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 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

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:

Ability to roll into a ball
Convex oblong body shape
Multiple black ocelli
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 primarily feed on decaying organic matter like leaves, decomposed wood fibers, and less commonly on other organic material like lichen. 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. By eating their own fecal matter, as well as other organisms’, it serves as an opportunity to digest items a second time and break them down further. 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.
Copper is an important nutrient for Armadillidium, as it is crucial in the transportation of oxygen through their body. Like humans have hemoglobin to carry oxygen, Armadillidium have hemocyanin, which uses copper to bind and transport oxygen. Because this mineral is significant to the movement of oxygen, pill bugs that are deficient in copper will target foods that have higher amounts of the molecule. However, those with sufficient copper levels will choose foods with lower amounts of the metal.

Distribution and habitat

Armadillidium are nocturnal, being most active during the night. They are usually found in moist areas such as decomposing leaf matter and soil. They can also be found in shaded nooks and crannies, such as those created by rocks. They typically live in places that are temperate in climate and have moist 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 to excrete their waste as a way to save water and reduce dangerous substances, however, Armadillidium are able to excrete their nitrogenous waste as ammonia gas – a more toxic substance than other forms of waste. They don’t experience any negative effects from this toxic form and are able to excrete their waste without losing any water. Although they do lose lots of hydrogen, which could impact their water balance so that there isn’t any large difference in terms of saving water via ammonia gas.
Although they have high amounts of copper and ammonia, both of which are extremely toxic, in their systems, they don't experience any adverse effects. This has led researchers to believe that the toxicity doesn’t interfere with any important body functions, they have developed resistances, or that they have developed ways to store these substances without harming themselves.
Armadillidium can also produce pungent fluids, which serve as deterrents for potential predators. Located on the dorsal surface along their body segments are glands that produce the repugnant secretions.
Armadillidium breathe through pseudotracheae: white, bean-shaped structures, located on the first two segments of the isopod's abdomen which help them to respire in air.
With an extremely straight tube for a gut and only two pairs of digestive glands, Armadillidium have a very simple digestive system that isn’t the best equipped for absorbing nutrients.
Armadillidium prefer to have something touching them on all sides, which leads to them aggregating with each other as well as huddling under rocks and in tight spaces.
An important part of Armadillidum defence is the ability to roll into a ball, called conglobation, to protect their soft, exposed underside. 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.
Some Armadillidium are capable of producing pheromones that are released in their feces, attracting others of the same species. These pheromones, also found in sections of the gut, are for species aggregation, which makes it easier for individuals to find mates. Along with producing their own aggregation pheromones, some Armadillidium respond to odors from other species as well as their own when searching for shelter.

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. Typically, Armadillidium don’t move very quick, but at low and moderate temperatures they will move considerably faster when presented with light. Certain Armadillidium species are also known to increase their activity when subjected to hot environments, instead of limiting their action as many animals do.
Because of their exoskeletons, Armadillidium are subject to high rates of transpiration and are therefore especially susceptible to drying out. Their pseudotracheae are also a major source of water loss, around 42% of their total, as the organ can’t be closed off. Along with protection from predators, volvation also decreases the amount of water lost through their exoskeleton via transpiration.
Armadillidium are organisms that can aggregate in groups. They have been known to aggregate around the same species more than compared to clay models of them. This response – controlled by touch and potentially influenced by pheromones – can be useful for decreasing water lost by transpiration.
Some Armadillidium have been found to have a circadian rhythm that determines their nocturnality but can also be influenced by moisture levels and light.