ILLACME PLENIPES, new species.
Body very slender and flexible, filiform, strongly and evenly convex, moderately pubescent on all of the exposed surfaces; number of segments attaining 192, with a length of 36 mm. and a width of 0.7 mm. in the largest female specimen; other individuals 26 to 29 mm. long, 0.5 to 0.6 mm. wide, with 136 to 152 segments.
Head rather narrowly triangular-cordate, the vertex more densely hirsute, the hairs rather short, the clypeus with longer and sparser hairs, nearly naked above the rather blunt-pointed labium; position of head nearly vertical, not strongly recurved under the body.
Antennae inserted at the sides of the head, moderately hirsute, abruptly capitate-clavate, subgeniculate, the terminal joints carried at the sides of the head, the second and third joints at the lateral margin of the first segment; joints 2 to 4 gradually thicker but much smaller and narrower than joints 5 and 6; joint 2 somewhat longer than joints 3 and 4, which are subequal and nearly as broad as long; joint 5 also about as broad as long, but much thicker than joint 4; joint 6 slightly narrower than joint 5, and distinctly longer, about one and one-half times as long as broad, cylindric-oval, slightly narrowed toward the end; joint 7 projecting as a rather broad frustum about one-sixth of the length of joint 6; olfactory cones not prominent.
First segment with the lateral margins evenly rounded and the anterior margin nearly parallel with the posterior; the surface more even than on other segments, which are abruptly convex behind the transverse constriction.
Penultimate segment without legs, the large pleura meeting in the middle and apparently united, but the sutures indicated by a fine median groove; last segment converging to a broadly rounded apex, not projecting beyond the margins of the valves, scarcely equal to the margin when viewed from the side.
Type.—Cat. No. 976, U.S.N.M.
Numerous specimens were collected by O. F. Cook in San Benito County, Calif., November 27, 1926, a short distance after crossing the divide between Salinas and San Juan Bautista. Only one colony was found, in a small valley of a northern slope wooded with oaks, under a rather large stone. The living animals were nearly white, moved very slowly, and rolled themselves into regular, close spiral coils when disturbed, the coils with three or four turns.
Catalog Number: USNM
Collection: Smithsonian Institution, National Museum of Natural History, Department of Entomology
Collector(s): O. Cook
Year Collected: 1926
Locality: Near divide between Salinas + San Juan bautista; San Benito, California, United States
Illacme plenipes is a siphonorhinid millipede found in the central region of the U.S. state of California. It has up to 750 legs, more than any other animal in the world. The only species in the genus Illacme, it was first seen in 1926, but was not rediscovered until 2005.
Although no known millipede species has one thousand legs, Illacme plenipes comes the closest with one recorded specimen having 750 legs. On average, they have over 600 legs, twice the average for millipede species. Despite having more legs than any other creature on Earth, it is actually quite small, even relative to other millipedes. Females grow to just over an inch; males are slightly smaller and have fewer legs.
The species was first discovered in San Benito County, part of the California Floristic Province, in 1926 by federal scientist Orator Cook and formally described by Cook and Harold F. Loomis in 1928. Cook and Loomis described the species without illustration, and it was not until 1996 that the first illustrations were provided by Rowland Shelley of the North Carolina Museum of Natural Sciences, who re-examined the original specimens. However, the species was not seen alive until it was rediscovered in November 2005, almost 80 years after its discovery, by Paul Marek, then a Ph.D. student at East Carolina University. Marek published his discovery in the journal Nature. Marek and colleagues produced a more detailed description of the anatomy and ecology of I. plenipes in 2012.
Illacme is a member of the order Siphonophorida and family Siphonorhinidae. It is the only known siphonorhinid in the western hemisphere, although Shelley suggested more species may be found in Southern California and Mexico. The closest relative of Illacme is undetermined, but a likely candidate is Nematozonium filum of South Africa which shares a long, narrow body shape and characteristics the gonopods.
- Marek, P.; Shear, W.; Bond, J. (2012). "A redescription of the leggiest animal, the millipede Illacme plenipes, with notes on its natural history and biogeography (Diplopoda, Siphonophorida, Siphonorhinidae)". ZooKeys 241 (241): 77–112. doi:10.3897/zookeys.241.3831. PMC 3559107. PMID 23372415.
- O. F. Cook & H. F. Loomis (1928). "Millipedes of the order Colobognatha, with descriptions of six new genera and type species, from Arizona and California". Proceedings of the United States National Museum 72 (18): 1–26, f. 1–6, pls. 1–2. doi:10.5479/si.00963801.72-2714.1.
- Shelley, Rowland M. (1996). "The milliped order Siphonophorida in the United States and northern Mexico". Myriapodologica 4 (4): 21–33.
- "ECU students rediscovers rare millipede". East Carolina University. June 15, 2006. Retrieved August 27, 2010.
- Paul E. Marek and Jason E. Bond (2006). "Biodiversity hotspots: rediscovery of the world's leggiest animal". Nature 441 (7094): 707. doi:10.1038/441707a. PMID 16760967.
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