MarekPaul E.ShearWilliam A.BondJason E.A redescription of the leggiest animal, the millipede Illacme plenipes, with notes on its natural history and biogeography (Diplopoda, Siphonophorida, Siphonorhinidae) Zookeys1411201220122417711210.3897/zookeys.241.3831 Illacme http://species-id.net/wiki/Illacme Cook & Loomis, 1928IllacmeCook and Loomis 1928: 12; Chamberlin and Hoffman 1958: 189; Buckett 1964: 29; Jeekel 1971: 39; Hoffman 1980: 116; Shelley 1996b: 23; Shelley 1996a: 1808; Hoffman 1999: 195; Jeekel 2001: 46; Marek and Bond 2006: 707; Shelley 2010: 45.Type species.

Illacme plenipes Cook and Loomis 1928: 12; by original designation.

Family placement.

Illacme is placed with other taxa in the family Siphonorhinidae based on the following characters: Head pear-shaped (♂) or triangular (♀), not elongate or bird beak-shaped, as in the Siphonophoridae (Fig. 2, Morphbank 805574, Appendix I). Antennae elbowed between antennomeres 3, 4 (Fig. 3, Mb-805578). Antennomeres 5, 6 with apical dorsal cluster of 7 or 8 basiconic sensilla (Bs2) in slight depression, not deep-set into circular pits, as in the Siphonophoridae (Fig. 4, Mb-805575). Posterior gonopods with distal podomere divided into 2 or 3 branches (Fig. 5, Mb-805576, Fig. 6c). See also diagnoses of Illacme in Shelley (1996b, p. 23) and of Siphonorhinidae in Shelley and Hoffman (2004, p. 218).

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.

Diagnosis.

Adults of Illacme are distinct from other siphonorhinid genera (and commonly-encountered millipedes co-occurring with Illacme plenipes) based on the combination of – Exoskeleton: Body light cream-colored, thread-like, extremely narrow and long (max. width: ♂ 0.55, ♀ 0.64; max. length: ♂ 28.16, ♀ 40.40). Adult individuals with 84 – 192 segments, and with 318 – 750 legs (VMNH paratype ♀ with 192 segments and 750 legs, more than any other organism known on Earth). Body with hirsute vestiture, appearing velvety (Fig. 2, Mb-805577). Antennae elbowed between antennomeres 3, 4 (Figs 2, 3, Mb-805578). Antennomeres 5, 6 enlarged, appearing oversized relative to other millipedes (Figs 2, 3, Mb-805579). Head pear-shaped (♂) or triangular/chevron-shaped (♀), eyeless (Figs 2, 3, Mb-805574, Appendix I). Mouthparts (gnathochilarium, mandibles) and labrum tightly appressed, tapered anteriorly to rounded apex, not bird beak-shaped, as in the Siphonophoridae (Fig. 3, Mb-805586). Labrum with triangular tooth-lined orifice (Fig. 7a, b; Mb-805580). Denticulate shelf-like carina, projecting dorsally from labrum-epistome margin (Fig. 8a, b; Mb-805588). Internal anatomy. Posterior one-quarter length of enteron loosely spiraled; when alive, visible through translucent cuticle (Fig. 9, Mb-805582). Male gonopods. 9th and 10th leg pairs modified into gonopods, each comprising 6 podomeres (Fig. 6a, b). Anterior gonopod thick, more robust than posterior gonopod (Fig. 10, Mb-805583, Fig. 6b). Anterior gonopodal apex (podomere 6, Fig. 6a, A6) shovel-shaped; in repose, cupped sheath-like around flagelliform posterior gonopodal apex (podomere 6, Fig. 11, Mb-805584, Fig. 6b, P6). Posterior gonopodal podomere 6 divided, comprising a bundle of 3 stylus-shaped articles (Fig. 5, Mb-805627, Fig. 6a, P6); remaining siphonorhinid taxa have 2 stylus-shaped articles with a small spine (Nematozonium filum) or 2 articles without a spine (Siphonorhinus Pocock, 1894 species and Kleruchus olivaceus Attems, 1938). 2 dorsal-most, longest articles of P6 laminate distally and recurved laterally, with denticulate posterior margins appearing claw-like (Fig. 12, Mb-805585, Fig. 6a, P6). Ventral-most, shortest article of P6 acuminate distally, spike-like. Habit in life. Movement very slow, nearly imperceptible (Appendix II, III). Antennae movement rapid, independent. Terminal antennomeres held flat and rapidly tap substrate and surroundings (Appendix IV).

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.

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.

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.

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.

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.

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