Woodlice (singular woodlouse), though often mistaken for insects, are small crustaceans belonging to the suborder Oniscidea (under the order Isopoda). They rank as the third most diverse group of terrestrial arthropods, following insects (Hexapoda) and spiders (Arachnida). These invertebrates are easily recognized by their flattened, segmented bodies with fourteen legs located in the thoracic region. Additionally, they possess specialized respiratory structures called pleopodal lungs, located in their abdominal legs, which enable them to breathe on land.
Originally aquatic, woodlice are believed to have emerged during the Carboniferous Period. Over time, they adapted to life on land, making them one of the few crustacean lineages to achieve a primarily terrestrial lifestyle.
Woodlice are nocturnal and feed on decaying organic matter, playing a crucial role in replenishing soil nutrients. Harmless by nature, these animals are often kept as household pets by animal enthusiasts and collectors.
On average, woodlice are around 10 mm (0.39 in) long and about 5mm (0.19 in) wide. One of the smallest species in Britain, the least pygmy woodlouse (Trichoniscus pygmaeus) reaches a maximum length of about 2.5 mm (0.09 in). In contrast, one of the largest species, the common sea slater (Ligia oceanica), may reach up to 30 mm (1.2 in).
These animals have heavily armored, dorsoventrally flattened bodies divided into two broad regions: the thorax (pereon) and the abdomen (pleon). They exist in a variety of colors, ranging from dull gray and brown to yellow and pink.
This region is divided into seven segments, each equipped with a pair of appendages or legs. The first segment is fused with the head, forming the cephalothorax. This structure supports the compound eyes and a pair of long, sensory antennae that end in delicate, flagellum-like structures, enhancing their ability to detect environmental cues.
The abdominal region is composed of five segments containing paddle-shaped appendages called pleopods. On the underside of the pleopods are trachea-like structures called pleopodal lungs, which aid in respiration on the land.
These animals derive their name from their preferred habitat, wood, and their resemblance to lice, which are parasitic insects. They are also addressed by various common names in different parts of the world, such as armadillo bugs, cheeselogs, cheesy bugs, roly-poly, potato bugs, and pill bugs (usually applicable to the genus Armadillidium).
Around 4,000 species of woodlice are currently grouped under 5 undisputed sections: Diplocheta, Tylida, Microcheta, Synocheta, and Crinocheta. There are a total of 34 families under these 5 sections. Only one family, Ligiidae, falls under the section Diplocheta, while all other families are currently believed to belong to the infraorder Holoverticata.
In an alternate taxonomy, the family Tylidae is placed in an infraorder called Tylomorpha. However, it is generally accepted that woodlice have five major lineages, as shown above.
The oldest fossils of these animals, dating back to the Mid-Cretaceous Period (around 100 million years ago), have been unearthed from amber deposits in Spain, France, and Myanmar. These fossils include specimens of the genera Ligia (from the Charentese amber of France) and Myanmariscus (from the Burmese amber of Myanmar), as well as species like Heraclitus helenae revealed from Spanish amber.
Although the highest fossil diversity is found in the Mid-Cretaceous, woodlice are believed to have originated before the breakup of the supercontinent Pangaea during the Carboniferous Period. Around this time, these animals, which initially had an aquatic lifestyle, gradually adapted to life on the land.
Woodlice are found in almost every terrestrial biome, ranging from subterranean lakes and oceanic littoral zones to deserts and elevations of about 15,500 ft (4,725 m) above sea level. Their highest species diversity is particularly observed in the Mediterranean Basin, the Afrotropics, Indomalaya, Australasia, and the Neotropics.
Although these animals usually prefer living in dark and damp places, like under rocks and tree logs, Hemilepistus reaumuri is found to live in deserts and semi-deserts in North Africa and the Middle East. This species is believed to occupy ‘the driest habitat conquered by any species of crustacean.’
Some evolutionarily ancient species, like the marine-intertidal sea slater (Ligia oceanica), are amphibious, switching between land and water, while others, like Typhlotricholigoides aquaticus of Mexico and Cantabroniscus primitivus of Spain, are assumed to be completely aquatic.
Of all woodlice species found worldwide, 35 species are endemic to the British Isles, an archipelago in the North Atlantic. Five woodlice varieties, popularly known as the ‘famous five species,’ are especially common in this region, including the common shiny woodlouse (Oniscus asellus), the common rough woodlouse (Porcellio scaber), the common striped woodlouse (Philoscia muscorum), the common pygmy woodlouse (Trichoniscus pusillus), and the common pill bug (Armadillidium vulgare).
They are primarily detritivorous and feed on dead and decaying plants, as well as fungi and even their own feces (coprophagy).
Although woodlice have an average lifespan of 2 to 3 years, they survive up to 5 years under ideal conditions.
At night, male woodlice climb onto receptive females and lick their heads prior to mating. The male drums on the back of the female with his legs (for about five minutes) and positions himself diagonally on her back to transfer sperm to the genital opening on the left side of her body. He then switches to the other side and deposits sperm in the genital opening on her right side. The transfer of sperm takes around ten minutes to complete.
The female typically stores around 25 to 90 (sometimes up to 200) fertilized eggs in a small pouch-like marsupium covering the underside of the thorax. With time, these eggs hatch into offspring that resemble tiny, white, curled-up balls, lacking the last pair of legs. They slowly grow into young (mancae) and are ready to leave the pouch in about 40 to 50 days.
When the mancae are released, they have six pairs of legs. However, after some time, they go through the first molt, which produces their seventh pair of legs, giving rise to juveniles.
Very rarely, a few species, such as the common pygmy woodlouse, reproduce asexually through parthenogenesis, a process in which females give birth to genetically identical offspring without fertilization by the males.
These arthropods are preyed upon by insectivorous invertebrates, like spiders of the genus Dysdera, such as the woodlouse spider (Dysdera crocata). Land planarians, like Luteostriata abundans, also feed on these animals.
Apart from humans, they are also threatened by other predators, like centipedes, toads, and shrews.
According to the IUCN Red List, the Celtic woodlouse (Metatrichoniscoides celticus) is Near Threatened (NT), while the spike yellow woodlouse (Pseudolaureola atlantica) is Critically Endangered (CR).
The species Acaeroplastes melanurus was considered extinct in the British Isles until it was rediscovered in County Dublin, Ireland, in the year 2022.
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