Myriapods (centipedes, millipedes)

Myriapoda is a subphylum of Arthropoda characterized by a high number of legs and a body divided into two distinct regions: a head and a long, multi-segmented trunk. Unlike crustaceans, myriapods are strictly terrestrial and lack a waxy cuticle, making them highly susceptible to desiccation (drying out).

General Anatomical Features

Myriapods share several fundamental arthropod traits but possess unique modifications for their life in soil and leaf litter.

• Body Structure: The body consists of a head followed by numerous similar segments. They lack a fused thorax (unlike insects).
• Uniramous Appendages: All limbs are single-branched (uniramous), a feature they share with insects but which distinguishes them from crustaceans.
• Respiration: They breathe through a system of tracheae (air tubes) that open to the outside through pores called spiracles.
• Excretion: Waste is removed via Malpighian tubules, similar to insects, to conserve water.
• Sensory Organs: They possess one pair of antennae and simple eyes (ocelli), though many soil-dwelling species are blind.

Classification: Chilopoda vs. Diplopoda

The two most significant classes under Myriapoda for UPSC Prelims are Chilopoda (Centipedes) and Diplopoda (Millipedes).

Class Chilopoda: The Centipedes

Centipedes are specialized hunters found in dark, moist environments such as under stones or logs.

• Venom Apparatus: The first pair of trunk appendages are modified into poison claws or forcipules. These are used to inject venom into prey (insects, worms, or small vertebrates).
• Segment Count: The number of segments is always an odd number, ranging from 15 to over 170.
• Ecological Role: They act as secondary consumers, controlling the population of soil-dwelling pests.

Class Diplopoda: The Millipedes

Millipedes are essential decomposers in the forest floor ecosystem.

• Diplosegments: Each apparent segment is actually a “diplosegment,” formed by the fusion of two embryonic segments, explaining why they have two pairs of legs per segment.
• Feeding Habits: They feed on dead plant material, playing a critical role in nutrient cycling and soil formation.
• Chemical Defense: Lacking venomous claws, they protect themselves by rolling into a tight ball and often secreting noxious liquids (including hydrogen cyanide in some species) from reputatorial glands.

Evolutionary and Ecological Significance

• Transition to Land: Myriapods represent one of the earliest lineages of arthropods to colonize terrestrial habitats.
• Soil Health: Along with earthworms, millipedes are primary “shredders” in the soil food web. They break down coarse leaf litter into smaller particles, increasing the surface area for microbial decomposition.
• Indicator Species: Because they are sensitive to moisture and soil chemistry, their presence and diversity can indicate the health of a forest ecosystem.

UPSC Trivia: Quick Facts for Prelims

• The “Thousand-Legger” Myth: While the name “Millipede” implies 1,000 legs, most species have between 40 and 400. However, the species Eumillipes persephone discovered in Australia can have over 1,300 legs.
• Lack of Cuticle: Unlike insects, myriapods lack a waterproof waxy layer on their exoskeleton. This is why they are rarely seen in daylight and must remain in humid micro-habitats to survive.
• Simple Eyes: Most myriapods have clusters of ocelli rather than the complex compound eyes found in crabs or bees.
• Reproduction: Most myriapods lay eggs in the soil. In some millipede species, the male uses modified legs called gonopods to transfer sperm to the female.