The Many-legged Myriapods
Whilst centipedes and millipedes have jaw-like mandibles on their heads for feeding like insects and crustaceans, they are classified in their own Arthropod subphylum: the Myriapoda. In additional to centipedes (Class Chilopoda) and millipedes (Class Diplopoda), the Myriapoda also includes two other little- known microscopic classes: symphalans (Class Symphyla) and pauropodans (Class Pauropoda).
Myriapods are immediately recognizable by their long, segmented bodies, with each segment possessing one or two pairs of jointed legs. The name Myriapoda is derived from Greek murias meaning ten thousand, + Latin pod meaning foot. As the name of this group suggests, these animals have a myriad of legs but whilst nowadays a ‘myriad’ denotes something countless or extremely great in number, a myriad classically referred to a unit of ten thousand – and no myriapod even comes close to possessing this many legs. Actually, some myriapod species have as few as 10 legs in total. The record for the greatest number of legs is held by a species of millipede: Illacme plenipes, which has 750 legs. This extremely rare, species of millipede is restricted to a tiny area in California. Illacme plenipes was thought to be extinct as it had not been seen for over 80 years since its initial discovery, and was only rediscovered in 2008. Illacme plenipes not only has the greatest number of legs of all myriapods, but also in fact holds the world record for the greatest number of legs of any animal! During locomotion, the legs move in waves that travel down the length of the body. It’s amazing that they can travel, often considerably rapidly, without getting all those legs tangled up! Their coordination is quite remarkable.
The body of a myriapod is comprised of two main sections (tagmata):
1) The head, which bears a single pair of antennae and a single pair of simple eyes
2) A long trunk of many segments that are all alike (homonomous). Each trunk segment bears legs.
Myriapods are exclusively terrestrial (there are no aquatic species). They live amongst leaf litter and soil, and you can often discover them hiding beneath a log, rock, or pile of debris.
Like all arthropods, myriapods have a chitinous external skeleton (exoskeleton). The myriapod exoskeleton is well-sclerotized, making the outer body hard, providing protection and support for the body. The exoskeleton also provides protection against desiccation (water loss and drying out).
About 12, 000 extant (living) species of centipedes and millipedes have been described worldwide.
Myriapods are a very ancient group of animals; in fact, the first animals to adopt a terrestrial (land-dwelling) life on Earth were millipedes! The first millipedes based on confirmed fossils appeared on land more than 425 million years ago (during the Silurian period); however trace fossils resembling millipede burrows suggest millipedes may have colonized the land even earlier in geological history. What is remarkable is that the ancient fossil forms are virtually indistinguishable from modern millipedes, indicating a remarkable amount of genetic and morphological evolutionary conservation in this group over the millions of years. Centipedes also are an ancient group, with the earliest fossils dating back to at least 380 million years ago (Devonian period). Like the millipedes, these early fossils forms also resemble extant forms.
So, what’s the difference between millipedes and centipedes?
Millipedes: Millipedes typically have more legs than centipedes. Whilst the prefix ‘milli’, is Latin for a thousand, suggests millipedes possess ‘thousands’ (or even ‘millions’) of legs, they actually have between 36 and 400 (maximum of 750 held by Illacme plenipes).
The name ‘Diplopoda’ (di Latin for two, + poda, Latin for foot) refers to how each leg-bearing segment appears to have 2 pairs of legs; however this condition actually arises because each ‘segment’ is in fact 2 segments fused together; for this reason each segment of a millipede is referred to as a ‘diplosegment.’ The first and last trunk segments are always legless. The legs are relatively short and held vertically beneath the body.
Each diplosegment contains a pair of glands containing an offensive noxious chemical that millipedes secrete to defend themselves against predators. When threatened they often roll up into a flat coil.
Millipedes are herbivorous, feeding on plant material or decomposing dead plant litter (detritus). They play an important role in ecosystems by converting plant and vegetable matter into humus, thereby producing and enriching soil. Millipedes have burrowing habits, foraging through soil, leaf litter or rotting timber in search of food. They are relatively slow-moving compared with centipedes.
Centipedes: Centipedes have only 1 pair of legs per trunk segment, and the legs of centipedes are generally longer than those of millipedes and extend laterally from the body. All body segments possess legs however the last pair extend backwards, held off the ground, and therefore are not used in locomotion. Centipedes are carnivorous and are typically more active and faster than millipedes. They are predatory, and their diet mainly consists of other invertebrates like cockroaches and crickets. They capture and kill their prey with a pair of large, poisonous claws (fangs) located on the first trunk segment below the mouth. It is believed that centipedes are the only animals in the world to have evolved legs modified into poison-injecting claws. When hunting, a centipede uses these formidable large claws (also known as prehensors or forcipules) to stab their prey, and then they inject venomous poison into the prey to further subdue and kill it. Therefore, it’s best to avoid centipedes and you should definitely not attempt to pick one up: when threatened, a centipede will never simply roll-up into a defensive posture but instead will attack, striking out with their poisonous claws to inflict a painful bite. Whilst not fatal, for humans a centipede bite produces acute local pain and swelling (similar to the reaction caused by a wasp or scorpion sting); however no Australian centipede is classified as dangerously venomous to humans. Centipedes are typically surface-dwelling rather than burrowing beneath the soil.
The table below summarizes the differences between millipedes (Class Diplopoda) from centipedes (Class Chilopoda)
|No. of extant (living) species described worldwide||10,000||2,800|
|Number of legs per body segment||2||1|
|Leg structure||Shorter, ventrally and vertically positioned||Longer, laterally positioned|
|Threat response||Curl into a defensive coil, discharge noxious chemical secretions||Inflict a poisonous bite|
|Habitat||Burrowing beneath the soil||Surface-dwelling|