Coloradesmus, Shear & Steinmann, 2019

Shear, William A. & Steinmann, David B., 2019, Cave millipedes of the United States. XV. Coloradesmus gen. nov. (Diplopoda, Polydesmida, Macrosternodesmidae), and four new species from caves in Colorado, USA, Subterranean Biology 32, pp. 15-32 : 18-20

Coloradesmus gen. nov.

Type species.

Speodesmus aquiliensis Shear, 1984.

Diagnosis.

Distinct from the similar Pratherodesmus Shear, 2009 in its much larger gonopod solenomere and endomerite, from Tidesmus Chamberlin, 1943, Sequoiadesmus Shear & Shelley, 2008 and Nevadesmus Shear, 2009 in having a simple, unbranched endomerite, or endomerite lacking. Packardesmus Shear & Shelley, 2019 has all gonopod branches clustered at the tip of an extended prefemoral stem.

Description.

Small, probably troglobiotic macrosternodesmines 4.0-11.0 mm long, lacking pigment. Nineteen trunk rings (collum + 17 pedigerous rings + telson). Head sparsely to densely setose. Antennae ( Fig. 22 ) relatively short, elbowed between antennomeres three and four, antennomere six much enlarged, with subapical accessory sensory organ. Order of length of antennomeres: 6>3=4>5>2>7>1>8. Collum with three rows of eight setae (number may vary on more posterior rings); anterior row at anterior margin of collum, middle row may be dispersed, posterior row at posterior margin of collum. Collum setae on low tubercles or sockets more or less flush with surface. Subsequent rings with anterior row posterior to anterior margin of metazonite, but posterior row at posterior edge of metazonite, rows of 6-10 setae or setae becoming more scattered on more posterior rings. Metazonites with narrow paranota bearing three short, marginal teeth corresponding to setal rows, posteriolateral metazonite corners acute to projecting. Limbus minutely dentate. Ozopores laterodorsal, at posteriolateral corners, opening in a distinct pore callus. Pore formula 5, 7, 9, 10, 12, 13, 15-18. Penultimate ring with 10-18 scattered setae, telson with 8-10 scattered setae, epiproct process short, blunt, with four spinnerets set in shallow depression. Paraprocts and hypoproct with two setae. Dorsal setae post-collum on more prominent tubercles, setae themselves may be long, acute, or short, clavate. Males with all legs having dorsally swollen prefemora, femora also dorsally swollen, curved, both podomeres ventrally with many sphaeotrichomes; postfemora and tibiae normal, with few ventral sphaerotrichomes or sphaerotrichomes absent ( Fig. 4 ). Gonopod socket rounded-cordate, often with anterior rim, midposterior portion extending slightly between ninth coxae. Gonopods joined by tough membrane anteriorly; small, more sclerotized strip represents sternal remnant, articulates mesally with coxae. Gonopod coxae not movable, tightly appressed in midline but not fused, anteriomesally excavate to receive telopodite. Telopodites movable only in plane parallel to body axis. Prefemorites transverse across posterior surface of coxae. Prefemoral process single, broad, prefemoral process and acropodite on short stem arising from prefemorite. Acropodite with long seminiferous branch, pore surrounded by minute cuticular extensions, these sometimes extending distad along concave mesal surface of distal zone. Distal zone long, tapering, acute, or short, blunt. Endomerite arising basally or midway on acropodite.

Etymology.

From the state of Colorado, to which the genus appears endemic, and the common combining stem - desmus in the order.

Species included.

In addition to the type species, the following new species: beckleyi , warneri , hopkinsae and manitou .

Distribution.

See Map 1 . Caves in central and northern Colorado, USA. Species distributions appear to be defined by major rivers and large canyons. Groaning Cave where C. hopkinsae occurs is across Deep Creek Canyon from La Sunder Cave where C. beckleyi occurs and this canyon is 2,000' deep. The Colorado River may define the limits of C. aquiliensis relative to C. beckleyi and C. hopkinsae because C. aquiliensis has only been collected south of the Colorado River, while the other two species were collected north of the river. It is likely that Coloradesmus will extend into Wyoming because C. warneri was collected 5 miles from the state line.

Notes.

Two species groups are recognizable in this genus, distinguished primarily by size and the presence or absence of a distinct endomerite. Coloradesmus aquiliensis , C. hopkinsae and C. manitou are 0.6 mm or greater in width and from 5-11 mm long, depending on the contraction or extension of the body. Coloradesmus beckleyi and C. warneri are considerably smaller, about 0.4 mm wide and 4 mm long, placing them among the smallest of all millipedes. The former three species have distinct endomerites, while endomerites seem lacking in the latter two. Division of Coloradesmus may prove desirable in the future but for now we prefer to group all the species in a single genus.

All specimens of Coloradesmus were collected in the dark zones of limestone caves. The preferred habitat for the genus appears to be caves with moist organic materials including wood, scat and guano. Millipedes representing Coloradesmus were found under rocks, burrowing in cave soils, and on wet cave formations. Many of the caves where Coloradesmus occurs are remote and at high-altitudes with temperatures of 2-4 °C.

Establishing troglobiosis is difficult in macrosternodesmines due to the small size and depigmented appearance of nearly all species except those of Tidesmus . Eyelessness is not a marker of troglobiotic adaptation in Polydesmida , since all known species of the order, some thousands, are eyeless. However, despite antennae and legs that seem not much elongated compared to those of litter-dwelling species of Chaetaspis , the species of Coloradesmus have a loose-jointed, elongated appearance as a whole that, along with the weak sclerotization of the rings, suggests a significant degree of adaptation.

As WAS has repeatedly emphasized in previous publications, collecting in caves has generally been more intense than surface collecting, especially in western parts of the United States. Further, suspect troglobionts tend to be very small and would only be found outside caves by very careful sifting of forest litter at an appropriate season of the year, if they exist there. Many caves in the Rocky Mountains are at high altitudes or are situated in surroundings that would not be very conducive to the survival of small, delicate arthropods on the surface. The mesovoid space of small cavities from a few centimeters to meters underground has not been explored by collectors in western North America.

Figures

Map 1. Northwestern Colorado, showing the distribution of Coloradesmus species. Green area indicates Colorado Rocky Mountain Forest Ecoregion. Symbols may represent more than one locality as some caves are very close to one another. Stars, C. aquilensis, triangles, C. hopkinsae, squares, C. manitou, circles, C. buckleyi, hexagon, C. warneri.

Figures 21 - 25. Coloradesmus species males. 21 C. manitou gonopods, posterior view 22 - 24 C. lasunda male 22 head, collum and anterior four rings, dorsal view 23 collum, dorsal view 24 midbody ring, dorsal view 25 telson, dorsal view. See text for explanation of labels.

Figures 1 - 6. Coloradesmus aquiliensis male. 1 Collum, dorsal view 2 midbody ring, dorsal view 3 telson, dorsal view 4 right leg 7, posterior view 5 gonopods, ventral view 6 gonopods, posterior view. See text for explanation of labels.