Myriapoda, a subphylum within the arthropod phylum, encompasses invertebrates characterized by their elongated, segmented bodies and numerous legs. The name “Myriapoda” originates from Greek words, with “myriads” meaning countless and “poda” meaning feet, directly referencing their many limbs. They are terrestrial, related to insects and crustaceans, and share arthropod traits like an exoskeleton made of chitin and jointed appendages. Myriapods have a long evolutionary history, with fossil evidence indicating their presence over 400 million years ago, dating back to the Silurian period.
The Four Classes of Myriapods
The subphylum Myriapoda includes four distinct classes. Chilopoda, commonly known as centipedes, are predatory myriapods with approximately 5,000 known species. They are recognized by their flattened bodies and a single pair of legs on most body segments.
Diplopoda, or millipedes, are the most diverse myriapod group, with over 12,000 described species. They have two pairs of legs on most body segments, a feature resulting from the embryonic fusion of adjacent segments. The other two classes, Pauropoda and Symphyla, are smaller and less commonly encountered. Pauropoda are tiny, typically ranging from 0.5 to 2.0 mm, while Symphyla are also small, usually 1 to 8 mm long. Both are translucent, soil-dwelling organisms, often found in damp environments.
Distinguishing Centipedes and Millipedes
Centipedes and millipedes, while both multi-legged arthropods, exhibit clear differences in their physical characteristics, diet, and behavior. Centipedes have one pair of legs per body segment, except for the first, which typically lacks legs. Their bodies are flattened, allowing for swift, undulating movements as they pursue prey.
Millipedes, in contrast, have two pairs of legs on most body segments, giving them a more cylindrical or rounded appearance. Their movement is typically slower and more deliberate, adapted for burrowing through soil. Centipedes are carnivores, hunting small invertebrates like insects, spiders, and other arthropods. They immobilize prey using venom delivered through modified front legs called forcipules.
Millipedes are primarily detritivores, consuming decaying plant matter and organic debris, which helps break down dead plant material. For defense, centipedes rely on their speed and venomous bite. Millipedes protect themselves by coiling their bodies into a tight spiral to shield their softer underside and may release defensive chemical secretions from glands along their bodies when threatened.
Anatomy and Physical Characteristics
Myriapods have a distinct head and an elongated, segmented trunk. Each trunk segment typically bears one or two pairs of legs. The head features a single pair of antennae, important sensory organs for navigating their environment and detecting chemical cues and vibrations.
Mouthparts are located on the underside of the head and include mandibles for chewing, along with an upper lip and a lower lip formed by maxillae. Respiration occurs through a system of tubes called tracheae, which connect to the outside via small openings known as spiracles, similar to those found in insects. Most myriapods undergo molting, shedding their rigid exoskeleton. During this process, many species add new body segments and legs, a developmental pattern known as anamorphosis.
Habitat and Ecological Role
Myriapods are terrestrial, preferring damp and dark environments to prevent desiccation. They are commonly found in moist habitats such as leaf litter, under rocks or logs, and within the soil. Their reliance on moisture limits their ability to migrate across dry areas.
Myriapods play distinct but complementary roles within their ecosystems. Millipedes, as detritivores, break down decaying plant material, which is important for nutrient cycling and maintaining soil health. Centipedes, as predators, help regulate populations of other invertebrates, including insects and spiders, contributing to ecosystem balance.