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The arthropod class Diplopoda (millipedes) contains some 10,000 described species2 and is one of the most ancient groups of terrestrial animals3,4,5. Confident reconstructions of diplopod phylogeny and the taxon's placement within the broader context of arthropod evolution remain elusive, however, due in part to the group's antiquity, but also to the rarity of key taxa6,7 — some are known from just a single specimen. The millipede I. plenipes (Arthropoda: Myriapoda: Diplopoda: Siphonorhinidae) has not been sighted since 1926 (ref. 1).

Over the course of three separate collecting expeditions, we captured four male, three female and five juvenile specimens of I. plenipes. The female specimens were 32.38–33.20 mm long and comprised 170 or 171 segments with a total of 662–666 legs — slightly fewer than the record-holding specimen first described1. Male specimens were smaller at 14.38–16.15 mm, with 84–105 segments and 318–402 legs.

Millipedes undergo anamorphosis, the addition of segments and legs during post-embryonic growth8. In siphonorhinid millipedes, this continues for an indeterminate length of time, possibly even after sexual maturity. This would result in continuous segment addition and elongation and would account for the length variation we observed in our adult specimens. The average body width for all specimens is a thread-like 0.57 mm.

The species' simple outward appearance (Fig. 1a) belies its ornate exoskeletal surface structure, which becomes evident through scanning electron microscopy (Fig. 1b-e). (For more images and for the only live video ever recorded, to our knowledge, of I. plenipes, see supplementary information.) Each segment is equipped with numerous hairs (setae) and previously undocumented cuticular projections. Many of the setae on the dorsal segmental plates (tergites) seem to secrete a ‘silk-like’ substance whose function is unknown. The trailing edge of each tergite and the ozopore — a gland opening — have bizarre, almost gothic ornamentation.

Figure 1: The millipede Illacme plenipes.
figure 1

a, Live female I. plenipes with 662 legs (captured by Rob Marek). b,c, Scanning electron micrographs of male I. plenipes: b, mid-section, dorsal aspect of segments (note dense setation and micro-sculpturation of intersegmental cuticle); c, mid-section, ventral aspect of segments showing paired appendages on each diplo-segment. d, Close-up view of ozopore, the opening to the repugnatorial gland, and surface structure of cuticular projection on posterior margin of segment. e, Male copulatory device (gonopods), which are modified legs (ninth and tenth pair); left anterior is shaded blue; right posterior is in yellow. Scale bars: a, 10 mm; b, c, 400 µm; d, 50 µm; e, 200 µm.

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The male copulatory device (Fig. 1e) is far more complicated than previously described9, comprising an anterior component that cups the posterior articles in situ. The distal article of the posterior gonopod, the sixth podomere, is bifurcate and each branch is armed with a serrated apex like a bird claw (Fig. 1e, structure shaded yellow).

The rediscovery of this remarkable animal in a known biodiversity hotspot, the California Floristic Province10,11, is noteworthy: other species of closely related12 and equally rare siphonorhinid genera are endemic to centres of biodiversity in Indo–Burma, southern Africa, Sundaland and Wallacea. The family's enigmatic distribution further supports the unique status of these hotspots as repositories of exceptional diversity and provides evidence that the siphonorhinid radiation pre-dates the break-up of Pangaea more than 200 million years ago.

Our rediscovery of I. plenipes at a time when sophisticated microscopes are available has revealed the fine-scale structure of a creature whose morphology is surprisingly intricate. The Diplopoda are an intriguing class of organisms that await new discoveries.