Armadillidium

Armadillidium () is a genus of the small terrestrial crustacean known as the woodlouse. It is one of 18 genera nested within the family Armadillidiidae. Armadillidium is also one of the groups commonly known as pill woodlice, leg pebbles, pill bugs, roly-poly, or potato bugs, and are often confused with pill millipedes such as Glomeris marginata. They are characterised by their ability to roll into a ball ("volvation") when disturbed.

With a penchant for damp and dark places, species in the Armadillidium genus can typically be found under rocks, in leaf litter, and in or around the soil. Aided by their dorsoventrally flattened body and small size – usually growing no bigger than 2.5 cm – these pill bugs are able to squeeze into tight cracks and are common household pests as a result.

Description

Unlike other terrestrial arthropods such as insects and spiders, pill bugs do not have a waxy cuticle that would reduce evaporation from their bodies. Pill bugs also use modified lungs, called pseudotrachea, for respiration, and the lungs must remain moist to function. Individual pill bugs typically live for two or three years, and females brood eggs once or twice each summer. In larger species and individuals, up to over a hundred eggs are brooded at a time in the marsupium, a pocket on the ventral side of the female pill bug. The marsupium provides nutrients and oxygen to the eggs until they hatch, resulting in something akin to live birth.

The colouration, especially of young A. klugii resembles the red hourglass marking of the Mediterranean black widow Latrodectus tredecimguttatus. It has been proposed to be a result of Batesian mimicry, to ward off predators that mistake the harmless animal for a venomous spider.

Behaviour

thumb|[[Armadillidium gestroi (Yellow Spotted Isopod) hiding under bark. Picture taken at the University of Alberta.]]

Pill bugs in the genus Armadillidium are characterized by their ability to roll up into a ball when alarmed. However, this is not their greatest defense, instead, their tendency to seek out dark, cramped spaces and hide during the day is what serves as a deterrent to being preyed upon in the first place. Armadillidium are also susceptible to drowning due to flooding and storms, and as such will move to high places such as trees to avoid this. In cases of extremely hot and dry environments, they are also known to avoid these conditions by moving underground or deeper into soil. Although they don't appear to move very fast, when in search of food during the summer time, some species have been known to travel up to 13 meters in only half a day. However, during winter, that travel rate is significantly reduced.

Identification

There are some morphological features that when seen together, help to identify members of Armadillidium from other closely related groups. These include:

alt=An image depicting Armadillidium vulgare and its component pieces.|thumb|343x343px|The anatomy of Armadillidium vulgare as depicted in A Monograph on the Isopods of North America by Harriet Richardson

Ability to roll into a ball
Convex oblong body shape
Multiple black ocelli (small, simple eyes)
Size range of 5mm - 25mm
Small first set of antennae, second set of antennae is approximately equal to half the length of the body
Epistome passes the frontal edge, is vertically directed, and often triangular in shape
Short clypeus that is not lobate, and with a slightly wavy anterior margin towards the mid-line
End segment of the abdomen either triangular or rectangular
Tracheae present on the opercular plates of the first two pairs of pleopoda
Short uropods

Feeding habits and diet

Pill bugs in the genus Armadillidium are primarily detritophagous omnivores, with a few instances of animal matter consumption. They are capable of switching feeding strategies, changing from herbivory to scavenging when plants become scarce, such as during especially dry periods.

Although they mainly feed on plant matter, they are also known to eat their own fecal pellets – a process called coprophagy – as well as those of other organisms. It has also been proven that the rate of growth of Armadillidium can be stunted if they stopped eating feces, which accounts for around 1/10th of their diet.

When presented with options, some species of Armadillidium have displayed a feeding preference for certain plants, such as A. vulgare preferring thistle, tarweed, and vetch over other leaves. They are usually found in moist areas such as decomposing leaf matter and soil. Armadillidium vulgare is the most abundant species in Europe, with a native distribution spanning from England to Russia, and has been introduced to several countries worldwide. Currently, there are large populations of A. vulgare within the US after being introduced to California. There are also A. vulgare populations present in Madagascar, South Africa, Australia, Canada, and Japan. However, the vast majority of species are endemic to small regions close to the Mediterranean Sea, in much lower numbers than common species such as A. vulgare, and hence are understudied.

Physiology

Typically, land animals utilize uric acid The breeding season can affect a female's ability to roll into a ball because the marsupium can become distended with the amount of young they carry. The female's ball form will not be fully complete, leading to an increased predation risk.

Chemoreception

Some species of Armadillidium, such as A. vulgare, are known to emit scents and have chemoreceptors located in the antennae to detect them.

Response to temperature, humidity, and transpiration

The behaviour of Armadillidium species differs at certain temperatures as well as certain light levels. However, it's been found that these conditions interact to affect how pill bugs respond. Because of this, they must shed their exoskeleton to get bigger. However, right after moulting, the pill bug is susceptible to many dangers, such as predation, as their cuticle – which comprises their new exoskeleton –is soft and their colouration is closer to white or translucent rather than their regular greys, blacks, and browns.

Over the course of their lives, Armadillidium are able to produce up to three broods per year, with each consisting of up to 200 eggs. Females have a unique adaptation called the marsupium or ‘brood-pouch’ that carries eggs for 1–2 weeks till they are ready to hatch. The marsupium is formed during the parturial molt by two layers of the pill bug's endoplasm and provides nutrients, calcium, water and oxygen to their young. Pill bugs are iteroparous, meaning they produce offspring and reproduce multiple times throughout their lives,Armadillidium mate during the intermolt phase: prior to moulting and producing a brood-pouch. Although some species have parthenogenic tendencies, most Armadillidium are not primarily parthenogenic and require contributions from both males and females to produce viable offspring. The growth rates of reproductive females are significantly less than those of non-reproductive females of similar sizes. In non-reproductive females, the total energy they contribute to growth is equivalent to the energy that reproductive females devote to both reproduction and growth. The fertility of female Armadillidium is also influenced by their size, with larger individuals having greater reproductive output as they have larger brood pouches and are able to carry more offspring. The parasite eggs are ingested via coprophagy and eventually develop into larvae which burrow into the pill bug's body cavity via the gut wall. As the parasite develops, it makes the pill bugs avoid their usual sheltered areas, seek out lighter coloured substrates (making their dark colouration stand out), as well as stay in drier areas as opposed to their usual moist habitats. All of these behavioural changes combine to increase the pill bug's chances of being preyed upon by the parasitic worm's primary host: the starling. They are sensitive to environmental changes and the effects of global warming. When soil pH, temperature, vegetation and water availability are not optimal, it can decrease A. vulgare distribution and body size. Soil pH is easily altered by pesticides and non-organic fertilizers which affects the amount of calcium which is needed to build up the pill bugs' chitinous exoskeleton.

In greenhouses and agriculture, Armadillidium is considered a pest, but they can also be helpful as a detritovores because they eat dead leaves, animals and leafmold. Common species that are traded include Armadillidium album, Armadillidium arcangelii and Armadillidium assimile.

Evolution

thumb|Fossilized isopod in amber from the early cretaceous.

Terrestrial isopods are believed to have evolved to live on land after first emerging from the ocean to the shore and littoral zone, Oniscidean isopods are the only Pancrustaceans, with the exception of hexapods, to have adapted to terrestrial life.

Armadillidium absoloni Strouhal, 1939
Armadillidium aegaeum Strouhal, 1929
Armadillidium aelleni Caruso & Ferrara, 1982
Armadillidium alassiense Verhoeff, 1910
Armadillidium albigauni Arcangeli, 1935
Armadillidium albomarginatum Verhoeff, 1901
Armadillidium album Dollfus, 1887
Armadillidium ameglioi Arcangeli, 1914
Armadillidium amicorum Rodriguez & Vicente, 1993
Armadillidium anconanum Verhoeff, 1928
Armadillidium angulatum Koelbel, 1891
Armadillidium apenninigenum Verhoeff, 1936
Armadillidium apenninorum Verhoeff, 1928
Armadillidium apfelbecki Dollfus, 1895
Armadillidium apuanum Taiti & Ferrara, 1995
Armadillidium arcadicum Verhoeff, 1902
Armadillidium arcangelii Strouhal, 1929
Armadillidium argentarium Verhoeff, 1931
Armadillidium argolicum Verhoeff, 1907
Armadillidium artense Strouhal, 1956
Armadillidium assimile Budde-Lund, 1885
Armadillidium atticum Strouhal, 1929
Armadillidium azerbaidzhanum Schmalfuss, 1990
Armadillidium badium Budde-Lund, 1885
Armadillidium baldense Verhoeff, 1902
Armadillidium banaticum Verhoeff, 1907
Armadillidium beieri Strouhal, 1937
Armadillidium bensei Schmalfuss, 2006
Armadillidium bicurvatum Verhoeff, 1901
Armadillidium boalense Cifuentes, Robla & Garcia, 2024
Armadillidium bosniense Strouhal, 1939
Armadillidium boukorninense Hamaied, Charfi-Cheikhrouha & Lombardo, 2018
Armadillidium brentanum Verhoeff, 1932
Armadillidium bulgaricum Frankenberger, 1941
Armadillidium calabricum Verhoeff, 1908
Armadillidium canaliferum Verhoeff, 1908
Armadillidium capreae Verhoeff, 1944
Armadillidium carniolense Verhoeff, 1901
Armadillidium carynthiacum Verhoeff, 1939
Armadillidium cavannai Arcangeli, 1960
Armadillidium cephalonicum Strouhal, 1929
Armadillidium chazaliei Dollfus, 1896
Armadillidium clausi Verhoeff, 1901
Armadillidium clavigerum Verhoeff, 1928
Armadillidium corcyraeum Verhoeff, 1901
Armadillidium cruzi Garcia, 2003
Armadillidium cythereium Strouhal, 1937
Armadillidium dalmaticum Strouhal, 1939
Armadillidium decorum Brandt, 1833
Armadillidium delattini Verhoeff, 1943
Armadillidium depressum Brandt in Brandt & Ratzeburg, 1831
Armadillidium djebalensis Vandel, 1958
Armadillidium dollfusi Verhoeff, 1902
Armadillidium elysii Verhoeff, 1936
Armadillidium epiroticum Strouhal, 1956
Armadillidium espanyoli Cruz, 1992
Armadillidium esterelanum Dollfus, 1887
Armadillidium etruriae Ferrara & Taiti, 1978
Armadillidium euxinum Verhoeff, 1929
Armadillidium fallax Brandt, 1833
Armadillidium ficalbii Arcangeli, 1911
Armadillidium flavoscutatum Strouhal, 1927
Armadillidium fossuligerum Verhoeff, 1902
Armadillidium frontemarginatum Strouhal, 1927
Armadillidium frontetriangulum Verhoeff, 1901
Armadillidium furcatum Budde-Lund, 1885
Armadillidium galiciense Schmölzer, 1955
Armadillidium germanicum Verhoeff, 1901
Armadillidium gestroi Tua, 1900
Armadillidium ghardalamensis Caruso & Hili, 1991
Armadillidium gionum Schmalfuss, 2012
Armadillidium granulatum Brandt, 1833
Armadillidium grimmi Schmalfuss, 2006
Armadillidium hauseni Schmalfuss, 1985
Armadillidium herzegowinense Verhoeff, 1907
Armadillidium hessei Verhoeff, 1930
Armadillidium hirtum Budde-Lund, 1885
Armadillidium humectum Strouhal, 1937
Armadillidium ibericum Cifuentes & Escarabajal Bernabé, 2022
Armadillidium ichkeuli Hamaied & Charfi-Cheikhrouha, 2017
Armadillidium insulanum Verhoeff, 1907
Armadillidium irmengardis Strouhal, 1956
Armadillidium janinense Verhoeff, 1902
Armadillidium jaqueti Dollfus in Jaquet, 1898
Armadillidium jerrentrupi Schmalfuss, 2008
Armadillidium jonicum Strouhal, 1927
Armadillidium justi Strouhal, 1937
Armadillidium kalamatense Verhoeff, 1907
Armadillidium kalamium Strouhal, 1956
Armadillidium klaptoczi Verhoeff, 1908
Armadillidium klugii Brandt, 1833
Armadillidium kochi Dollfus, 1887
Armadillidium kossuthi Arcangeli, 1929
Armadillidium kuehnelti Schmalfuss, 2006
Armadillidium laconicum Strouhal, 1938
Armadillidium lagrecai Vandel, 1969
Armadillidium laminigerum Verhoeff, 1907
Armadillidium lanzai Taiti & Ferrara, 1996
Armadillidium littorale Taiti & Ferrara, 1996
Armadillidium lobocurvum Verhoeff, 1902
Armadillidium lymberakisi Schmalfuss, Paragamian & Sfenthourakis, 2004
Armadillidium maccagnoi Arcangeli, 1960
Armadillidium maculatum (Risso, 1816)
Armadillidium maniatum Schmalfuss, 2006
Armadillidium mareoticum Budde-Lund, 1885
Armadillidium marinense Verhoeff, 1902
Armadillidium marinensium Verhoeff, 1928
Armadillidium marmoratum Strouhal, 1929
Armadillidium marmorivagum Verhoeff, 1934
Armadillidium messenicum Verhoeff, 1902
Armadillidium meteorense Schmalfuss, 2012
Armadillidium mohamedanicum Verhoeff, 1929
Armadillidium nahumi Garcia, 2020
Armadillidium narentanum Verhoeff, 1907
Armadillidium nasatum Budde-Lund, 1885
Armadillidium nigrum Arcangeli, 1956
Armadillidium obenbergi Frankenberger, 1941
Armadillidium odhneri Verhoeff, 1930
Armadillidium oglasae Ferrara & Taiti, 1978
Armadillidium opacum (C. Koch, 1841)
Armadillidium ormeanum Verhoeff, 1931
Armadillidium paeninsulae Ferrara & Taiti, 1978
Armadillidium pallasii Brandt, 1833
Armadillidium pallidum Verhoeff, 1907
Armadillidium pangaionum Schmalfuss, 2008
Armadillidium panningi Strouhal, 1937
Armadillidium pardoi Vandel, 1956
Armadillidium pelionense Strouhal, 1928
Armadillidium peloponnesiacum Verhoeff, 1901
Armadillidium peraccae Tua, 1900
Armadillidium petralonense Schmalfuss, 2008
Armadillidium phalacronum Schmalfuss, 2008
Armadillidium pictum Brandt, 1833
Armadillidium pieperi Schmalfuss, 2008
Armadillidium pilosellum Dollfus, 1896
Armadillidium ponalense Verhoeff, 1934
Armadillidium portofinense Verhoeff, 1908
Armadillidium pretusi Cruz, 1992
Armadillidium pseudassimile Taiti & Ferrara, 1980
Armadillidium pseudovulgare Verhoeff, 1902
Armadillidium pulchellum (Zenker in Panzer, 1799)
Armadillidium quinquepustulatum Budde-Lund, 1885
Armadillidium rhodopinum Verhoeff, 1936
Armadillidium rojanum Verhoeff, 1936
Armadillidium rosai Arcangeli, 1913
Armadillidium ruffoi Arcangeli, 1940
Armadillidium rupium Verhoeff, 1928
Armadillidium sanctum Dollfus, 1892
Armadillidium savonense Verhoeff, 1931
Armadillidium saxivagum Verhoeff, 1901
Armadillidium scaberrimum Stein, 1859
Armadillidium scabrum Dollfus, 1892
Armadillidium schmalfussi Caruso & Lombardo, 1982
Armadillidium serrai Cruz & Dalens, 1990
Armadillidium serratum Budde-Lund, 1885
Armadillidium sfenthourakisi Schmalfuss, 2008
Armadillidium siculorum Verhoeff, 1908
Armadillidium silvestrii Verhoeff, 1931
Armadillidium simile Strouhal, 1937
Armadillidium simoni Dollfus, 1887
Armadillidium sordidum Dollfus, 1887
Armadillidium stagnoense Verhoeff, 1902
Armadillidium steindachneri Strouhal, 1927
Armadillidium stolikanum Verhoeff, 1907
Armadillidium storkani Frankenberger, 1941
Armadillidium strinatii Vandel, 1961
Armadillidium stymphalicum Schmalfuss, 2006
Armadillidium sulcatum H. Milne Edwards, 1840
Armadillidium tabacarui Gruia, Iavorschi & Sarbu, 1994
Armadillidium teramense Verhoeff, 1933
Armadillidium tigris Budde-Lund, 1885
Armadillidium tirolense Verhoeff, 1901
Armadillidium torchiai Taiti & Ferrara, 1996
Armadillidium traiani Demianowicz, 1932
Armadillidium tripolitzense Verhoeff, 1902
Armadillidium tuberculatum Schmalfuss, 2008
Armadillidium tunetanum Verhoeff, 1907
Armadillidium tunisiense Hamaïed & Charfi-Cheikhrouha, 2007
Armadillidium tyrrhenum Taiti & Ferrara, 1980
Armadillidium vallombrosae Verhoeff, 1907
Armadillidium valonae Arcangeli, 1952
Armadillidium verhoeffi Rogenhofer, 1915
Armadillidium versicolor Stein, 1859
Armadillidium versluysi Strouhal, 1937
Armadillidium virgo Caruso & Bouchon, 2011
Armadillidium vulgare (Latreille, 1804)
Armadillidium werneri Strouhal, 1927
Armadillidium xerovunense Strouhal, 1956
Armadillidium zangherii Arcangeli, 1924
Armadillidium zenckeri Brandt, 1833