millipede

Wikipedia:

Millipedes are arthropods that have two pairs of legs per segment (except for the first segment behind the head which does not have any appendages at all, and the next few which only have one pair of legs). Each segment that has two pairs of legs is a result of two single segments fused together as one. Most millipedes have very elongated cylindrical bodies, although some are flattened dorso-ventrally, while pill millipedes are shorter and can roll into a ball, like a pillbug.

The name "millipede" is a compound word formed from the Latin roots mille ("thousand") and pes ("foot"). Despite their name, no known millipede has 1,000 legs, although the rare species Illacme plenipes has up to 750. Common species have between 36 and 400 legs. The class contains around 10,000 species in 13 orders and 115 families. The giant African millipede (Archispirostreptus gigas), is the largest species of millipede.

Millipedes are detritivores and slow moving. Most millipedes eat decaying leaves and other dead plant matter, moisturising the food with secretions and then scraping it in with their jaws. However, they can also be minor garden pests, especially in greenhouses where they can cause severe damage to emergent seedlings. Signs of millipede damage include the stripping of the outer layers of a young plant stem and irregular damage to leaves and plant apices, the very top of a plant.

Millipedes can be easily distinguished from the somewhat similar and related centipedes (Class Chilopoda), which move rapidly, and have a single pair of legs for each body segment.

Evolution

This class of arthropod is thought to be among the first animals to have colonised land during the Silurian geologic period. These early forms probably ate mosses and primitive vascular plants. The oldest known land creature, Pneumodesmus newmani, was a 1 centimetre (0.39 in) long millipede, and lived 428 million years ago. In the Upper Carboniferous (340 to 280 million years ago), Arthropleura became the largest known land invertebrate of all time, reaching lengths of up to 2.6 metres (8 ft 6 in).

Characteristics

Millipedes range from 2 to 280 millimetres (0.079 to 11 in) in length, and can have as few as eleven, to over a hundred segments. They are generally black or brown in colour, although there are a few brightly coloured species.

The millipede's most obvious feature is its large number of legs. Having very many short legs makes millipedes rather slow, but they are powerful burrowers. With their legs and body length moving in a wavelike pattern, they easily force their way underground head first. They also seem to have some engineering ability, reinforcing the tunnel by rearranging the particles around it. Their bodies have segmented sections which makes them move in a wave-like form.

The head of a millipede is typically rounded above and flattened below and bears large mandibles. The body is flattened or cylindrical, with a single chitinous plate above, one at each side, and two or three on the underside. In many millipedes, these plates are fused to varying degrees, sometimes forming a single cylindrical ring. The plates are typically hard, being impregnated with calcium salts. Because they lack a waxy cuticle millipedes are susceptible to water loss, and must spend most of their time in moist or humid environments.

Unlike centipedes and other similar animals, each segment bears two pairs of legs, rather than just one. This is because each is actually formed by the fusion of two embryonic segments, and is therefore properly referred to as a "diplosegment", or double segment. The first few segments behind the head are not fused in this fashion, and the first segment is legless, called a collum segment while the second to fourth have one pair each. In some millipedes, the last few segments may also be legless. The final segment bears a telson.

Millipedes breathe through two pairs of spiracles on each diplosegment. Each opens into an internal pouch, and connects to a system of tracheae. The heart runs the entire length of the body, with an aorta stretching into the head. The excretory organs are two pairs of malpighian tubules, located near the mid-part of the gut.

The head contains a pair of sensory organs known as the Tömösváry organs. These are found just posterior and lateral to the antennae, and are shaped as small and oval rings at the base of the antennae. They are probably used to measure the humidity in the surroundings, and they may have some chemoreceptory abilities too. Millipede eyes consist of a number of simple flat lensed ocelli arranged in a group on the front/side of the head. Many species of millipedes, including cave-dwelling millipedes such as Causeyella, have secondarily lost their eyes.

According to Guinness World Records the African giant black millipede Archispirostreptus gigas can grow to 38.6 centimetres (15.2 in).

Diet

Most millipedes are herbivorous, and feed on decomposing vegetation or organic matter mixed with soil. A few species are omnivorous or carnivorous, and may prey on small arthropods, such as insects and centipedes, or on earthworms. Some species have piercing mouth parts that allow them to feed on plant juices.

The digestive tract is a simple tube with two pairs of salivary glands to help digest the food. Many millipedes moisten their food with saliva before eating it.

Reproduction

Male millipedes can be differentiated from female millipedes by the presence of one or two pairs of legs modified into gonopods. These modified legs, which are usually on the seventh segment, are used to transfer sperm packets to the female during copulation. A few species are parthenogenetic, having few, if any, males.

The genital openings are located on the third segment, and are accompanied in the male by one or two penises, which deposit the sperm packets onto the gonopods. In the female, the genital pores open into a small chamber, or vulva, which is covered by a small hood-like cover, and is used to store the sperm after copulation.

Females lay between ten and three hundred eggs at a time, depending on species, fertilising them with the stored sperm as they do so. Many species simply deposit the eggs on moist soil or organic detritus, but some construct nests lined with dried faeces.

The young hatch after a few weeks, and typically have only three pairs of legs, followed by up to four legless segments. As they grow, they continually moult, adding further segments and legs as they do so. Some species moult within specially prepared chambers, which they may also use to wait out dry weather, and most species eat the shed exoskeleton after moulting. Millipedes live from one to ten years, depending on species.

Defense mechanisms

Due to their lack of speed and their inability to bite or sting, millipedes' primary defense mechanism is to curl into a tight coil — protecting their delicate legs inside an armored body exterior. Many species also emit poisonous liquid secretions or hydrogen cyanide gas through microscopic pores called odoriferous glands along the sides of their bodies as a secondary defense. Some of these substances are caustic and can burn the exoskeleton of ants and other insect predators, and the skin and eyes of larger predators. Animals such as Capuchin monkeys have been observed intentionally irritating millipedes in order to rub the chemicals on themselves to repel mosquitoes. At least one species, Polyxenus fasciculatus, employs detachable bristles to entangle ants.

As far as humans are concerned, this chemical brew is fairly harmless, usually causing only minor effects on the skin, the main effect being discoloration, but other effects may also include pain, itching, local erythema, edema, blisters, eczema, and occasionally cracked skin. Eye exposures to these secretions causes general eye irritation and potentially more severe effects such as conjunctivitis and keratitis. First aid consists of flushing the area thoroughly with water; further treatment is aimed at relieving the local effects.

Classification

The living members of the Diplopoda are divided into fifteen orders in three subclasses. The basal subclass Penicillata contains 160 species whose exoskeleton is not calcified, and which are covered in setae or bristles. All other millipedes belong to the Chilognatha in the strict sense.

The subclass Pentazonia contains the short-bodied pill millipedes, which are capable of rolling themselves into a ball ("volvation"). The subclass Helminthomorpha contains the great majority of the species.

The subgroups of millipedes in phylogenetic sequence, from most basal to most advanced, are: