Polydesmida

Flat-backed Millipedes

Polydesmida is the largest order of , containing over 5,000 described . Members are commonly known as flat-backed millipedes due to the presence of lateral keels (paranota) on most body segments. This order includes all millipedes known to produce hydrogen cyanide (HCN) as a chemical defense. Polydesmids are found on every continent except Antarctica and exhibit the highest morphological diversity among millipede orders. They are characterized by a unique developmental mode called teloanamorphosis, in which reach a fixed number of segments and cease molting.

Rhysodesmini by (c) Zoe O., some rights reserved (CC BY), uploaded by Zoe O.. Used under a CC-BY license.

Pseudopolydesmus pinetorum by no rights reserved, uploaded by Calinsdad. Used under a CC0 license.

Sphaeriodesmidae by (c) Jorge Armín Escalante-Pasos, some rights reserved (CC BY), uploaded by Jorge Armín Escalante-Pasos. Used under a CC-BY license.

Pronunciation

How to pronounce Polydesmida: //ˌpɒl.iˈdɛz.mɪ.də//

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Identification

Flat-backed are identified by their flattened body profile with prominent lateral keels (paranota), absence of , and relatively short compared to some other millipede orders. They can be distinguished from cylindrical millipedes (orders such as Spirobolida and Spirostreptida) by their dorsoventrally compressed shape. Within Polydesmida, are distinguished by structure, segment number variations, and other subtle morphological features. The presence of hydrogen cyanide defense is diagnostic for the order but requires chemical testing or observation of defensive . identification typically requires examination of male gonopods under magnification.

Images

Habitat

Polydesmids are predominantly found in leaf litter and soil , where they burrow by levering with the end of the body. They occupy diverse environments from tropical rainforests to temperate woodlands, grasslands, and caves. Many are associated with moist microhabitats under rocks, logs, and decaying wood. Some lineages have adapted to cave environments, exhibiting troglomorphic features such as elongated appendages and loss of pigmentation. The order shows particular diversity in tropical and subtropical regions.

Distribution

distribution on all continents except Antarctica. Highest diversity occurs in tropical and subtropical regions, particularly in Southeast Asia, the Neotropics, and Madagascar. The order is well-represented in North America, Europe, Africa, and Australia. Specific distribution patterns vary by ; for example, Chelodesmidae shows strong Neotropical affinity, while Xystodesmidae is primarily Holarctic.

Seasonality

Activity patterns vary by and climate. In temperate regions, activity peaks during moist periods in spring and autumn, with reduced surface activity during dry summer months or cold winter periods. Some species retreat to deeper soil layers (up to 15 cm) during unfavorable conditions. In tropical regions, activity may be year-round with peaks during wet seasons. Mating activity in studied species is concentrated in early morning and late afternoon hours.

Diet

Feeds on decaying vegetation, particularly leaf litter. Some may consume decaying wood and other plant detritus.

Life Cycle

Development occurs through teloanamorphosis: juveniles hatch with 7 segments and 3 leg pairs, then undergo a fixed series of (typically 7-8 molts) adding segments until reaching the -specific segment count, at which point molting ceases. Most species reach adulthood with 20 segments after 8 developmental stages. Species with fewer segments mature earlier; those with more segments undergo additional molts. Adults reproduce and die without further molting. time varies from one to two years in studied species.

Behavior

Burrows through leaf litter using levering motion with the body. When threatened, many curl into a tight spiral or ball, shielding the surface while exposing chemical defense glands. Defensive secretions containing hydrogen cyanide are released from lateral glands on most body segments. Some species exhibit seasonal between microhabitats, moving between grassland and forest in response to moisture availability. Mating involves pre-copulatory and variable duration copulation (1-25 minutes in studied species). Males typically pursue females in a polygynandrous mating system.

Ecological Role

Primary decomposers in forest and grassland . Accelerate leaf litter decomposition and regulate soil carbon and phosphorus cycling through consumption and fragmentation of decaying plant material. Serve as prey for specialized including certain carabid (Promecognathus) and funnel-web spiders. Their sensitivity to environmental conditions makes them potential indicators of soil health and ecosystem disturbance.

Human Relevance

Ecologically important as decomposers in forest . Some produce hydrogen cyanide, which can cause irritation to humans handling specimens in confined spaces. The order includes the only with documented cyanide defense, which has attracted research interest in chemical and -prey . Some cave-dwelling species are of conservation concern due to vulnerability. The bright coloration of certain species, such as the "shocking pink dragon millipede," has generated public interest in millipede biodiversity.

Similar Taxa

SpirobolidaCylindrical with rounded cross-section, lacking the lateral paranota characteristic of Polydesmida. Typically have more segments and longer .
SpirostreptidaCylindrical with very long, slender bodies and numerous segments (often 40+), lacking the flattened appearance and paranota of flat-backed millipedes.
JulidaCylindrical with relatively short legs and rounded body profile; some superficially resemble small polydesmids but lack paranota and have different structure.

More Details

Chemical Defense

Polydesmida is the only order containing that produce hydrogen cyanide (HCN). The cyanogenic compounds are stored as benzaldehyde cyanohydrin in paired glands along the sides of most segments, then enzymatically converted to HCN and benzaldehyde when discharged. This defense has driven with specialized such as Promecognathus , which have evolved exceptional cyanide .

Teloanamorphosis

The fixed-segment developmental mode of Polydesmida (teloanamorphosis) contrasts with the continuous molting and segment addition seen in most other orders (euanamorphosis and hemianamorphosis). This trait is diagnostic for the order and has significant implications for growth patterns and evolution.

Segment Number Variation

While 20 segments is typical for , extensive variation exists within the order. Some have 18-19 segments due to early maturation, while cave-dwelling species in the Devillea have up to 23 segments, and the Brazilian cave species Dobrodesmus mirabilis has 40 segments in males—the highest known for the order.