and moderately flattened along its dorsal plane. Isopods have a cephalic shield (incompletely fused carapace) that is less durable than the fully fused carapace of other crustaceans. They have three tagmata: the head, which bears their cephalon (fused maxillipeds), the pereon (thorax), and the pleon (abdomen).

Isopod heads have unstalked eyes, unlike the compound eyes of most crustaceans, as well as one pair of antennae that bear setae. A secondary pair of smaller antennae is present but vestigial and serves no known biological purpose. The pereon is divided into seven somites (divisions), each of which has a pair of pereopods (short walking legs) protruding from it. The second through fifth ventral somite plates form the female's marsupium. Isopods primarily acquire oxygen via a thickened cuticle composed of a fibrous matrix of calcium carbonate that allows for both gas diffusion and water conservation. The pleon supports two pairs of oval-shaped respiratory structures called pleopods. They are located on the first two ventral segments of the pleon, and are hypothesized to have once been a pair of appendages. The pleopods trap air with sponge-like structures called pseudotracheae, giving them a white appearance. This is not to be confused with the white dorsal calcium carbonate plates formed during the pre-ecdysis stage of molting. The pleon also supports several tail projections, which transport water to the mouth of the isopod. Like most other crustaceans, isopods have flattened plate-like uropods (flattened appendages used for movement) and a telson (rigid structure used for backwards thrust) which are fused to form a posterior tail fan.

Armadillidium vulgare can be distinguished from other terrestrial isopods by observing both clearly visible antennae that protrude during conglobation and relatively short pereopods that cannot be seen from their dorsal surface. Compared to other species within the same genus, A. vulgare has a thicker cuticle and denser endothelium between the respiratory cavity and lung fluids. Although not visible externally, these morphological adaptations may have contributed to its increased resistance to desiccation, and thereby its cosmopolitan distribution.

Armadillidium vulgare has an oval body shape approximately twice as long as it is wide. Two- to three-month-old A. vulgare juveniles are generally between 5 to 7 mm in length. Typical young adults are 10 mm long and 5 mm wide. Sexually mature individuals can generally be distinguished by having a length greater than 0.7 mm. Males and females have approximately equivalent mass.

Pigmentation in Armadillidium vulgare is determined by two distinct pigments in the integument (outer shell): ommochrome pigment that produces dark body coloration, and pteridine pigments that produce distinct colored spots in the dorsal region. The presence of dense pteridine pigments usually results in slightly yellowish spots, although brown or red coloration also occurs. Most individuals have a dull, dark gray universal body color due to the distribution of these pigments but variants occur. Individuals infected with IIV-31 may instead be light blue, purple, or violet. Some populations of A. vulgare have drastically reduced and less dense ommochrome pigment such that they do not display the darker coloration at all.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Development

The Armadillidium vulgare life cycle involves an egg stage, a juvenile stage termed a manca, and finally a reproductive adult stage. There is no nauplius stage, which most crustaceans have. Instead, embryos hatch as immaturely developed adults.

Armadillidium vulgare eggs are thin-walled and possess a yolk. After release from the oviduct, they are stored in the marsupium, a fluid-filled pouch present in reproducing females. Eggs are enclosed in both an inner vitelline membrane (composed of protein fibers and species-specific sperm receptors) and an outer chorion. The chorion is shed as a protein envelope when the egg's embryo has consumed half the original yolk. Within the embryo is a poorly understood 'dorsal organ' sensory structure common to many crustaceans. This structure is hypothesized to regulate ion and acid exchange for the developing embryo.

Egg size increases with the size of the mother. When the yolk is fully consumed, the dorsal organ atrophies and the embryo undergoes blastokinesis. After one to two days, the vitelline membrane is shed and the manca hatches. Only half of the eggs produced result in fully developed mancas. After three to four days, the mancas crawl out of the marsupium.

The thickened cuticle of Armadillidium vulgare consists of an exocuticle containing rows of crystallized calcite and an endocuticle made up of amorphous calcium carbonate. The cuticle must be periodically shed to allow for continued growth. The molting cycle of males and non-reproducing females, called the normal intermolt, takes about 29 days. It begins with a 2 day period following the last molt, where the new, soft cuticle calicifies. The pillbug cannot move or eat, and is vulnerable to predation and dessication. For 12 to 14 days, the calcium builds up in the exoskeleton. Then a 10 to 12 day premolt, where the new molt cycle begins. The hemolymph reabsorbs calcium from the old exoskeleton, and the new exoskeleton forms while the old exoskeleton separates from the epidermis. The remaining part of the cycle is a 2 to 4 day ecdysis of intermolt, where the splitting of the old exoskeleton occurs and is shed from the body.

Armadillidium vulgare females enter a separate molting cycle during their reproductive phase. This cycle is called the preparturial intermolt, with the actual process of ecdysis at the end of the cycle called the parturial molt. During parturial molts, females entirely repress food consumption. The roughly 43 day preparturial intermolt begins the same as the normal intermolt, with a two day period following the previous molt. For 12 to 14 days, the calcium builds up as it does in the normal intermolt, and females forage more during this time. The marsupium also differentiates during this time, and ovarian maturation occurs. For 15 days, the new molt cycle begins, same as the normal intermolt. For about 10 days after this, the female's sexual receptivity is at its highest. Finally, there are 2 to 4 days of ecdysis.

Reproduction

Mating Armadillidium vulgare pairs can potentially form up to a few days before the female's receptive period begins. However, males are more attracted to females with prominent calcium plates, which correspond to their higher reproductive receptivity period. Terrestrial isopods as a whole generally mate in spring. Warmer conditions usually lead to earlier reproduction. In areas with mild winters, particularly Mediterranean climates, they can remain sexually active throughout the year. A. vulgare females can store sperm from multiple males, who leave the female after mating and are free to mate again. Thus, there are no truly permanent mating pairs, making this species polygynandrous.

Mating is brief, completed within a few seconds, and synchronized with the beginning of the female's parturial molting cycle. Mating occurs as Armadillidium vulgare males climb onto the backs of females, bend their pleon downwards, and use their first pair of pleopods to transfer sperm to the female's ventral gonopores. Within the oviduct, the sperm are immobilized within an epicuticular envelope bundle. Bundles from each mating incident form rings within the oviduct, so that muscle cells around the oviduct can pressurize the bundles to release the immobilized spermatozoa onto oocytes that pass through these rings during oviposition. Sperm from one mating incident can be stored in this manner for an entire year for use in subsequent broods, with older sperm bundles taking precedence over more recent genetic material when broods are laid. After mating, female individuals exhibit a 'refractory period' during which further male mating attempts are rejected.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

While male A. vulgare individuals are sexually active at all times (except during molting), they do not initiate courtship behaviors unless females are currently secreting 'aggregate' pheromones, which indicate female receptivity. Female sexual activity is limited to the receptive period of their preparturial intermolt cycle. Female parturial molts are not limited to any particular season, but they occur most often in early spring. It is unclear how often parturial molting cycles occur or at what exact time females produce their eggs, but since females can sometimes have three broods a year it is clear that parturial molts are not restricted to once-per-year occurrences. Eggs are retained in the marsupium for two to three months until the mancas hatch. The hatched mancas remain within the marsupium for three to four days and then emerge. After undergoing a few molts, they are considered independent.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• sperm-storing sperm-storing

After mating, A. vulgare males leave to continue to feed, molt, and mate again, thus offering no parental investment in the brood. While the mancas remain in the marsupium, the mother remains in or close to her natal burrow. After the mancas emerge they may separate from the mother and live in branching burrow tunnels alone, or they may remain with the mother, who offers them protection within her natal tunnel. Either way, the mancas remain in the burrows for several successive molts, until their shells stiffen and their last pair of pereopods grows.

• Parental Investment
• precocial precocial
• female parental care female parental care
• pre-hatching/birth
  • protecting
    • female
• pre-independence
  • protecting
    • female

Lifespan/Longevity

Individuals have an average lifespan of a year and a half, but some have longer lifespans of up to a few years. Studies have suggested that A. vulgare relies on a highly social lifestyle for proper longevity; those that are isolated from others in biologically viable conditions have a very high mortality rate.

Behavior

Much of the behavior of Armadillidium vulgare revolves around a constant struggle to preserve body moisture and prevent unnecessary desiccation. Armadillidium vulgare individuals move slowly during periods of high humidity and more rapidly during dry periods as they search for more humid areas. In drier environments, they spend more time sheltering as opposed to feeding or other more energy-demanding activities. Individuals travel roughly twice as much during summer (13 meters per day) as winter (6.6 meters per day), and they are usually more active at night to further reduce desiccation risk. It is unclear whether movement is constant, allowing the individuals to forage while they go, or if periods of movement are interspersed with periods of foraging and rest. The species has been described as negatively photo-kinetic (when presented with bright light, they cease moving), likely in an effort to prevent any nonessential moisture loss. When air temperature ranges between 20°C to 30°C, pheromones activate causing conspecifics to be attracted to one another and bunch together in a group. This behavior helps decrease the surface area of any one individual in the group, and thereby exposure to the moisture-depleting external heat.

Armadillidium vulgare exhibits a behavior of curling posterior joints in towards anterior joints in a sphere-like shape. This behavior, conglobation, has been observed in response to situations when the body is exposed directly to hard pressures or strong vibrations in the immediate area. Conglobation has been hypothesized to be either a defense mechanism used to shield the softer inner abdomen and limbs with the more stiff outer shell or an effective method of further preventing desiccation.

Forward movement can be initiated from weak ground vibrations or forces that reach the body. This response is considered an escape mechanism from potential predators. While moving, A. vulgare exhibits a behavior termed ‘turn alternation’, in which they invariably alternate between gradual left and right turns to move indirectly forward in a straight line until encountering an obstruction. It is hypothesized that turn alternation results from A. vulgare being placed in an unfamiliar environment in which potential hazards or resources are unknown. Water is a common obstruction or inhibitory structure to A. vulgare movement, as they cannot swim.

It has been suggested that A. vulgare practices a 'priming' method of habitat selection in which juveniles leave their natal home range in search of habitats more desirable than those they were born into. If they fail to locate a more desirable habitat after a yet-undetermined time threshold, they will return to their natal habitat, presumably to decrease the survival risks involved with lengthy, long-distance movements.

• Key Behaviors
• fossorial fossorial
• nocturnal nocturnal
• motile motile

Communication and Perception

This species has limited abilities of sight, smell, and touch, provided through ommatidia, esthetascs and tactile setae, respectively. Each small, non-stalked eye of A. vulgare contains a complex of ommatidia, linear structures which capture light through photoreceptors. Vision is limited to the detection of the presence or absence of light, so they generally have poor visual acuity. Esthetascs (olfactory hairs) are used to locate food and recognize other crustaceans and their sexual states. Tactile setae are used to detect objects and are found on their antennae, mouthparts, and some pleopods.

Chemical sensing, particularly how the antennae handle air-borne 'aggregate pheromone' chemicals, is likely this species' most important perceptive tool. The aggregate pheromone has many specific functions, including desiccation prevention, body growth rate acceleration, and metabolic rate reduction, but it may be used primarily as a way of locating conspecifics. The aggregate pheromone is secreted by digestive tissues, evacuated in excrement, and found in subtle traces both on the outer shell and within 'marking trails' that are produced as the individual moves through its habitat. These marking trails can be detected by the antennae of conspecifics, and allow A. vulgare to locate one another purely through this chemical sensing technique. This pheromone also signifies the desirability of a habitat. If an A. vulgare individual enters a new habitat and senses the presence of aggregate pheromone in feces, molt fragments, or the marking trails, it would indicate to the individual that other members of its species are successfully surviving in that habitat. Aggregate pheromone concentration varies with humidity, but it is always highest during mating seasons. This has caused the pheromone to be hypothesized to serve as a potential mate-finding tool, or as an aid in synchronization of female molting cycles.

• Communication Channels
• chemical chemical

• Other Communication Modes
• pheromones pheromones

• Perception Channels
• visual visual
• tactile tactile
• vibrations vibrations
• chemical chemical

Food Habits

Terrestrial isopods like Armadillidium vulgare are usually detritivores, although during drought periods they adopt more scavenger-like tendencies. A. vulgare breaks down the decaying leaf litter of many plant species, such as Acer platanoides (Norway maple), Salix fragilis (brittle willow), Quercus robur (English oak), and dried lime leaves ( Tilia sp. ).

Herbivory and carnivory have also been observed. A. vulgare can also feed on small pieces of garden roots such as carrots, as well as fruit, and laboratory colonies have been maintained on lettuce leaves. Commercial potting soil contains a high organic content, which yields another food source for A. vulgare . Some data suggest that almost ten percent of the pill-bug's diet comes from self-coprophagy, which recirculates microbes and nutrients that were not fully processed during original digestion. During social interactions, individual woodlice can even cannibalize weaker individuals from the same or different species if the prey is injured or caught in the middle of a molting period. Armadillidium vulgare has also been known to be granivorous, although seeds are likely only eaten in absence of other, more desirable food sources, such as spring and early summer when leaf litter is scarce. A. vulgare is known to eat the seeds of herbs such as Chenopodium album (Lamb's quarters), Capsella bursa-pastoris (Shepard's purse), Stellaria media (common chickenweed), and Veronica persica (Persian speedwell).

Food deprivation reduces growth rates, but is not as serious a threat to survival as might be assumed. In one experiment, A. vulgare individuals starved of food for three months were still able to survive under laboratory conditions.

• Primary Diet
• detritivore detritivore

• Animal Foods
• terrestrial non-insect arthropods

• Plant Foods
• leaves
• roots and tubers
• seeds, grains, and nuts
• fruit

• Other Foods
• detritus detritus
• dung

Predation

Armadillidium vulgare has a variety of defenses against predation. This species has evolved heavily encrusted armor, repugnatorial glands located on the pereon which release unpleasant secretions, and conglobation. Their earthen tone coloration also provides some degree of camouflage against wood or rock substrates. These defenses are inadequate against large predators, such as birds, including the Common Starling , but few smaller predatory arthropods are able to successfully overcome them. A genus of tropical ants, Leptogenys sp., have elongated mandibles that allow them to pry apart conglobated individuals.

The Dysdera genus of spiders are some of the few nocturnal, ground-dwelling predators that prey on nocturnally active woodlice such as A. vulgare . Woodlice remnants have been found in Dysdera silk retreats and digestive tracts. Lack of woodlice in the diet of the young Dysdera hungarica can impede growth and development. It is hypothesized that the ability of D. hungarica to prey on woodlice is due to its evolution of elongated chelicerae.

Ecosystem Roles

Terrestrial isopods as a whole are traditionally detritivores that feed largely on plant litter, seeds, or scavenged dung. Seed predators play a critical role in the growth of spermatophyte populations, and Armadillidium vulgare can be the dominant seed predators in systems where other seed predator groups (such as birds, ants, slugs, or crickets) are scarce or temporarily inactive.

Individuals in Japan, Madagascar, the United Kingdom, Australia, South Africa, India, and the United States have been observed to carry non-enveloped virions (viral structures or particles) of Iridovirus IIV-31 (invertebrate iridescent virus 31). Blue pillbugs, which are infected with IIV-31, have a shorter lifespan and decreased photo- and water-responsiveness than non-infected individuals. Other terrestrial isopod species have also been observed to be IIV carriers, and infection appears possible in all terrestrial isopods regardless of phylogenetic relationship. Specific methods of IIV-31 transmission remains unclear, but hypotheses range from cannibalism, coprophagy, inter-species aggression, or parasitism by nematode vectors.

Many arthropods commonly bear populations of Wolbachia sp., an endosymbiotic bacterium that usually reinforces the host's immune system and is passed down through generations through maternal gametes. This transmission is limited to maternal gametes due to Wolbachia sp.'s tendency to induce feminization of genetic males by forcing zygotes to develop into female adults regardless of their sex chromosome composition. Infection can also be transmitted through cannibalism of weak individuals, although this is far less common than gamete transmission. A. vulgare is particularly susceptible to Wolbachia sp. as the bacterium can reside inside its hemocytes. Unlike the benefits usually provided to other arthropods, presence of Wolbachia sp. in A. vulgare has been linked with immunodepression of hemocytes, particularly in older individuals. Since Wolbachia species have been observed to survive transition through the digestive tract of A. vulgare , the only defenses against infection appear to be normal lysosome activity and cell replacement. Due to the possibility of a scarcity of males, an eventual high prevalence of Wolbachia sp. could lead to A. vulgare extinction, but current Wolbachia sp. prevalence levels do not indicate this as an imminent threat.

Terrestrial isopods such as Armadillidium vulgare have a less common, but more deadly, relationship with the bacterium Photorhabdus luminescens . Upon entering an isopod host, the entomopathogenic P. luminescens rapidly distributes a toxic protein into the blood that strongly reduces concentration of isopod hemocytes. This reduction in the host's immune protection is lethal, and after injections of P. luminescens in a laboratory setting, four out of six A. vulgare individuals were dead after 48 hours of exposure. This rapid host decline is notable because the bacterium itself could potentially perform little or no multiplication during that time.

Some pillbug species, Armadillidium vulgare among them, act as an intermediate host in the life cycle of an acanthocephalan worm species, Plagiorhynchus cylindraceus . Feces from the primary bird hosts of P. cylindraceus , notably Sturnus vulgaris , contain the eggs. A. vulgare , being coprophagic, can ingest the eggs, which hatch inside the pillbugs' digestive tract. P. cylindraceus is small, usually only 2 to 3 mm at maximum length, but large infestations can crowd internal organs. Plagiorhynchus cylindraceus infestations also render female isopods sterile, and directly alter the behavior of its host. Infected individuals leave their natural habitats and move into wide open spaces where bird predation is more likely.

• Ecosystem Impact
• biodegradation biodegradation

Economic Importance for Humans: Positive

In the rare situation where Armadillidium vulgare performs seed predation on agricultural weeds, such as in the agroecosystems of central Europe, this species serves as a form of biological weed control. A. vulgare also makes for a valuable species used in laboratory research, as colonies are relatively easily maintained and are long-standing. As detritivores, the eating habits and burrowing activities of soil fauna such as A. vulgare encourage soil microbe activity, which releases nutrients pent up within duff material to be recycled through the ecosystem. This microbial activity increases soil quality and thereby contributes towards agricultural success.

• Positive Impacts
• research and education
• controls pest population

Economic Importance for Humans: Negative

Large populations in gardens or greenhouses may cause noticeable damage to young plant shoots, but this threat is minimal.

• Negative Impacts
• crop pest

Conservation Status

This species is not an endangered or protected species.

Other Comments

Common names of Armadillidium vulgare also include common pill woodlouse, roly poly, and German 'Kugelassel'. The genus was once Armadillo , named after the similarly conglobating well-known armored mammal species.

Woodlice/pillbugs are not the same as sowbugs, which live in more heavily forested and semiarid regions.

Isopods such as A. vulgare are estimated to have diversified into terrestrial species between 200 (Jurassic period) and 60 (Cretaceous period) million years ago.

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

Asa Holland (author), Sierra College, Jennifer Skillen (editor), Sierra College, Angela Miner (editor), Animal Diversity Web Staff.

References

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