Myriapods comprise millipedes, centipedes, and their relatives and have many body segments, each segment bearing one or two pairs of legs (or in a few cases being legless). They are sometimes grouped with the hexapods.
The name Myriapoda appears in Latreille’s “Histoire naturelle générale et particulière des crustacés et insectes” in 1802; the term is a “learned” neologism from the Greek words myrias, myriada (“ten thousand”) and pus, podos (“foot”).
Myriapods are united by a body structure characterized by the presence of a cephalic capsule (quite similar to that of the hexapods in the number and arrangement of segments and in the type of appendages) and an elongated trunk with numerous crescents, that is, the body is divided into many rings, all of which are equal except for the first and last, and each of which carries a pair of legs (in the case of millipedes and two pairs in the case of centipedes). The different classes that make up this subphylum show considerable differences, in fact some authors do not consider it a valid systematic grouping.
Myriapods are divided into four classes according to their characteristics: chylopods, pauropods, diplopods and sympatric. They have a body divided into two parts: the head and the trunk, which is divided into segments from which a pair or two of appendages emerge, depending on the class to which they belong.
- Chilopods [Chilopoda]: they are nocturnal, cannot see, have a flattened trunk, can reach 30 cm, and two pairs of legs emerge from each segment, have two massillipeds equipped with glands that secrete venom. The chilopods include two of the most common species: Scolopendra escutigera.
- Pauropods [Pauropoda]: they are white, they reach a maximum length of 2 mm, they live in damp soil, they have a compressed body composed of 11 segments, they cannot see, but they are equipped with antennae with a double bifurcation.
- Diplopods [Diplopoda]: their trunk is divided into an abdomen and a thorax; the latter is made up of numerous segments from which a pair of appendages emerge, while in the abdomen, which is made up of double segments, two pairs of appendages emerge. Despite the large number of legs, they move very slowly.
- Symphylans [Symphyla]: not more than 1 cm long, with a whitish body, they live in the soil and are vegetarian, although in the case of Scutigerella they can sometimes damage crops.
Fossil remains of myriapods date from the Silurian Period (about 400 million years ago), but some molecular analyses suggest that this group of arthropods may have already differentiated in the Cambrian Period (about 500 million years ago). Also from this period is a multisegmented centipede called Xanthomyria spinosa, which may represent the oldest fossil remains of myriapods. Other myriapod-like organisms (Pseudoiulia cambriensis) have been found in even older strata (Lower Cambrian, 520 million years ago). During the Carboniferous period (about 300 million years ago), myriapods evolved enormously (Acantherpestes, Euphoberia) and also gave rise to giant forms over two meters long (Arthropleura) due to the considerable oxygen density in the atmosphere.
The millipede Illacme plenipes is considered the myriapod with the most pairs of legs, in fact it has 750, the females being longer than the males, reaching 300 mm in length. These specimens live in California and were first found in 1926.
In August 2020 it was discovered in Australia at a depth of sixty meters, the first millipede with 1306 legs, that is 653 pairs of limbs, 330 segments, a diameter of 0.95 millimeters and a length of 95.7 millimeters, this myriapod was named Eumillipes Persephone.