Idagona westcotti Buckett and Gardner 1967

Shear, William A., 2007, Cave millipeds of the United States. V. The genus Idagona Buckett & Gardner (Chordeumatida, Conotylidae, Idagoninae), Zootaxa 1463, pp. 1-12 : 4-6

Idagona westcotti Buckett and Gardner 1967

Figs. 1, 2 , 5, 6, 9, 10, 15, 16

Idagona westcotti Bucket and Gardner 1967 , p. 120, figs. 1-8. Shear, 1972, p. 270, figs. 479-481.

Types. Male holotype in Department of Entomology, University of California, Davis, California. “…one male paratype sent to H. F. Loomis, Miami, Florida; allotype retained in the authors’ private collection; remainder of paratypes divided between University of Idaho, Moscow, Idaho, and the authors’ private collection ( Buckett and Gardner 1967, p. 124).” The entire type series consisted of the male holotype, one female allotype, three male and three female paratypes. The series was collected in Crystal Falls Cave, 20 mi. (32.2 km) northeast of Dubois, Clark Co., Idaho, 16 July 1965, by R. L. Westcott. An additional female specimen, not designated as a paratype, was collected by Westcott on 18 August in Boy Scout Cave, Craters of the Moon National Monument, Butte Co., Idaho ( Buckett and Gardner 1967).

New Records: Idaho: Clark Co., Blowhole-Sand Lake Cave, 14 July 2000, D. A. Hubbard, males, females; Ice Capades Cave, 14 July 2000, D. A. Hubbard, males, females. Blaine Co., Government Cave, 16 July 2000, D. A. Hubbard, males, females. Fremont Co., London Tunnel Cave, 15 July 2000, D. A. Hubbard, juveniles presumably this species. Lincoln Co.: Giant Arch Cave, 16 July 2000, D. A. Hubbard, males, females; Pot O’Gold Cave, 16 July 2000, D. A. Hubbard, males, females (Giant Arch Cave and Pot O’Gold Cave may be parts of the same lava tube, or different names for the same cave). These specimens are at present in the author’s personal collection but will be deposited in the Field Museum of Natural History, Chicago, when studies are complete.

Notes. The drawings in Shear (1972, figs. 479–481) are in general accurate and serve for recognition of the species. Other drawings and a detailed written description are given in Buckett and Gardner (1967). Scanning electron micrographs are provided here for additional detail; they show that what Shear (1972) interpreted as fimbriate, or hairy, margins on the anterior gonopods are actually arrays of stout cuticular teeth.

The species may be general in lava tubes through the extensive Snake River Plain lava beds of southern Idaho (Map 1). Comparing the specimens from the listed localities, I found no differences of note and therefore treat the populations as a single species. Even those from the two most distant clusters of records have gonopods, and third and tenth legpair modifications that are virtually identical ( Figs. 1, 2 , 5, 6, 9, 10, 15, 16). It is worth noting,however, that the available collections represent samples from within a larger potential distribution; more collecting and examination of molecular evidence is needed to clarify the actual status of these populations. Howarth (1973; see also Peck 1973, 1982) emphasized that troglophilic or troglobitic animals in lava beds may occupy small spaces in the lava inaccessible to collectors and use these spaces to spread to new lava tubes. Thus I. westcotti in the Idaho lava beds may well be a single panmictic population, rather than a series of discrete populations isolated in larger lava tubes. Because of their northerly location and the insulating properties of the porous lava, many, but not all, of the tubes in which I. westcotti occurs have permanent ice and constant temperatures around 4°C. This does not seem to have an adverse effect on the diverse fauna found there, which includes beetles, centipeds, and opiliones as well as millipeds ( Briggs 1973, 1974; Peck, 1973).

Idagona westcotti shows few consipicuous adaptations to cave life, such as enlongate appendages, depigmentation and reduction or loss of eyes. Most specimens are only slightly paler than epigean conotylid species found in the Rocky Mountains and the Pacific Northwest, and while ocelli appear to be somewhat reduced in number and size, the change is not very conspicuous. However, given the bleak sagebrush desert and dry juniper woodland habitat surrounding the lava tubes, it seems unlikely that surface populations exist, and that the animals are now effectively limited to subterranean habitats.

The prevailing theory of the colonization of western North American lava tubes emphasizes that the arthropod faunas of these caves are derived from taxa that are mesic forest litter inhabitants. During glacial maxima in the Pleistocene, such forest habitats occupied much lower elevations than they do now, and litter animals were available to enter, survive, and reproduce in caves. As the climate became warmer and much drier under interglacial conditions, the forests retreated to higher elevations, leaving behind remnant animal populations in caves, the surrounding environment having become inhospitable for them. Depending on such factors as time since isolation, population size and selective regime, the isolates may diverge and eventually speciate ( Peck 1973, 1982). The pattern of distribution seen in I. westcotti indicates that the species was widespread on the surface prior to environmental change and since then the lava tube populations have diverged little, at least morphologically.

Figures

FIGURES 1 – 4. Gonopods of Idagona species. 1, Idagona westcotti, Giant Arch Cave, posteriorventral view. 2. I. westcotti, Ice Capades Cave, posterioventral view. 3. I. jasperi, n. sp. posterioventral view. 4. I. lehmanenesis, n. sp., posterior view.