Myriapods, the Many-Legged Arthropods

Myriapods (Myriapoda) are a group of arthropods that includes millipedes, centipedes, pauropods, and symphylans. About 15,000 species of myriapods are alive today. As their name implies, myriapods (from the Greek myriads, a myriad, plus photos, foot) are noted for having many legs, though the number varies widely from species to species. Some species have fewer than a dozen legs, while others have many hundreds of legs. The Illacme pipes, a millipede that inhabits central California, is the current record holder for myriapod leg count: This species has 750 legs, the most of all known myriapods.

Oldest Evidence

The earliest fossil evidence for myriads dates back to the late Silurian Period, about 420 million years ago. Molecular evidence, however, indicates that the group evolved before this, perhaps as early as the Cambrian Period, more than 485 million years ago.

Some Cambrian fossils show similarities to early myriapods, indicating that their evolution could have been underway at that time.

Characteristics

The key characteristics of myriapods include:

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• Many pairs of legs
• Two body sections (head and trunk)
• One pair of antennae on the head
• Simple eyes
• Mandibles (lower jaw) and maxillae (upper jaw)
• Respiratory exchange occurring through a tracheal system

Myriapods' bodies are divided into two tagmata, or body sections—a head and a trunk. The trunk is further divided into multiple segments, each having a pair of appendages, or legs. Myriapods have a pair of antennae on their head and a pair of mandibles and two pairs of maxillae (millipedes only have one pair of maxillae).

Centipedes have a round, flat head with one pair of antennae, a pair of maxillae, and a pair of large mandibles. Centipedes have limited vision; some species have no eyes at all. Those that have eyes can perceive differences in light and dark but lack true vision.

Millipedes have a rounded head that, unlike centipedes, is flat only on the bottom. Millipedes have a pair of large mandibles, a pair of antennae, and (like centipedes) limited vision. The body of millipedes is cylindrical. Millipedes are detritivores, feeding on detritus such as decomposing vegetation, organic material, and feces, and are prey for a variety of animals including amphibians, reptiles, mammals, birds, and other invertebrates.

Millipedes lack the venomous claws of centipedes, so they must curl into a tight coil to protect themselves. Millipedes generally have 25 to 100 segments. Each thoracic segment has a pair of legs, while the abdominal segments bear two pairs of legs each.

Habitat

Myriapods inhabit a variety of habitats but are most abundant in forests. They also inhabit grasslands, scrublands, and deserts. Although most myriapods are detritivores, centipedes are not; they are mainly nocturnal predators.

The two less familiar groups of myriapods, the sauropods and the symphylans, are small organisms (some are microscopic) that live in soil.

Classification

Myriapods are classified within the following taxonomic hierarchy:

1. Animals
2. Invertebrates
3. Arthropods
4. Myriapods

Myriapods are divided into the following taxonomic groups:

• Centipedes (Chilopoda): There are more than 3,000 species of centipedes alive today. Members of this group include stone centipedes, tropical centipedes, soil centipedes, and house centipedes. Centipedes are carnivorous and the first segment of their body is equipped with a pair of venomous claws.
• Millipedes (Diplopoda): About 12,000 species of millipedes are alive today. Members of this group include polyxenidans, chordeumatidans, platydesmidans, siphonophoridans, polydesmidans, and many others.