Millipedes are a famous group of arthropods characterized by their segmented body and two pairs of jointed legs on almost every segment; they are scientifically categorized into the class Diplopoda, a name derived from this unique feature.
In millipedes, each double-legged segment results from two single segments joined together. Most millipedes come in flattened or elongated cylindrical bodies with over 20 segments. At the same time, there are the pill millipedes, which are shorter and are capable of rolling into a ball.
Although the name of this animal, “millipede,” is derived from the Latin word for “thousand feet,” no known species has 1,000. The only record of 750 legs is for the Illacme plenipes.
There are about 12,000 named millipede species, and they are classified into 16 orders and about 140 families. That makes Diplopoda the most significant class of myriapods, a group of arthropods that includes centipedes and several other multi-legged creatures.
Scientific Classification
- Kingdom: Animalia
- Phylum: Arthropoda
- Subphylum: Myriapoda
- Class: Diplopoda
Subclasses
- Penicillata
- Chilognatha
- †Arthropleuridea
Most species of millipedes are very slow-moving detritivores. They mostly feed on decaying leaves and several other dead plant matter.
Some of them eat fungi or suck up the juice from plants, and very few of them are predatory. Even though a lot of people fear millipedes, they are almost harmless to humans, although some of them can become pests in homes or gardens.
Millipedes are considered unwanted, mostly in greenhouses, where they mainly cause severe damage to seedlings.
Most millipedes are rumored to secret acidic liquid when touched, but in reality, they defend themselves with various chemicals they secrete from pores.
Although the millipedes with tiny bristles are covered with hundreds of detachable bristles. In most millipede species, reproduction uses modified male legs known as gonopods, which inject sperm packets into females.
First, in the Silurian period, it was proven that millipedes were one of the oldest land animals known to man. Some of the prehistoric groups of millipedes grew bigger than 2 ft (6 ft 7 in).
Currently, the largest species grow to a max length of 27 to 38 cm (11 to 15 in). According to the study, the longest extant millipede species is the giant African millipede (Archispirostreptus gigas).
Among all myriapods, only millipedes have been traditionally considered the most closely related to tiny pauropods, although a couple of molecular studies have challenged this relationship.
Millipedes can easily be distinguished from the somewhat similar but quite distantly related centipedes (they belong to the class Chilopoda), which are venous, move rapidly, carnivorous, and only have a single pair of legs on each segment of their body.
Diplopodology is the scientific study of millipedes, and a diplopodologist is the name for a scientist who studies them.
Classification
Approximately 12,000 different species of millipedes have been described. The exact number of millipede species on earth has been estimated to range from 15,000 to 80,000.
Very few millipede species are widespread, mostly because they possess inferior dispersal abilities, which depend on how they do in humid habitats and terrestrial locomotion.
Though seemingly a drawback, these factors have favored the genetic isolation and rapid speciation of these creatures, thus producing many lineages of millipedes with restricted ranges.
The existing members of the Diplopoda are grouped into sixteen orders belonging to two subclasses.
The basal subclass is Penicillata, which contains a single order: Polyxenida ( housing only bristle millipedes).
Every other species of millipedes falls under the subclass Chilognatha, which is made up of two infraclasses: Helminthomorpha (the famous worm-like millipedes) and the most considerable number of millipede species and Pentazonia, which contains the short-bodied pill millipedes only.
Millipedes come in different shapes and sizes, ranging from around 2 mm (0.08 in) to 35 cm (14 in) in length, and they can have anywhere from eleven to more than a hundred segments.
Millipedes are generally brown or black, although it is not strange to find a few brightly colored ones, and some of them come with aposematic coloring to warn predators that they are toxic.
Species of Motyxia secret cyanide are their only chemical defense, and they can easily be spotted because they are bioluminescent.
Amongst major millipede groups, body styles vary greatly. The basal subclass of millipedes, which consists of the Penicillata, housing the tiny bristle millipedes, the exoskeleton is very soft and uncalcified, entirely covered in obvious setae or bristles.
All other millipede species belonging to the subclass Chilognatha come with a hardened exoskeleton.
The chilognaths are further divided into two infraclasses: the Pentazonia, housing relatively short-bodied groups of millipedes like the pill millipedes, and the second infraclass Helminthomorpha (“worm-like” millipedes), which houses the more significant majority of species, with long bodies featuring many segments.
Head
The heads of all millipedes are typically rounded on top and flattened under. The head bears a pair of bold mandibles in front of a plate-like structure known as a gnathochilarium (“jaw lip”).
They also have a single pair of antennae on their heads, with seven or eight segments and a few sensitive cones at the tip. Many orders also have a pair of sensory organs called the Tömösváry organs.
These organs look like small oval rings posterior and are lateral to the base of the antennae. The function of these organs is unknown, but they are also found in some types of centipedes and are possibly used to measure light levels or humidity in their environment.
The eyes of all species of Millipedes consist of several simple flat-lensed ocelli neatly arranged in a group or simple patch on each side of the head. These patches are also called ocular fields or ocellaria.
Many of the millipede species, including the whole Polydesmida order and cave-dwelling millipedes like Trichopetalum and Causeyella, had ancestors with eyes that could see. Still, subsequently, they have lost their eyes and have become blind.
Body
Paranota of polydesmidan (left) and platydesmidan millipedes
Millipedes may have Flattened bodies or cylindrical ones and are composed of numerous tiny segments, each bearing an exoskeleton made up of four chitinous plates: one single plate above (known as the tergite), one plate at each side (known as the pleurites), and a single plate on the under (known as the sternite) where the millipede’s legs are attached.
In many species of millipedes, these different plates are fused to varying degrees, sometimes creating one cylindrical ring.
The plates are naturally hard, being packed full of calcium salts. Because they do not have a waxy cuticle and cannot close their permanently open spiracles, all species of millipedes are vulnerable to water loss and have to spend most of the day in moist or humid places.
The first segment behind a millipede’s head has no legs and is called a collum ( derived from the Latin word for neck or collar). The next three segments that follow each have a single pair of legs and are called “haplosegments” (sometimes the three haplosegments are referred to as a “thorax”).
After the fourth segment, all others from the fifth to the end are appropriately called double segments or diplosegments, created by the fusion of two embryonic segments. As found in centipedes, each diplosegment features two pairs of legs instead of just one.
In some species of millipedes, there may be no legs in the last few segments. The terms “body ring” or segments are often used interchangeably for the haplo- and diplosegments.
The final segment of a millipede’s body is known as the telson. It is made up of a preanal ring that has no legs, a pair of anal valves (naturally closeable plates surrounding the anus), and a tiny scale under the anus.
Millipedes in different orders possess keel-like extensions of their body wall called paranota, which can widely vary in size, shape, and texture; modifications include ridges, lobes, papillae, spines, crests, and notches.
Paranota may offer millipedes a chance to wedge more easily into crevices, protect the legs, or make it harder for predators to swallow the millipede.
The legs of all millipedes are composed of seven separate segments and are fixed on the underside of their body.
The legs of one millipede are generally quite similar, although the males often have them longer than the females, and the males of some millipede species may have a reduced or enlarged pair of their first legs.
As discussed below, the most conspicuous modifications of millipede legs are involved in reproduction.
Even though it is common for people to say millipedes have thousands of legs, no species of millipede has been found with 1,000 legs: the most common species are known to have between 34 and 400 legs, with the record for the most feet held by the Illacme plenipes.
Individuals of the Illacme plenipes possess as many as 750 legs – more than any other living creature.
Internal Organs
Millipedes can breathe via two pairs of spiracles located on each segment close to the base of their legs. Each of them opens into an internal pouch, connecting to a tracheae system.
The millipede heart runs the full length of its body, with an aorta reaching into the head. Two pairs of malpighian tubules make the excretory organ close to the gut’s mid-part.
The millipede digestive tract is a straightforward tube with two pairs of salivary glands to help digest food.
Reproduction
Millipedes are interesting to study regarding reproduction as they show a diversity of structures and mating styles.
Millipedes in the basal order Polyxenida (bristle millipedes) engage in mating indirectly: males of these types deposit their spermatophores onto webs, which they secrete with special glands, and females will have to pick up the spermatophores subsequently.
In all the other millipede groups, males have one or two pairs of modified legs known as gonopods, which transfer sperm to female millipedes during copulation.
There is always a difference in the location of the gonopods between groups: the gonopods are located at the rear of the body in the males of the Pentazonia, and they are known as telopods.
They may also function effectively in grasping females, but in the Helminthomorpha – the vast majority of species – they can be found on the seventh segment of the body. A few species of millipedes are parthenogenetic, with just a few if any, males.
Millipedes are not a healthy pet option, so we do not recommend keeping one as a family member.
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