Millipede facts for kids
Quick facts for kids Millipedes |
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Diplopoda
de Blainville in Gervais, 1844
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Helminthomorpha |
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Millipedes are a group of arthropods that are characterised by having two pairs of jointed legs on most body segments; they are known scientifically as the class Diplopoda. Most millipedes have very elongated round or flattened bodies with more than 20 segments, while pill millipedes are shorter and can roll into a ball. Although the name "millipede" comes from the Latin for "thousand feet", no known species has 1,000; the record of 750 legs belongs to Illacme plenipes.
There are approximately 12,000 named species classified into 16 orders and around 140 families, making Diplopoda the largest class of myriapods, an arthropod group which also includes centipedes and other multi-legged creatures. Most millipedes are slow-moving detritivores, eating decaying leaves and other dead plant matter. Some eat fungi or suck plant fluids, and a small minority are predatory.
Millipedes are generally harmless to humans, although some can become household or garden pests. Millipedes can be unwanted especially in greenhouses where they can cause severe damage to emergent seedlings. Most millipedes defend themselves with a variety of chemicals released from pores along the body, although the tiny bristle millipedes are covered with tufts of detachable bristles.
Approximately 12,000 millipede species have been described. Estimates of the true number of species on earth range from 15,000 to as high as 80,000.
Contents
Evolution
Millipedes are among the first animals to have colonised land during the Silurian period. Early forms probably ate mosses and primitive vascular plants. There are two major groups of millipedes whose members are all extinct: the Archipolypoda ("ancient, many-legged ones") which contain the oldest known terrestrial animals, and Arthropleuridea, which contain the largest known land invertebrates.
The earliest known land creature, Pneumodesmus newmani, was a 1 cm (0.4 in) long archipolypodan that lived 428 million years ago in the upper Silurian, and has clear evidence of spiracles (breathing holes) making clear evidence of its air-breathing habits. Arthropleura became the largest known land-dwelling invertebrate on record, reaching lengths of at least 2 m (6 ft 7 in). Millipedes also exhibit the earliest evidence of chemical defence.
Distinction from centipedes
The differences between millipedes and centipedes is a common question from the general public. Both groups of myriapods share similarities, such as long, multi-segmented bodies, many legs, a single pair of antennae, and the presence of postanntennal organs, but have many differences and distinct evolutionary histories. The head alone demonstrates the differences; millipedes have short, elbowed antennae for probing, a pair of powerful mandibles and a single pair of maxillae (paired structures present on the head as mouth-parts) fused into a lip; centipedes have long, threadlike antennae, a pair of small mandibles, two pairs of maxillae and a pair of large poison claws.
Distinction from pill bugs
Oniscomorpha is a superorder of millipedes that are commonly called pill millipedes due to their resemblance to certain woodlice (Oniscidea), also called pill bugs or "roly-polies". However, millipedes and woodlice are not closely related; rather, this is a case of developing similar visual traits in the same environment, during evolution.
Characteristics
Millipedes come in a variety of body shapes and sizes, ranging from 2 mm (1⁄16 in) to around 35 cm (14 in) in length, and can have as few as eleven to over three hundred segments. They are generally black or brown in colour, although there are a few brightly coloured species, and some have aposematic colouring to warn that they are toxic. Species of Motyxia produce cyanide as a chemical defence and are bioluminescent.
Body styles vary greatly between major millipede groups. In the basal subclass Penicillata, consisting of the tiny bristle millipedes, the exoskeleton is soft and uncalcified, and is covered in prominent setae or bristles. All other millipedes, belonging to the subclass Chilognatha, have a hardened exoskeleton. The chilognaths are in turn divided into two infraclasses: the Pentazonia, containing relatively short-bodied groups such as pill millipedes, and the Helminthomorpha ("worm-like" millipedes), which contains the vast majority of species, with long, many-segmented bodies.
They have also lost the gene that codes for the JHAMTl enzyme, which is responsible for catalysing the last step of the production of a juvenile hormone that regulates the development and reproduction in other arthropods like crustaceans, centipedes and insects.
Head
The head of a millipede is typically rounded above and flattened below and bears a pair of large mandibles in front of a plate-like structure called a gnathochilarium ("jaw lip"). The head contains a single pair of antennae with seven or eight segments and a group of sensory cones at the tip. Many orders also possess a pair of sensory organs known as the Tömösváry organs, shaped as small oval rings posterior and lateral to the base of the antennae. Their function is unknown, but they also occur in some centipedes, and are possibly used to measure humidity or light levels in the surrounding environment.
Millipede eyes consist of several simple flat-lensed ocelli arranged in a group or patch on each side of the head. These patches are also called ocular fields or ocellaria. Many species of millipedes, including the entire orders Polydesmida, Siphoniulida, Glomeridesmida, Siphonophorida and Platydesmida, and cave-dwelling millipedes such as Causeyella and Trichopetalum, had ancestors that could see but have subsequently lost their eyes and are blind.
Body
Millipede bodies may be flattened or cylindrical, and are composed of numerous metameric segments, each with an exoskeleton consisting of four chitinous plates: a single plate above (the tergite), one at each side (pleurites), and a plate on the underside (sternite) where the legs attach. In many millipedes, such as Merocheta and Juliformia, these plates are fused to varying degrees, sometimes forming a single cylindrical ring. The plates are typically hard, impregnated with calcium salts. Because they can't close their permanently open spiracles and most species lack a waxy cuticle, millipedes are susceptible to water loss and with a few exceptions must spend most of their time in moist or humid environments.
The first segment behind the head is legless and known as a collum (from the Latin for neck or collar). The second, third, and fourth body segments bear a single pair of legs each and are known as "haplosegments" (the three haplosegments are sometimes referred to as a "thorax"). The remaining segments, from the fifth to the posterior, are properly known as diplosegments or double segments, formed by the fusion of two embryonic segments. Each diplosegment bears two pairs of legs, rather than just one as in centipedes. In some millipedes, the last few segments may be legless. The terms "segment" or "body ring" are often used interchangeably to refer to both haplo- and diplosegments. The final segment is known as the telson and consists of a legless preanal ring, a pair of anal valves (closeable plates around the anus), and a small scale below the anus.
Millipedes in several orders have keel-like extensions of the body-wall known as paranota, which can vary widely in shape, size, and texture; modifications include lobes, papillae, ridges, crests, spines and notches. Paranota may allow millipedes to wedge more securely into crevices, protect the legs, or make the millipede more difficult for predators to swallow.
The legs are composed of seven segments, and attach on the underside of the body. The legs of an individual are generally rather similar to each other, although often longer in males than females, and males of some species may have a reduced or enlarged first pair of legs. The most conspicuous leg modifications are involved in reproduction, discussed below. Despite the common name, no millipede was known to have 1,000 legs until 2021: common species have between 34 and 400 legs, and the record is held by Eumillipes persephone, with individuals possessing up to 1,306 legs – more than any other creature on Earth.
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Internal organs
Millipedes breathe through two pairs of spiracles located ventrally on each segment near the base of the legs. 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 digestive tract is a simple tube with two pairs of salivary glands to help digest the food.
Reproduction and growth
Millipedes show a diversity of mating styles and structures. In the basal order Polyxenida (bristle millipedes), mating is indirect: males deposit spermatophores onto webs they secrete with special glands, and the spermatophores are subsequently picked up by females. In all other millipede groups, males possess one or two pairs of modified legs called gonopods which are used to transfer sperm to the female.
Gonopods occur in a diversity of shapes and sizes, and in the range from closely resembling walking legs to complex structures quite unlike legs at all. In some groups, the gonopods are kept retracted within the body; in others they project forward parallel to the body. Gonopod morphology is the predominant means of determining species among millipedes: the structures may differ greatly between closely related species but very little within a species. The gonopods develop gradually from walking legs through successive moults until reproductive maturity.
Females lay from ten to three hundred eggs at a time, depending on species. Many species deposit the eggs on moist soil or organic detritus, but some construct nests lined with dried faeces, and may protect the eggs within silk cocoons. In most species, the female abandons the eggs after they are laid, but some species in the orders Platydesmida and Stemmiulida provide parental care for eggs and young.
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, a mode of development known as anamorphosis. Some species moult within specially prepared chambers of soil or silk, and may also shelter in these during wet weather, and most species eat the discarded exoskeleton after moulting. The adult stage, when individuals become reproductively mature, is generally reached in the final moult stage, which varies between species and orders, although some species continue to moult after adulthood. Furthermore, some species alternate between reproductive and non-reproductive stages after maturity, a phenomenon known as periodomorphosis, in which the reproductive structures regress during non-reproductive stages. Millipedes may live from one to ten years, depending on species.
Habitat and distribution
Millipedes occur on all continents except Antarctica. Typically forest floor dwellers, they live in leaf litter, dead wood, or soil, with a preference for humid conditions. In temperate zones, millipedes are most abundant in moist deciduous (tree's that shed leaves annually) forests, and may reach densities of over 1,000 individuals per square metre. Other habitats include coniferous forests, deserts, caves, and alpine ecosystems. Some species can survive freshwater floods and live submerged underwater for up to 11 months. A few species occur near the seashore and can survive in somewhat salty conditions.
Interactions with humans
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Millipedes generally have little impact on human economic or social well-being, especially in comparison with insects, although locally they can be a nuisance or agricultural pest. Millipedes do not bite, and their defensive secretions are mostly harmless to humans — usually causing only minor discolouration on the skin — but the secretions of some tropical species may cause pain, itching, local erythema, edema, blisters, eczema, and occasionally cracked skin.
Eye exposures to these secretions causes general irritation and potentially more severe effects. This is called millipede burn. First aid consists of flushing the area thoroughly with water; further treatment is aimed at relieving the local effects.
Some millipedes are considered household pests which can infest thatched roofs in India, and which periodically invade homes in Australia. Other species exhibit periodical swarming behaviour, which can result in home invasion and crop damage.
Some millipedes can cause significant damage to crops: the spotted snake millipede is a noted pest of sugar beets and other root crops. Some of the larger millipedes are popular as pets.
Millipedes have also inspired and played roles in scientific research. In 1963, a walking vehicle with 36 legs was designed, said to have been inspired by a study of millipede locomotion. Experimental robots have had the same inspiration, in particular when heavy loads are needed to be carried in tight areas involving turns and curves.
Images for kids
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Approximate relative diversity of extant millipede orders, ranging from ca. 3,500 species of Polydesmida to 2 species of Siphoniulida
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Octoglena sierra (Colobognatha, Polyzoniida)
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Anadenobolus monilicornis (Juliformia, Spirobolida)
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Harpaphe haydeniana (Polydesmida)
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Pauropods are thought to be the closest relative of millipedes.
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A Sceliages beetle transporting a millipede carcass
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Ammodesmus nimba from Guinea, West Africa, curled in a defensive coil
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Flat millipede found in the Mount Cameroon Forest
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See also
In Spanish: Diplopoda para niños
