Type specimens: ♂ holotype (USNM), 1♂, 3♀ paratypes (FSCA) and 3♀ paratypes (VMNH)—from United States, California, San Benito County, from “near divide between Salinas and San Juan Bautista” [an imprecise location probably on the north side of the Gabilan Range on San Juan Grade Road or Old Stage Road in a radius of 4 km around the coordinates 36.831371°N, -121.562808°W], 27.xi.1926 (Coll. O.F. Cook). Non-type specimens: California, San Benito County: 1♂ (SPC000924), 2♀ (SPC000930, -931), Gabilan Range, San Juan Bautista, 29.xi.2005 (Colls: P. and R. Marek); 3♂ (SPC000932, -933, -934), 1 juvenile (SPC000935), loc. ibid., 8.xii.2005, (Coll: J. Bond). 2♀ (SPC001187, MIL0020), Gabilan Range, San Juan Bautista, 16.xii.2007, 13:00 (Colls: P. and R. Marek).

(See generic diagnosis.)

(♂) USNM TYPE NO. 976 – Counts and measurements: p = 143. a = 2. l = 562. (143 + 2 + T). HW = 0.30. HL = 0.34. ISW = 0.20. AW = [antennae missing]. CW = 0.42. W1 = 0.53. W2 = 0.55. W3 = 0.55. L1 = 0.20. L2 = 0.20. L3 = 0.18. H1 = 0.31. H2 = 0.30. H3 = 0.33. AS1 = 0.45. A5W = 0.05. P5W = 0.04. BL = 28.16. Head pear-shaped, tapered anteriorly to round point at a 160° angle anterior from antennal sockets; occipital area posterior from antennal sockets gradually curved medially towards neck (Figs 2, 3, Mb-805574—note: all SEMs herein are images of specimen #SPC000932, not the holotype). Head pilose, covered with long, slender setae (Fig. 2, Mb-805577). Mouthparts (gnathochilarium, mandibles) and labrum tightly appressed, tapered anteriorly to round point (Fig. 3, Mb-805586). Gnathochilarium elements (stipes, promentum, etc.) indistinguishably fused, tightly appressed to the ventral surface of the head, leaving a small opening anteriorly. Lateral opening apparent between gnathochilarium and head capsule (Fig. 2a, Mb-805587). Mandibles thin, stylet-like, with heavily calcified apices (viewed dorsally through translucent head capsule at 400× with a compound microscope). 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). Gnathochilarium, mandible and head capsule noticeably separate at base, 1/3 head length distally from mandibular joint (Fig. 2a, Mb-805589). Antennae sub-geniculate, elbowed between antennomeres 3, 4, comprising 7 antennomeres (Fig. 3, Mb-805578). Antennae massive distally; antennomeres 5, 6 enlarged (Fig. 3, Mb-805579). Five sensillum types: 4 apical cones (AS) oriented in a trapezoidal cluster on 7th antennomere, with longitudinally grooved outer surface and apical circular invagination (Fig. 13, Mb-805590). Chaetiform sensilla (CS) widely spaced on antennomeres 1-7, each sensillum with 2 or 3 barbules (Fig. 14a, Mb-805591). Trichoid sensilla (TS) oriented apically encircling antennomeres 1–7, lacking barbules (Fig. 14b, Mb-805592). Small basiconic sensilla (Bs₂) in clusters of 7 or 8; in slight depressions oriented apical dorsally (retrolaterally) on antennomeres 5 and 6; smooth, finger-shaped, 1/2 length of chaetiform sensillum (Fig. 4, Mb-805593). 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 (Fig. 13b, Mb-805594). Antennae extend posteriorly to middle of 3rd tergite. Relative antennomere lengths 6>2>5>3>4>1>7. Segments:Collum not covering head, with straight cephalic edge, gradually tapering laterally (Fig. 2b, Mb-805595). Collum with carina present on anterolateral margin, appearing scaly (Fig. 15, Mb-805596). Carina repeated serially on lateral tergal and pleural margins (absent from telson). Lateral tergal and pleural carinae jagged, pronounced on midbody segments (Fig. 16a, Mb-805597). Lateral margin of collum round. Tergites: Metazonites rectangular, 3× wider than long, slightly convex (Fig. 17, Mb-805598). Paranota absent. Metazonite dorsal surface pilose, covered with long, slender setae (Fig. 2, Mb-805599). Tergal setae hollow, cavity diameter 1/8 that of setae diameter; tipped with silk-like exudate, tangled, appearing adhered to neighboring setae (Fig. 18, Mb-805600). (NB: Tergal silk-like exudate observed in scanning electron micrographs, and by the observation of fine strands issuing from the metaterga of live individuals, viewed while magnified at 80× with a stereomicroscope. Silk stickiness was indicated by increased adherence of soil particles after handling and live observation of the millipede’s coiled body becoming stuck together.) Metazonite posterior margin (limbus) lined with posteriorly projecting anchor-shaped spikes and a row of conical spikes just dorsal to anchor-shaped spikes (Fig. 17a, Mb-805601). Anchor-shaped spikes alternating in size (large, small) along row. Ozopores oriented dorsally, located near limbus, absent from tergites 1 – 3 and telson. Ozopores elevated slightly (porosteles absent), with 2 stout posteriorly projecting spines and encircled by 13 – 15 robust setae (Fig. 19, Mb-805602). 3 or 4 stout flat tubercles opposite ozopore near anterior margin, lunate arrangement encircling ozopore (Fig. 17b, Mb-805603). Posterior tergites more convex, covered with a greater density of long, slender “silk”-exuding setae (Fig. 20, Mb-805604). Lunate-arranged tubercles opposite ozopores on posterior metazonites: conical and spiked, not flat. Apodous segments lacking sterna, pleurites contiguous in midline. Apodous tergites densely setose, covered with unevenly distributed spikes (Fig. 21, Mb-805605). Telson densely covered with irregularly oriented and unevenly distributed stout spines; posterior margin lined with variably-shaped posterodorsally oriented anchor-shaped spikes. Tergal tubercles and spikes: consistently projecting posteriorly, occasionally posterodorsally. Prozonite highly sculptured, with 5 rows of discoidal flat tubercles; anterior 3 rows staggered and posterior 2 rows aligned (Fig. 22, Mb-805606). Pleurites quadrate, flat, with jagged scaly lateral, posterior and medial margins (Fig. 16, Mb-805609). Pleurite medial margin broad, with scaly carina (Fig. 16b, Mb-805610). Left and right pleurites plate-like, comprising 4/5’s of ventral segment space. Left and right pleurites broadly overlapping sternite, covering spiracles (Fig. 23, Mb-805612). Sternites free, separate from pleurites; heart-shaped, wider anteriorly. Sternal surface with broad, jagged scales. Medial sternal ridge projecting ventrally, with spiracles and legs oriented ventrally (Fig. 24, Mb-805614). Spiracles circular, orifice open; oriented dorsally above legs (Fig. 25, Mb-805615). Anterior and posterior sternites separate. Tergites, pleurites and sternites separated by arthrodial membrane (Fig. 20, Mb-805616). Arthrodial membrane between tergites and pleurites wider posteriorly. Telson pilose, covered with long, slender posteriorly recurved setae (Fig. 20, Mb-805628). Paraprocts semihemispherical, anterior margins slightly scaly. Epiproct absent. Hypoproct small, one-eighth area of paraproct, with row of posterior projecting setae. Legs: six subequally shaped podomeres, with coxa slightly shorter and tarsus slightly longer. Legs with sparse setae, appearing similar to trichoid sensilla, with 2 or 3 barbules. Coxae nearly contiguous medially, separated by thin sternal ridge. Large posteroventral D-shaped opening for eversible sac (Fig. 26, Mb-805618). Eversible sacs membranous, bulging slightly from opening (Fig. 24b, Mb-805620). Pregonopodal tarsus with stout bifurcate claw; dorsal subdivision thicker, more arcuate (Fig. 27, Mb-805621). Postgonopodal tarsus with two separate claws, co-terminal on tarsal apex; dorsal claw thick and arcuate, ventral claw thin and setiform (Fig. 16c, Mb-805623). 2nd leg pair with posteriorly oriented coxal gonapophyses; rounded, protuberant, one-third length of prefemur. Gonopods: 9^th, 10^th 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) shovel-shaped; in repose cupped sheath-like around flagelliform posterior gonopodal apex (podomere 6, Fig. 11, Mb-805584). Posterior gonopodal podomere 6 divided, comprising a bundle of 3 stylus-shaped articles (Fig. 5, Mb-805627, Fig 6a). 2 dorsal-most, longest articles of P6 laminate distally, recurved laterally, denticulate posterior margins, appearance similar to a chicken foot in rigor mortis (Fig. 12, Mb-805585, Fig 6a). Ventral-most, shortest article of P6 acuminate distally, spike-like. Thin ridge-shaped sterna present between left and right gonopods, thicker between anterior gonopods.

(♀) VMNH – Counts and measurements: p = 190. a = 2. l = 750. (190 + 2 + T). HW = 0.37. HL = 0.44. ISW = 0.30. AW = antennae missing. CW = 0.44. W1 = 0.58. W2 = 0.58. W3 = 0.57. L1 = 0.23. L2 = 0.21. L3 = 0.23. H1 = 0.46. H2 = 0.44. H3 = 0.48. AS1 = 0.44. BL = 40.40. Anatomical description similar to male holotype. In combination with its measurements, the following structures differ from male holotype. Head triangular, chevron-shaped, tapered anteriorly to round point at a 135° angle anterior from antennal sockets; occipital area posterior from antennal sockets straight, not curved medially towards neck. Cyphopods large, area 1/6 the segmental area in widest cross-section; almond-shaped, bivalvular, narrow apex oriented ventrolaterally. Valves transparent, glassy. Ventral valve thickened and clam-like, with 4 or 5 thick setae; dorsolateral valve thin and flat, with 2 or 3 spines. Oviduct connected posteriorly to cyphopod, opening oriented ventromedially and located between valves. Oviduct tube wrinkled, appearing highly expandable in width, cross-section 1/8 area of cyphopod. Receptacle, suture and operculum absent.

Cook and Loomis (1928) named this species “in highest fulfillment of feet”. Il = “in” (Latin); acme, άκμή (Greek) = “the highest point, or culmination”; pleni = “full” (Latin); pes = “foot” (Latin).

There is negligible variation in coloration among live specimens. (FSCA paratype specimens that have been stored in alcohol for 86 years are dark mahogany brown, which is likely an unnatural color and a result of alcohol preservative, vial stopper and age.) The predominant source of variation between specimens is segment and leg counts (Tables 1–3). Females have between 486-750 legs with a standard deviation of 78, and males between 318–562 legs with a standard deviation of 107. The segments of Illacme plenipes (males and females) are uniform in length, width and height along the trunk, and are slightly taller, and more convex, in posterior segments—potentially to accommodate the spiraled metenteron.

Illacme plenipes specimens were collected during the day in a small valley adjacent to cattle pasture. The woodland habitat was primarily composed of California live-oak, Quercus agrifolia (Fig. 28). Understory flora included ferns (bracken, Pteridium aquilinum; California polypody, Polypodium californicum; and California maiden-hair, Adiantum jordanii), California blackberry (Rubus ursinus), and poison oak (Toxicodendron diversilobum) (Fig. 29). Specimens were found beneath large moss-covered boulders, typically with a mass > 30 kg (Fig. 30). The millipede Tylobolus uncigerus (Wood, 1864) (order Spirobolida) was found co-occurring with Illacme plenipes at this locality. Other arthropods encountered include: Aptostichus and Calisoga trapdoor spiders (Mygalomorphae), Evalljapyx (Diplura), and Promecognathus ground beetles (Carabidae). Edaphic setting: Specimens collected in 2007 were found beneath a large stone (Fig 30, about 30 kg). When the stone was removed, individuals were seen corkscrewing outward into the cavity from the soil (Fig. 31). The soil, consisting of moist small-grained substrate, was dark chocolate brown in coloration and somewhat sandy (Fig. 31). The soil did not contain clay particles and seemed to drain water quickly. During the 16 December 2007 collections, soil moisture extended 15 cm below the surface.

Illacme plenipes is only known from a small area, ca. 4.5 km in diameter, in the northwestern foothills of the Gabilan Range in San Benito County, California.