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A redescription of the leggiest animal, the millipede Illacme plenipes, with notes on its natural history and biogeography (Diplopoda, Siphonophorida, Siphonorhinidae) - PubMed

A redescription of the leggiest animal, the millipede Illacme plenipes, with notes on its natural history and biogeography (Diplopoda, Siphonophorida, Siphonorhinidae)

Paul E Marek et al. Zookeys. 2012.

Abstract

With up to 750 legs, the millipede Illacme plenipes Cook and Loomis, 1928 is the leggiest animal known on Earth. It is endemic to the northwestern foothills of the Gabilan Range in San Benito County, California, where it is the only known species of the family Siphonorhinidae in the Western Hemisphere. Illacme plenipes is only known from 3 localities in a 4.5 km(2) area; the 1926 holotype locality is uncertain. Individuals of the species are strictly associated with large arkose sandstone boulders, and are extremely rare, with only 17 specimens known to exist in natural history collections. In contrast with its small size and unassuming outward appearance, the microanatomy of the species is strikingly complex. Here we provide a detailed redescription of the species, natural history notes, DNA barcodes for Illacme plenipes and similar-looking species, and a predictive occurrence map of the species inferred using niche based distribution modeling. Based on functional morphology of related species, the extreme number of legs is hypothesized to be associated with a life spent burrowing deep underground, and clinging to the surface of sandstone boulders.

Keywords: California Floristic Province; Gabilan Range; Salinas Valley; San Benito County; Silicon Valley; burrowing; conservation; endemic; paleoendemic; sandstone; silk.

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Figures

Figure 1.
Figure 1.

Niche-based distribution model inferred in Maxent. The model indicates predicted habitat suitability for Illacme plenipes based on climatic variables extracted from known geographical coordinates of the species. High levels of habitat suitability are denoted in blue and low levels in red (reverse heat map). Coordinates of recently collected specimens are indicated by a circle around the locations (northwest of the Gabilan Range) to preserve the confidentiality of sensitive habitat. Localities surveyed for additional populations of Illacme plenipes: 1 Frank Raines Park 2 Henry Coe State Park 3 Fremont Peak State Park 4 Pinnacles National Monument 5 Mount Madonna County Park 6 Alum Rock 7 Joseph D. Grant County Park 8 El Rancho Cienega del Gabilan.

Figure 2–5.
Figure 2–5.

2 Lateral (right) view of head and segments 1–5 (♂). a Lateral opening apparent between gnathochilarium and head capsule; gnathochilarium, mandible and head capsule noticeably separate at base, 1/3 head length distally from mandibular joint b Collum not covering head, with straight cephalic edge, gradually tapering laterally. Scale bar 0.5 mm. 3 Ventral view of head, antennae and segments 1 – 5 (♂).Scale bar 0.3 mm. 4 Lateral (right) view of antennomeres 5, 6 (♂).Arrow, small basiconic sensilla (Bs2) in cluster of 7 or 8 oriented apical dorsally (retrolaterally) in slight depression on antennomeres 5, 6. Scale bar 0.05 mm. 5 Oblique (right) view of right posterior gonopod (♂).Posterior gonopodal podomere 6 divided, comprising a bundle of 3 stylus-shaped articles. Scale bar 0.05 mm.

Figure 6.
Figure 6.

Illustration of anterior and posterior gonopods (♂). a Posterior gonopod with podomeres labeled P1-6 b Anterior gonopod with podomeres labeled A1-6 c 3 stylus-shaped articles. Scale bar 0.05 mm.

Figure 7.
Figure 7.

Dorsal view of anterior region of head and labrum (♂). a Scanning electron micrograph: arrow, labrum with triangular tooth-lined orifice b Line drawing: shaded area, triangular tooth-line orifice; arrow, gnathochilarium. Scale bar 0.02 mm.

Figure 8.
Figure 8.

Lateral (right) view of antennal and cephalic apices (♂). a Scanning electron micrograph: arrow, denticulate shelf-like carina, projecting dorsally from labrum-epistome margin. Scale bar 0.1 mm b Line drawing: top arrow, shelf-like carina; middle arrow, triangular tooth-lined orifice; bottom arrow, gnathochilarium. Scale bar 0.01 mm.

Figure 9.
Figure 9.

Illacme plenipes ♀ with 170 segments and 662 legs (specimen # SPC000931). Top inset, 2× magnified view of posterior segments with corkscrew-shaped metenteron visible through cuticle; bottom inset, 3× magnified illustration of corkscrew-shaped metenteron. Scale bar 1 mm.

Figures 10–15.
Figures 10–15.

10 Ventral in situ view of gonopods (♂). Arrow, anterior gonopod thick, more robust than posterior gonopod. Scale bar 0.1 mm 11 Medial view of right gonopods (♂). Arrow, Anterior gonopodal apex (podomere 6) shovel-shaped; in repose cupped sheath-like around flagelliform posterior gonopodal apex (podomere 6). Scale bar 0.05 mm. 12 Oblique (right) view of right posterior gonopodal apex (♂). 2 dorsal-most, longest articles laminate distally and recurved laterally, with denticulate posterior margins appearing claw-like. Scale bar 0.02 mm. 13 Antennomere 7 apex (♂). a Four apical cones (AS) oriented in a trapezoidal cluster on 7th antennomere, with longitudinally grooved outer surface and apical circular invagination b Spiniform basiconic sensilla (Bs3) in cluster of 5, oriented apical dorsally on 7th antennomere; tips facing apical cones (on longitudinal axis with Bs2 on antennomeres 5, 6); each sensillum with 2 barbules. Scale bar 0.02 mm.14 Lateral (right) view of right antenna (♂). a Chaetiform sensilla (CS) widely spaced on antennomeres 1-7, each sensillum with 2 or 3 barbules b Trichoid sensilla (TS) oriented apically encircling antennomeres 1–7, lacking barbules. Scale bar 0.1 mm. 15 Lateral (right) view of head, collum and segments 2, 3 (♂).Arrow, collum with carina present on anterolateral margin, appearing scaly. Carina repeated serially on lateral tergal and pleural margins (absent from telson). Scale bar 0.1 mm.

Figure 16–21.
Figure 16–21.

16 Ventral view of segments (♂). a Lateral tergal and pleural carinae jagged, pronounced on midbody segments b Pleurite medial margin broad, with scaly carina c Postgonopodal tarsus with thinner claw and without bifurcation, but with stout seta. Scale bar 0.4 mm. 17 Dorsal view of segments (♂). a Metazonite posterior margin (limbus) lined with posteriorly projecting anchor-shaped spikes and a row of conical spikes just dorsal to anchor-shaped spikes b 3 or 4 stout flat tubercles opposite ozopore near anterior margin, lunate arrangement encircling ozopore. Scale bar 0.4 mm. 18 Dorsal view of tergites (♂). Square, tergal setae tipped with silk-like exudate, tangled, appearing adhered to neighboring setae. Scale bar 0.05 mm. 19 Dorsal view of left ozopore (♂).Square, ozopores elevated slightly, with 2 stout posteriorly projecting spines and encircled by 13 – 15 robust setae. Scale bar 0.05 mm. 20 Right lateral view of posterior segments and telson (♂). Arrow, tergites, pleurites and sternites separated by arthrodial membrane. Scale bar 0.4 mm. 21 Oblique (right) ventrolateral view of 2 apodous segments, telson, hypoproct and paraprocts (♂).Apodous segments lacking sterna, pleurites contiguous in midline. Apodous tergites densely setose, covered with unevenly distributed spikes. Scale bar 0.2 mm.

Figure 22–27.
Figure 22–27.

22 Lateral view of fifth metatergite and prozonite (♂).Square, prozonite highly sculptured, with 5 rows of discoidal flat tubercles; anterior 3 rows staggered and posterior 2 rows aligned. Scale bar 0.1 mm. 23 Ventral view of mid-length sternites, pleurites and legs (♂).Left and right pleurites broadly overlapping sternite, covering spiracles. Scale bar 0.3 mm. 24 Ventral view of mid-length sternites and leg bases (♂). a Medial sternal ridge projecting ventrally, with spiracles and legs oriented ventrally b Eversible sacs membranous, bulging slightly from opening. Scale bar 0.1 mm. 25 Oblique (right) lateral view of sterna and spiracle (♂). Square, spiracles circular, orifice open; oriented dorsally above legs. Scale bar 0.05 mm. 26Ventral (right) view of legs, with posteroventral eversible sac opening (♂). Arrow, large posteroventral D-shaped opening for eversible sac. Scale bar 0.1 mm. 27 Oblique (right) lateral view of pregonopodal legs (♂).Arrow, pregonopodal tarsus with stout bifurcate claw. Scale bar 0.1 mm.

Figure 28.
Figure 28.

Habitat of Illacme plenipes. Top left, view of oak forest where Illacme plenipes were encountered. Top right, close up of oak forest and sandstone pinnacle where Illacme plenipes occur. Bottom, landscape view of oak forest, cattle trails evident (composite stitched image of three photos, image sides slightly distorted).

Figure 29.
Figure 29.

Oak forest understory habitat of Illacme plenipes. Top, base of sandstone pinnacle (from Fig. 28), where specimens were found. Bottom, mossy oak forest—close-up of habitat where Illacme plenipes individuals were encountered.

Figure 30.
Figure 30.

Sandstone microhabitat of Illacme plenipes.Top left, 50 kg sandstone from 29.xi.2005 rediscovery locality of Illacme plenipes; one ♀ with 666 legs was discovered from beneath the stone (scale bar = 5 cm, hand shovel shown for scale). Bottom left, 30 kg sandstone from the 16.xii.2007 locality, two ♀ (specimen #s: SPC001187, MIL0020) were discovered below the stone (scale bar = 5 cm, 15 cm ruler shown for scale). Top right, surface close up of sandstone from 16.xii.2007 locality with ♂ Illacme plenipes, not collected (scale bar = 5 mm). Bottom right, surface close up of sandstone from 29.xi.2005 locality with ♂ Illacme plenipes (specimen #: SPC000924, scale bar = 5 mm). Millipedes shown in right two pictures were found clinging to the surface of the stone.

Figure 31.
Figure 31.

Subterranean soil microhabitat of Illacme plenipes. Left, sandstone crater; dotted line indicates crater’s edges, arrows indicate two ♀ Illacme plenipes shown in situ upon removal of stone (specimen #s: SPC001187, MIL0020—lower individual with anterior trunk segments embedded in soil, upper individual with middle segments embedded in soil). Bottom middle, close up of lower individual from left image. Top right, dark sandy soil from microhabitat. Bottom right, close up of soil showing sandy grain structure.

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