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Comprehensive Description

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Original Description

Illacme, new genus

Type. — Illacme plenipes, a new species from California.

Diagnosis. — Similar to Siphonophora and Siphonacme but readily distinguished by the triangular-cordate head, narrowed gradually to an acute angle below, but showing no tendency to form a projecting beak.

Description. — Body extremely slender, moniliform, about fifty times as long as broad; composed of a very large number of segments, in the longest specimens approaching 200; dorsum strongly convex, nearly semicylindrical; the surface shining, moderately hirsute.

Head triangular-cordate in outline, resembling that of Polyzonium and without any suggestion of a beak, the sides continuous and sharply converging to a rather blunt angle, but not produced into a snout. Position of the head nearly vertical to the body, not reflexed under the body.

Antennae inserted at the sides of the head, abruptly capitate-clavate, subgeniculate; joints 1 to 4 very small, joints 5 and 6 abruptly thicker and larger, held parallel to the sides of the head; joint 6 the longest, but scarcely as broad as joint 5; joint 7 broad and short.

First segment slightly narrowed, about half again as long as the second; oblong, the lateral margins evenly rounded, anterior margin nearly parallel with the posterior; the surface nearly even, lacking the abrupt convexity of the posterior subsegments on the rest of the body; segments 2 to 4 shorter than the following, the transverse constriction with an irregular row of rounded tubercles on the anterior slope; surface of anterior subsegments rather coarsely reticulate, surface of posterior subsegments finely hirsute, each of the hairs subtended by a short sublunate transverse groove, giving the general effect of an indistinct network; supplementary margin distinct, with regular truncate divisions.

Repugnatorial pores difficult to detect, located on slight prominences near the posterior margins of the segments, not close to the lateral margins, pores of segment 5 in the same position as the others; lateral sutures open, bordered by slight parallel ridges; surface of pleurae with a rather open reticulation, sparsely hirsute along the posterior margins.

Last three segments somewhat longer than the preceding; the anal segment nearly as long as the penultimate, the sides converging to a broadly rounded apex, even with the valves, not projecting; surface of the anal valves rather strongly and evenly convex, the margins not prominent; preanal scale not distinct.

Legs rather short, scarcely exceeding the sides of the body, the basal joints prominent and swollen on the inner side, sometimes with an extruded membrane or exudate, nearly in contact on the median line.

This genus separates at once from Siphonacme and Siphonophora by the absence of a beak. The antennae are different from those of Siphonacme in having joints 1 to 4 much narrower and more slender, while joint 5 is abruptly larger and more nearly equal to joint 6. Also the body is much more slender and the number of segments is much greater, approaching twice as many, and perhaps exceeding any other milliped. A diplopod with 192 segments, as counted on one of these specimens, has a total equipment of 750 legs, and may be the nearest approach to a literal " thousand legs."

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