Millipedes are medium to large-sized invertebrates, arthropods of the class Diplopoda. They are also some of the oldest animals known to live on land, their first appearance in historical sources being traced as far back as the Silurian period. There are approximately 12,000 recognized species of millipedes (in reality, numbers are estimated to be around 15,000 – 20,000 and can go as high as 80,000), classified into 2 subclasses, 16 orders, and 140 families. Diplopoda is the largest class of myriapods (an arthropod group that includes centipedes and numerous other multi-legged creatures). The subclass Penicillata contains a single order, Polyxenida (also known as the bristle millipedes), and the subclass Chilognatha contains all the others. Although similar, millipedes can be easily distinguished from the distantly related centipedes (belonging to the class Chilopoda), which move more rapidly, are carnivorous, and only have one pair of legs on each body segment.
Description and distribution of millipedes
They come in a variety of shapes and sizes, with most of them having very long bodies of a cylindrical, yet somewhat flattened form. They can range from as little as 2mm to as large as 35cm in length and can have as few as eleven and as many as hundreds of segments. The current largest known species, the giant African millipede (Archispirostreptus gigas) reaches a maximum length of 27 – 38cm. When it comes to colouring, millipedes are generally black or brown, but there are certain species that are very brightly coloured.
As far as body composition is concerned, all millipedes of the subclass Chilognatha have hardened exoskeletons, while those belonging to the subclass Penicillata have soft and uncalcified skeletons, covered by prominent bristles, hence the reason why they are called bristle millipedes.
A millipede’s body can be separated into two sections: the head and the actual body. The head is rounded on the upper side and flattened below and typically contains a pair of large mandibles and a pair of antennae with seven or eight segments and a group of sensory cones at the tip. In many species, a pair of sensory organs called the Tömösváry organs is present; they are shaped as small oval rings and are positioned posterior and lateral to the base of the antennae. Although their true function is unknown, scientists suspect that they might be used to measure humidity or light levels in the environment. A millipede’s head also contains two eyes, which consist of simple flat-lensed ocelli arranged in a patch on each side of the head, being also called ocular fields or ocellaria. Many species, however, such as those of the order Polydesmida, Causeyella or Trichopetalum, have no eyes and are thus completely blind.
As previously mentioned, the body of a millipede is composed out of numerous segments, each having an exoskeleton consisting of five chitinous plates: one above (the tergite), one at each side (the pleurites), and one on the underside, where the legs attach (the sternite). The first segment behind the head is normally referred to as a collum, being the substitute of a neck and bearing no legs. The following three segments are considered to be the thorax and only have a pair of legs each, reason for which they are known as haplosegments (from the Greek word “haplo”, which means “single”). Then, the remaining segments are called diplosegments, containing two pairs of jointed legs each, and are thought to be the result of two segments fusing together. Finally, the last segment is called the telson and presents a legless pre-anal ring and a pair of anal valves.
The legs that millipedes are so well known for attach on the underside of the body and are composed out of seven segments. Often enough, they are longer in males than in females; in some species, males may also have either a reduced first pair of legs or an enlarged one. When millipedes walk, the legs work together, moving in a wave-like motion. When it comes to internal organs, a millipede’s constitution is fairly simple. They breathe through two pairs of spiracles located on the underside of each segment, near the base of the legs; these connect to an internal system of tracheae. Interestingly enough, the heart runs the length of the entire body; an aorta stretches into the head. The digestive tract is made out of a simple tube and two pairs of salivary glands that help digest the food, while the excretory organs are made out of two pairs of malpighian tubules.
Millipedes are cosmopolitan on all continents except Antarctica, able to survive in habitats set as far north as the Arctic Circle and as far south as the Santa Cruz Province in Argentina, but being predominantly more common in the southern hemisphere. They prefer high humidity and are nocturnal, living on forest floors, under rocks, in leaf covers, dead wood or even soil, being very abundant in moist forests (with up to 1,000 individuals per square meter) but also in coniferous forests, caves, deserts, and alpine ecosystems. Some species can live underwater up to 11 months and others can even survive in somewhat salty habitats, such as near the seashore. Around homes, millipedes live in flowerbeds and gardens, or under structures such as storage sheds or dog houses.
Breeding and dietary information
Because of the diversity of the species themselves, millipedes show a variety of mating structures and styles, ranging from direct to indirect. Simply put, though, reproduction in all species belonging to the subclass Chilognatha is done via modified male legs (called gonopods), which have the function of transferring packets of sperm to females. The exact position of these gonopods depends on the species, and so do the shape and size, these three characteristics being the main, easiest way in which millipedes can be categorized. In some species, gonopodes are maintained retracted inside the body, while in others, they are kept parallel to it. The male has one or two penes, which they use to deposit the sperm packets onto the gonopodes, and the females have small sacs (called cyphopods or vulvae) that are used to store the sperm following copulation. In all millipedes, the genital openings of both sexes are located on the underside of the third body segment and, with the exception of bristle millipedes, copulation always occurs with the individuals facing one another.
In the case of bristle millipedes, mating is indirect; males create webs out of secretions of special glands and deposit spermatophores onto them, leaving it to the females to pick them up. A small variety of millipedes are parthenogenetic, which means they reproduce asexually, without fertilization, and thus have few or no males at all.
Depending on the species, female millipedes may lay up to 300 eggs at once, usually fertilizing them at the same time. Some species deposit the eggs on moist soil or near organic waste, others take the time to build ‘nests’ made out of dried feces, and some may even choose to protect the eggs using silk cocoons. Most females immediately abandon the eggs, but there are some species (such as those belonging to the orders Platydesmida and Stemmiulida) that provide parental care for both the eggs and the resulting younglings.
It generally takes a few weeks for the eggs to hatch. Young millipedes only have three pairs of legs, and add more as they grow and molt. Those protected by silk cocoons moult within them and may wait for proper environmental conditions before coming out. The gonopodes develop gradually, through these successive moults, from walking legs; the moment the transition is complete is the moment the millipede has reached reproductive maturity, which usually happens in the second year of life, but can take up to five years in some species. Depending on the species, millipedes may live between one and ten years.
Most millipedes are detritivores, which means they obtain nutrients by consuming detritus: decomposing plant and animal parts, as well as feces. Some millipedes eat fungi and plant fluids, and others are herbivorous (feeding on living plants), which often leads to them becoming pests to certain crops. A small minority of millipedes is predatory, omnivorous and even carnivorous; these feed on insects, earthworms, snails, or centipedes. Some even have a poisonous bite that they can use to kill the prey before eating it.
Millipedes are preyed upon by a variety of animals, though, such as reptiles, birds, mammals, and even insects. The function of defence is usually accomplished by the instinct to curl into a tight coil (which protects their sensitive legs) and by a variety of chemicals secreted from pores along the body (such as alkaloids, benzoquinones, phenols, terpenoids, hydrogen cyanide, etc.); some of these substances may simply have a disgusting smell which deters predators from approaching them or may be caustic and capable of burning the exoskeleton of various insects, such as ants, and the skin and eyes of larger enemies. The bristle millipedes of the Pencillata subclass have a slightly different defence system, involving the detachable bristles that cover the length of their entire bodies. Some predators, however, have adapted to all of these survival mechanisms; meerkats and other mammalian predators, for example, have learned to roll millipedes on the ground or rub them off on their bodies in order to remove secretions before eating them, while poison dart frogs are suspects of actually having the ability to incorporate the toxic secretions into their system and use them, later on, for their own defence.
Millipedes as pests
Millipedes are generally harmless to humans, and have little impact on the economic or social well-being of a community, but they can be considered local household and garden pests. This is true especially when it comes to greenhouses, where they are known to cause severe damage; the spotted snake millipede (Blaniulus guttulatus), for example, is known as a pest of root crops such as sugar beets. Some species, such as Xenobolus carnifex and Ommatoiulus moreleti invade roofs and homes, while others present periodical swarming behaviors, which can result in home invasions, crop damage, train delays and even train crashes (when the tracks become slippery with the crushed remains of thousands of millipedes).
Millipedes do not bite and do not sting, and when it comes to most species, secretions only cause minor, temporary discoloration of the skin. However, some tropical species may have secretions that cause pain, itching, eczema, edema, blisters, or lead to sensitive, cracked skin. If eyes are exposed to these secretions, irritation is sure to follow, with the potential for more severe effects such as conjunctivitis and keratitis.
On the other hand, due to their preferred diet, millipedes can be considered useful, especially when it comes to the breakdown of leaf covers, seeing as they consume the litter, fragment it in their gut, and excrete pellets of leaf fragments, algae, fungi, and bacteria – all of which are then much more easily decomposed by microorganisms. Collectively, a population of millipedes may actually consume nearly all the leaf litter in one region.
Millipedes have been found to migrate during fall or after a drastic change in their environment, such as heavy rain that may have flooded their habitat, often moving towards or into homes in search for warmth and calmer weather. Once there, they may gather on porches or patios, crawling on the foundation of buildings and very easily finding ways inside, through basement doors and windows, vents, open garage doors, etc., only to then hide under furniture or storage boxes. To prevent this from happening, see what preventive measures you can take, or how you can exclude millipedes from your property, in the eventuality of a serious infestation. You can find more details on ‘How to get rid of Millipedes’ and how to ‘Prevent infestation with Millipedes’ in our related articles.