MYRIAPODS: CHARACTERISTICS, CLASSIFICATION, EXAMPLES OF SPECIES - BIOLOGY - 2025

The millipedes are a superclass of the phylum Arthropoda which consists centipedes and millipedes, among other lesser known members. The main characteristic of myriapods is that they present a clearly segmented body, from which various appendages emerge that can fulfill locomotion or other functions such as chewing or capturing prey.

Likewise, some of the myriapod species synthesize toxins or poisons that they use to inoculate their prey and thus be able to swallow them without problem. However, on occasions the human being has had encounters with some poisonous species and has been the victim of its bite. In humans, the venom can cause intense allergic reactions and local inflammation.

Specimen of myriapod. Source: Ferran Pestaña from Barcelona, ​​Spain

Taxonomy

The taxonomic classification of the myriapods is as follows:

- Domain: Eukarya.

- Animalia Kingdom.

- Phylum: Arthropoda.

- Subphylum: Mandibulata.

- Infrafilo: Tracheata.

- Superclass: Myriapoda.

characteristics

Myriapods are eukaryotic organisms, as well as multicellular. Its cells, in which DNA is delimited within the cell nucleus, have specialized in various functions, be they digestive, excretory or reproductive, among others.

Likewise, if its embryonic development process is studied, it can be seen that during it the three germ layers appear (endoderm, mesoderm and ectoderm). Because of this they are called triblastic animals.

By drawing an imaginary line along the longitudinal plane of the animal, two exactly equal halves are obtained, which allows us to affirm that they present bilateral symmetry.

Likewise, myriapods are dioecious organisms. That is, the sexes are separated. There are female individuals and male individuals. Likewise, they are oviparous, since they reproduce through eggs that are laid by the female after fertilization.

Morphology

-External anatomy

The main characteristic of myriapods is that, like all arthropods, they have a body divided into segments called tagmas. Specifically, the body of myriapods is segmented into three of them: head, thorax and abdomen. However, this division is not distinguishable with the naked eye, especially between the thorax and the abdomen.

Head

It is constituted in the first place by the acron. This is nothing more than a region that is not segmented. Apart from the acron, the head is also made up of several segments, which can be 5 or 6.

The element that stands out the most from this part of the animal is the pair of antennae it presents. At the base of these, there are pores that communicate with structures called Tömösvary organs.

These are sensory organs that are located in pairs and that, although their function has not been demonstrated, it is believed that it has to do with the detection of chemical substances (taste, smell) and hearing, among others.

Likewise, two appendages are detached from the head that have a very thickened and hard basal area that can be anatomically modified with elements that can be cutting or chewing. In some species, these mandibular appendages are modified to fulfill excavation functions.

After the mandibles, one or two pairs of maxillae may also be present. Of course, this depends on the species of myriapod.

Head enlargement of Scolopendra cingulata. (Appreciate the powerful calipers). Source: Fritz Geller-Grimm

A very important element in the anatomy of the head of a myriapod is the presence of modified appendages known as calipers. These are usually thick at the base and have a pincer shape.

At their tip they are pointed and tend to have a blackish coloration. They are associated with some venom-synthesizing glands. The calipers serve to inoculate the venom to the possible prey.

Thorax - abdomen

They make up the rest of the animal's body. It is important to note that between the thorax and the abdomen there is no anatomical element that can be taken to establish the limit between one region and another. In such a way that many specialists decide to call this area simply trunk.

The trunk is divided into segments, which are known as metamers. From each of them there is a specific number of appendages, depending on the species. For example, chilopods have only one pair of appendages, while diplopods have two pairs of appendages.

It is important to note that these appendages that emerge from each segment have functions related to the animal's locomotion. Likewise, the number of metamers is variable, according to the species. This is how there are myriapods whose body consists of about 10 segments, while there are others that can have more than 150.

Specimen of myriapod. The segmentation of the body and the appendages that emerge from each segment are evident. Source: Nahuel Cito

-Internal anatomy

The internal anatomy of myriapods is a bit complex. They present structures that throughout their development are specialized to fulfill specific functions such as digestion, respiration and excretion, among others.

Digestive system

The system dedicated to digestion is one of the simplest that can be observed among individuals of the phylum of arthropods. As in most of these, the digestive system is divided into three specialized areas: the stomodeum, the mesodeo, and the proctodeo.

It is made up of a cavity called the mouth, which continues with the pharynx and later the esophagus. Some species have crop and gizzard. It also has a midgut and the final or proctodean segment.

It is important to mention that at the level of the mouth it is possible to find salivary glands, whose function is the synthesis and secretion of saliva. In it, various chemical substances are dissolved, such as digestive enzymes that help in the processing of the food they eat.

Likewise, the cells that make up the midgut secrete a series of digestive enzymes that act on the components of the bolus, degrading it even more.

The last segment, the proctodeum, culminates in the anal orifice, into which the Malpighi tubes, which are part of the excretory system, also open.

Nervous system

The nervous system of myriapods could be considered highly specialized, when compared to that of other less evolved arthropods. It follows the same pattern consisting of a brain-type nerve formation, ventrally located nerve cords that extend the entire length of the animal and nerve ganglia in each metamer.

Brain formation is the result of the union of three neuronal clusters: protobrain, deutobrain and tritobrain.

The proto-brain is responsible for everything related to the secretion of endocrine-type substances and the information collected by the organs of sight (in those species that have them).

The deutobrain processes all the information that is captured through the receptors present in the antennas and it is believed that, to a lesser extent, in information regarding the senses of smell and taste.

The tritobrain collects information from the different appendages that the animal has, either the legs or the mouth appendages.

Regarding the sense organs, in the head one can find, apart from the Tömösvary organs, a kind of rudimentary eyes. These are characterized by not presenting ommatidia (sensory receptors that can distinguish between colors). Similarly, some species have a false compound eye.

Circulatory system

As in all arthropods, the circulatory system is open, with a kind of lagoon (hemocele) where the hemolymph reaches, which is the circulating fluid. In this liquid, the only specialized cell are the amoebocytes, which are responsible for coagulation among other functions.

Myriapods have a heart that is cylindrical in shape and extends over the entire length of the animal. For each segment, the heart has a pair of ostioles, as well as arteries.

An important element that occurs in this type of animal is the aorta artery, which is cephalad in direction.

Excretory system

The excretory system of myriapods is simple. It is made up of the so-called Malpighi tubes. These, of which there are one or two pairs, are blind and flow at the proctodean level, where they release waste substances.

Among the substances that myriapods discard is nitrogen in the form of uric acid.

Likewise, at the level of the head, specifically in the gnatoquillary, there are maxillary glands that are also excretory in nature.

Respiratory system

Myriapods have a tracheal type of respiratory system. They have a network of tubes called tracheas that are distributed throughout their anatomy. These tracheas communicate with the outside through holes known as spiracles.

Inside the animal, the tracheae branch out into tubes whose diameter is smaller and smaller, reaching each cell for gas exchange.

Habitat and distribution

The group of myriapods is widely distributed throughout the planet. They have managed to colonize a great variety of ecosystems, except for the poles.

Likewise, they are clearly terrestrial animals, so it is not possible to find them in aquatic environments. Despite this, it has been established that myriapods need to live near environments with ample availability of water, such as places around lakes or rivers.

Likewise, specialists have recorded that myriapod species are particularly abundant and diverse in tropical areas, while in areas further away and near the poles they are not so abundant.

Myriapods are nocturnal animals, so it is common during the day to find them in dark places such as under rocks. The large predators of this superclass generally hunt at night.

Feeding

Within the group of myriapods, food preferences are widely varied. There are species that are predatory carnivores such as Scolopendra cingulata, which feeds on small invertebrates.

Similarly, there are species that are herbivorous, such as those belonging to the Symphyla class. As well as there are omnivorous species that feed on small invertebrates and plants.

In the same vein, species of the Pauropoda class are saprophagous, that is, they feed on decomposing organic matter.

Now, regarding the type of digestion, both internal and external digestion are observed in myriapods.

Internal digestion is that in which the animal engulfs the plant or prey, after inoculating it with the poison, and the entire digestive process occurs within the body of the myriapod.

In this sense, the food is subjected to the action of digestive enzymes inside the mouth and pharynx to be converted into substances easily absorbed by the animal's body.

On the other hand, in external digestion the animal secretes a series of digestive enzymes that act directly on the food to be ingested, processing it and turning it into a kind of porridge that the animal finally ingests.

Regardless of the type of digestion, it is at the level of the midgut where the nutrients produced by food processing are absorbed. Finally, it is through the proctodeum, specifically the anus, where the substances not assimilated during the digestion process are released.

Reproduction

Myriapods reproduce sexually, with the fusion of male and female gametes. Likewise, the type of fertilization is indirect; that is to say, that despite occurring inside the body of the female it is not required that there be copulation between the individuals. However, there are a few species in which copulation does occur.

The reproduction process is as follows: the male releases a structure called spermatophore, in which his sperm is contained. The female then picks it up and introduces it, leading to self-fertilization.

Once this occurs, the female lays the eggs, usually in a safe place such as some hole she has prepared in the ground. A distinctive characteristic of myriapods is that once the eggs are laid, the female guards them, protecting them from possible predators until they hatch.

The group of myriapods is characterized by having a direct development. This implies that when the eggs hatch, the individual that emerges from them has characteristics that are very similar to those of adult individuals. That is, they do not experience larval stages.

Specimen of female centipede guarding her eggs. Source: Marshal Hedin

Of course, the young individual does not yet have the size reached by the adult specimens, in such a way that during his life he will experience several molting processes in which he must generate a new exoskeleton that each time adapts to its new dimensions. This will occur until the standard size of the adults of each species is reached.

Breathing

The type of respiration of myriapods is tracheal, that is, it occurs through a set of branched ducts that reach each cell directly.

Air enters through holes called spiracles and travels through the entire network of ducts until it reaches the cells. At the level of the tracheoles, which are the smallest ducts, is where gas exchange occurs.

In this, oxygen from the air passes into the cell and carbon dioxide, the cell's metabolic waste, leaves the cell to be expelled through the spiracles.

It is important to note that gas exchange occurs through a passive transport process called diffusion, which occurs in favor of the concentration gradient. This means that each gas will diffuse from where it is more concentrated to the place where there is less concentration of it.

Classification

Myriapods are classified into four classes: Chilopoda, Pauropoda, Diplopoda, and Symphila.

- Chilopoda: it is the class that includes all the so-called centipede species, as well as the famous scolopendras. Members of this group have approximately 21 body segments and are nocturnal. They are known for their powerful poisonous calipers.

- Pauropoda: they are the smallest myriapods that exist, since their size is only a few millimeters. They are saprophagous and inhabit mainly humid places where they have abundant food availability. They have a soft exoskeleton.

- Diplopoda: it is the class that is made up of millipedes. The distinctive characteristic of this group of individuals is that it has two pairs of appendages for each segment of the body. The body segments are fused two by two.

- Symphila: they are a group of small sized myriapods (up to 8 mm in length). Its body color is whitish and can even be translucent. They can have up to 12 pairs of legs. They are mainly found in dark and damp places such as in litter or under rocks.

Examples of species

Myriapods form one of the most diverse groups of arthropods. It covers more than 16,000 species. Among these, the most representative or prominent are:

- Diplopoda: Nipponesmus shirinensis, Oxidus gracilis and Epibolus pulchripes, among many others.

- Chilopoda: Scolopendra cingulata, Lithobius castaneus, Scutigera coleoptrata and many more.

Oxidus gracilis in its habitat. Source: Joseph Berger, Bugwood.org

References

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