Myriapoda

Myriapoda (from the Greek myrias, "ten thousand," and pous, "foot") is a subphylum of terrestrial arthropods comprising approximately 16,000 described extant species divided across four monophyletic classes: predatory Chilopoda (centipedes, ~3,300 species), detritivorous Diplopoda (millipedes, ~12,000 species), microscopic soil-dwelling Pauropoda (~850 species), and eyeless root-feeding Symphyla (~200 species). Phylogenomic analyses place Myriapoda as the basal sister group to Pancrustacea (Crustacea + Hexapoda), uniting the two clades into Mandibulata—defined by chewing mandibles on the fourth head segment (as opposed to the pincer-like chelicerae of Chelicerata).

In the fossil record, myriapods hold a landmark distinction in the history of life: Silurian millipede body fossils—most notably Pneumodesmus newmani (~428–414 million years ago from Stonehaven, Scotland), which preserves the earliest known air-breathing spiracles—together with Giant Carboniferous arthropleurids (Arthropleura, reaching 2.6 meters in length and 50 kg in weight across equatorial coal swamps), establish Myriapoda as the first animal lineage to colonize dry land.

Unlike insects (three tagmata: head, thorax, abdomen) and chelicerates (two tagmata: prosoma and opisthosoma), all myriapods share a two-tagma body plan consisting of an anterior head capsule followed by an elongated, homonomous multi-segmented trunk bearing uniramous walking legs:

1. Head Segment Homology across Mandibulata:
   • Segment 1 (Ocular / Protocerebral): Bears clusters of simple lateral ocelli (aggregated into pseudofaceted compound-like eyes only in house centipedes, Scutigeromorpha, and extinct Devonian–Carboniferous euthycarcinoids/archipolypods).
   • Segment 2 (Deutocerebral): Bears a single pair of uniramous antennae (branched only in Pauropoda).
   • Segment 3 (Tritocerebral / Intercalary): Appendageless (reduced to an embryonic premandibular segment, exactly as in Hexapoda, whereas Crustacea retain second antennae on this segment).
   • Segment 4 (Mandibular): Bears a pair of articulated, multi-segmented mandibles that bite transversely inside a preoral cavity.
   • Segments 5 & 6 (First and Second Maxillary): Bear one or two pairs of food-manipulating maxillae (fused into a single broad lower lip, the gnathochilarium, in Diplopoda and Pauropoda; retained as paired first and second maxillae in Chilopoda and Symphyla).
2. Malpighian Tubules, Tracheae, and Cuticular Water Balance: Like insects (by convergent evolution during independent terrestrialization events), myriapods excrete nitrogenous waste as insoluble uric acid (or ammonia in moist soil forms) via Malpighian tubules arising from the midgut–hindgut junction, and breathe via segmental spiracles connected to branching chitin-lined tracheae. However, most myriapods lack a waterproof epicuticular wax layer and cannot close their spiracles completely, confining them to humid soil, leaf litter, caves, and rotting wood.

1. Chilopod Forcipules and Ion-Channel Neurotoxins: Every centipede possesses a unique evolutionary novelty on its first trunk segment: the first pair of walking legs has migrated forward beneath the head capsule and fused with its sternite to form a pair of massive, curved, pincer-like venom claws called forcipules (toxicognaths). Inside each forcipule sits a muscular venom gland secreting a complex cocktail of pore-forming proteins, metalloproteases, hyaluronidases, and disulfide-rich neurotoxic peptides (scoloptoxins). As demonstrated by Shilong Yang, Glenn F. King, and colleagues (2013, 2018) in the golden head centipede Scolopendra subspinipes mutilans:
   • Centipede venom contains μ-SLPTX-Ssm6a, a 46-residue three-disulfide peptide that selectively inhibits the human pain-transducing voltage-gated sodium channel Na_v1.7 (SCN9A) with an IC₅₀ of 25 nM (delivering analgesic efficacy exceeding morphine in rodent pain models without affecting cardiac Na_v1.5);
   • Conversely, to subdue vertebrate and insect prey 15 times its own body weight within seconds, Scolopendra injects Ssm Spooky Toxin (SsTx), a 53-residue peptide that potently blocks KCNQ (K_v7) potassium channels, triggering immediate vasospasm, respiratory arrest, and seizures in prey.
2. Diplosegmentation and the 1,306-Legged Eumillipes persephone: During millipede embryonic and anamorphic post-embryonic development (where hatchlings emerge with only 3 pairs of legs and add new segments ahead of the telson at each molt), dorsal tergal plates fuse across pairs of ventral segments to form diplosegments. Each diplosegment contains two pairs of walking legs, two pairs of ventral ganglia, and two pairs of tracheal spiracles:
   • Providing twice the leg-muscle pushing thrust per unit body length to bulldoze head-first through compacted soil and rotting wood while flexing the calcified ring armor into a tight defensive sphere or spiral.
   • For nearly a century, the California millipede Illacme plenipes held the animal record at 750 legs (375 pairs); in 2021, Paul E. Marek and colleagues discovered the blind, thread-like subterranean millipede Eumillipes persephone 60 meters underground in Western Australia—bearing 330 segments and 1,306 legs (653 pairs), the first true "thousand-footed" animal ever documented!
3. Two-Chambered Hydrogen Cyanide Reactor Glands in Polydesmid Millipedes: Because millipedes walk too slowly to outrun predators, helminthomorph millipedes deploy chemical warfare from paired serial repugnatorial glands (ozadenes) opening through lateral ozopores:
   • Cylindrical julid and spirobolid millipedes secrete caustic 1,4-benzoquinones, cresols, and phenols that stain skin and repel ants, birds, and capuchin monkeys (which deliberately rub Orthoporus millipedes into their fur as a natural mosquito repellent!).
   • Flat-backed Polydesmida (Apheloria, Harpaphe, Motyxia bioluminescent millipedes) operate a two-chambered binary chemical reactor first elucidated by Thomas Eisner: an inner storage reservoir holds harmless mandelonitrile (synthesized from L-phenylalanine via cytochrome P450 oximes); when a predator bites the millipede, a muscular sphincteric valve squeezes mandelonitrile into an outer vestibule lined with the enzyme hydroxynitrile lyase (mandelonitrile lyase), instantaneously dissociating it into benzaldehyde (bitter-almond odor) and lethal hydrogen cyanide gas (HCN) at doses capable of killing mice or birds in confined spaces.

Why do centipedes always have an odd number of pairs of walking legs?

Although the name "centipede" means "hundred-footed," no living species of centipede has exactly 100 legs (50 pairs)! All adult Scutigeromorpha, Lithobiomorpha, and Craterostigmomorpha have exactly 15 pairs of walking legs (30 legs); Scolopendromorpha have either 21 or 23 pairs (42 or 46 legs); and elongated burrowing Geophilomorpha have between 27 and 191 pairs (54 to 382 legs)—always an odd number of pairs. Developmental genetics of the geophilomorph Strigamia maritima (Chipman, Arthur, & Akam, 2004) showed that during embryonic segmentation, the Notch/Delta–hairy segmentation clock generates double-segment (pair-rule) periodic units, each subdividing into two leg-bearing segments; combined with the transformation of the first trunk segment into venom forcipules and the modification of the terminal genital segments, subtracting one pair of forcipules from an even number of double-segment units always leaves an odd number of walking leg pairs.

Why does only the house centipede (Scutigera coleoptrata) have compound-like eyes and a dorsal tracheal lung?

Whereas most centipedes live beneath stones or soil with reduced ocelli (or complete blindness in Geophilomorpha) and lateral spiracles, Scutigeromorpha (Scutigera coleoptrata) evolved as cursorial surface pursuit predators reaching 40 cm/s (15 body lengths per second). In doing so, they re-evolved large, multifaceted compound-like eyes (formed from clustered ommatidia with crystalline cones), extraordinarily long multi-annulated flagellar legs that lasso flies mid-air, and a unique respiratory system: seven unpaired middorsal stomata (spiracles) opening along the back of the tergal plates into dense mats of hundreds of short tracheal tubules bathed directement in pericardial blood (tracheal lungs), where oxygen binds high-affinity hexameric hemocyanin—making Scutigeromorpha the only centipede clade that combines tracheal lungs with oxygen-transporting blood proteins.

1. Shear, W. A., & Edgecombe, G. D. (2010). The geological record and phylogeny of the Myriapoda. Arthropod Structure & Development, 39(2-3), 174–190. doi:10.1016/j.asd.2009.11.002
2. Chipman, A. D., Ferrier, D. E. K., Brena, C., Qu, J., Hughes, D. S. T., Schröder, R., ... & Richards, S. (2014). The first myriapod genome sequence reveals conservative arthropod gene content and genome organisation in the centipede Strigamia maritima. PLoS Biology, 12(11), e1002005. doi:10.1371/journal.pbio.1002005
3. Yang, S., Xiao, Y., Kang, D., Liu, J., Li, Y., Undheim, E. A. B., ... & King, G. F. (2013). Discovery of a selective Na_V1.7 inhibitor from centipede venom with analgesic efficacy exceeding morphine in rodent pain models. Proceedings of the National Academy of Sciences, 110(43), 17534–17539. doi:10.1073/pnas.1306285110
4. Luo, L., Li, B., Wang, S., Wu, F., Wang, X., Liang, P., ... & Yang, S. (2018). Centipedes subdue giant prey by blocking KCNQ channels. Proceedings of the National Academy of Sciences, 115(7), 1646–1651. doi:10.1073/pnas.1714760115
5. Marek, P. E., Buzatto, B. A., Shear, W. A., Means, J. C., Black, D. G., Harvey, M. S., & Rodriguez, J. (2021). The first true millipede—1,306 legs long. Scientific Reports, 11, 23126. doi:10.1038/s41598-021-02447-0