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Montana Field Guide

Montana Field Guides

A Millipede - Orthogmus oculatus

Species of Concern
Native Species

Global Rank: G1G3
State Rank: S1S3

Agency Status


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General Description
The validity of this species uncertain. The male holotype apparently immature, lacking one molt to maturity and with 28 body segments (Shear 1976, 2003). The female paratype is mature (with 30 body segments) and attributable to a valid new genus (Loomis and Schmitt 1971; but see Shear 1976, 2003), although the family affiliation remains unclear.

Genus Orthogmus: Body strongly pigmented; eyes fully developed in a subtriangular group; antennae quite short and stout, the 3rd joint longest. Gnathochilarium with an undivided mentum. Segments with ateral keels well developed, evident to body segment 26. Dorsal setae larger and about half as long as thickness of body, rising from large tubercles borne on swellings of the surface, the 3 setae each side of middle in an almost straight but slightly oblique line. Metazonites convex over much of length but flattened for a space along posterior margin, except for a few seqments at ends of body. Anterior gonopods reduced to low, simple, subhemispherical structures. Legs 9 apparently 3-jointed, in utline much resembling those of Scoterpes but with 3rd (or outer) joint indistinct and with a tiny claw. Pregenital legs and sterna unmodified except that outer joint of legs 1 and 2 with a comb of small setae along ventral surface. Leg 10 with pore opening inward from mesodistal face of coxa, other legs and strna behind gonopods unmodified. (Loomis and Schmitt 1971).

[From Loomis and Schmitt 1971] Orthogmus oculatus: Males to about 10.0 mm length, females to about 13.0 mm, about 1.5 mm width. Gunmetal gray in alcohol. Eyes composed of subequally large, well-formed ocelli in a triangular group of 4 rows: 7, 5, 4, 2, counting from back of head; little variation in number of ocelli. Antennae rather stout, joints increasing in length as follows: 1, 7, 6, 2, 4, 5, 3, with joints 2 and 3 more slender than any that follow. Gnathochilarium with undivided mentum. Body segment 1 with row of setae each side bowed obliquely forward from outer one in lateral angle to the submedian one far in front of posterior margin. Succeeding body segments with rows of setae rapidly becoming straighter with scarcely any evident curve posterior of segments 3 or 4; rows somewhat oblique, however. Setae rise from conspicuous hemispherical tubercles borne on strong swellings of surface, the outer 2 tubercles and swellings largest. Metazonites (except near body ends) with anterior 3/4 or more strongly convex, remainder flat. Preanal scale about 1/2 long as wide and almost semicircular, the apex extending well beyond the 2 subterminal setae. Anterior gonopods as described for genus, what appears a basal joint on leg 9 is part of sternum; anterior male legs and sterna unmodified except for a comb of fine setae beneath outer joint of legs 1 and 2. Legs 10 as described for genus.

Limited information, in part because of uncertain taxonomy. Adults reported mid-June to early August, young and large immatures in early August (Loomis and Schmitt 1971).

Diagnostic Characteristics
See genus in the General Description above. Similar to Scoterpes but with larger body, strongly developed eyes, body well pigmented, anterior gonopods much simpler; differs from Endopus by the interior portion of metazonites swollen, dorsal setae in a straight transverse row, the lateral keels larger.

Range Comments
Montana endemic. Reported from Sanders County (Loomis and Schmitt 1971), at about 670 m elevation. May be more widespread (depending on taxonomic identity).

Observations in Montana Natural Heritage Program Database
Number of Observations: 2

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

Not described for this species (Loomis and Schmitt 1971), but same collection locality under Orophe cabinetus mentions conifer forest of western red cedar with devil's club in understory.

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Citation for data on this website:
A Millipede — Orthogmus oculatus.  Montana Field Guide.  .  Retrieved on , from