Millipedes

There have been around 7,000 species of millipedes (sometimes spelled millipeds) described scientifically, with a projected estimate of possibly 80,000 total species occurring around the globe. Around 900 species have been described from the United States and Canada. However, the family Parajulidae, the largest group from North America, has not been studied thoroughly, suggesting that numerous more species exist in North America. Arkansas has a diverse array of millipede species. Some of them are fairly common, while others are rare.

Millipedes—their name meaning “thousand feet”—make up a group of invertebrates (lacking a backbone) in the phylum Arthropoda. Within the Arthropoda phylum, millipedes belong in the subphylum Myriapoda, which includes four classes: Chilopoda (centipedes), Diplopoda (millipedes), Paurapoda (paurapods), and Sumphyla (symphylans). Of the four classes, most people are familiar with the centipedes and the millipedes.

The class Diplopoda has three unique characteristics: aflagellate spermatozoa, the presence of four or more apical sensory cones on each antenna, and the diplosegment condition. Millipedes can be found as they move slowly through dark, moist habits, such as under logs, beneath stones, and within leaf litter. When disturbed, they commonly roll up into a ball to protect their legs and head from predators. They are herbivores, mainly eating dead plant material.

Millipedes have two main body types: a primarily cylindrical body form, such as in the family Parajulidae, or the dorsoventrally flattened body types, such as in the family Eurymerodesmidae. A millipede’s head consists of a clump of simple eyes on each side and a pair of antennae, mandibles, and maxillae. A millipede’s body has between twenty-five and 100 segments with either a single or double row of feet on each segment, giving the illusion of a thousand feet (although millipedes only have up to 750 legs). The seventh segment appendages are often specialized into copulatory organs called gonopods. Many species of millipedes can be identified only by using morphological characteristics when viewing the male copulatory structures. Millipedes breathe through structures called spiracles. Each abdominal segment contains two spiracles, which open into air chambers connected to tracheal tubes. After millipedes mate, females lay eggs and guard them. Hatchlings hatch from eggs and have only a few pairs of legs. Each time they molt (shed their exoskeleton), they gain more segments and legs.

A very common species of millipede in Arkansas is Pseudopolydesmus pinetorum (Bollman). This species has a flattened body and is a brownish color. Other common millipedes include the families Parajulidae and Eurymerodesmidae. A few genera of millipedes within Arkansas are rather colorful, with yellow and orange markings on the body segments, such as in Apheloria, Auturus, and Eurymerodesmus. One species occurring within Arkansas, Narceus americanus, is a very large millipede with a brownish body with orange and green markings along the periphery of the segments. Brachycebe lecontei is a millipede that has a pink body.

Published collections of millipedes date back to the late 1800s. In 1888, Charles H. Bollman published the “preliminary checklist of the Myriapoda of Arkansas, with descriptions of new species” in Entomologica Americana. Dr. Nell Bevel Causey, who held a faculty position at the University of Arkansas (UA) in Fayetteville (Washington County), authored numerous species and genera, many of which are endemic to Arkansas. Much of the knowledge of millipede species occurring in Arkansas can be contributed to this early work. Other diplopodologists who have contributed to the knowledge of millipedes are William A. Shear and Richard L. Hoffman. As recent research conducted in Arkansas by Rowland M. Shelley and Chris T. McAllister has increased the known species and distributional limits within the state, there is not a current comprehensive species list for Arkansas.

Some millipede species in Arkansas, such as Euryurus leachii (Koch), are typically more commonly found on the eastern side of the Mississippi River but do occur within the state. Yet, others, such as Abacion texense (Loomis), are more commonly found in the central portion of the United States but do occur east of the Mississippi River.

Arkansas is home to nineteen species of endemic millipedes; however, some of these species need to be re-described and may contain synonyms (same species with multiple scientific names). They include Abacion wilhelminae (Shelley, McAllister, and Hollis), Boraria profuga (Causey), Causeyella causeyae (Shear), Causeyella youngsteadtorum (Shear), Cleidogona arkansana (Causey), Eurymerodesmus compressus (Causey), Eurymerodesmus newtonius (Chamberlain), Eurymerodesmus polkensis (Causey), Eurymerodesmus pulaski (Causey), Nannaria davidcauseyi (Causey), Nannaria depalmai (Causey), Okliulus beveli (Causey), Petaserpes bikermani (Causey), Tiganogona glebosa (Causey), Tiganogona ladymani (Causey), Tiganogona moesta (Causey), Tiganogona steuartae (Causey), and Trigenotyla parca (Causey).

Abacion wilhelminae is endemic to Rich Mountain in Polk County. Boraria profuga is found within the Ouachita Mountains uplift and is probably endemic to that physiographic region. Causeyella causeyae is a cave-dwelling millipede that is eyeless and unpigmented; it occurs within the Ozark Mountains of Arkansas. Causeyella youngsteadtorum is also endemic to the Ozarks of Arkansas, occurring in Newton, Boone, and Searcy counties. Cleidogona arkansana is known only from the type locality in Dallas County. Eurymerodesmus compressus is known from Union County. Eurymerodesmus newtonius occurs in Benton, Newton, and Washington counties. Eurymerodesmus polkensis occurs in Montgomery, Polk, and Scott counties. Eurymerodesmus pulaski is known from Saline and Pulaski counties. Nannaria davidcauseyi is known from Newton County, and Nannaria depalmai is known from Carroll County. Okliulus beveli is known only from Union County. Petaserpes bikermani (Causey) produces a defensive secretion that smells like camphor. Tiganogona glebosa occurs in Washington County, Tiganogona ladymani occurs in Clay County, Tiganogona moesta occurs in Carroll and Washington counties, and Tiganogona steuartae occurs only in Sebastian County. Trigenotyla parca occurs primarily in caves and is known from Carroll County.

Due to their affinity for dark, moist habitats, millipedes are often found within cave systems or mines. These can include rare species that are found only within caves (troglobites) and more common surface species. Some rare millipedes found in caves include the genus Causeyella, which is found only within cave systems in the Ozarks, and Tingupa pallida, known from caves in Randolph and Sharp counties. Another common cave millipede is Cambala minor, which is not a true troglobite but does have a lightly colored body and is eyeless.

Of potential concern to cave ecosystems is the exotic and invasive greenhouse millipede Oxidus gricilus, which can be quite common in some cave systems. This species, however, is not limited to cave environments, and the potential harm this exotic millipede might bring to native fauna is not known at this time. Native environments, especially cave systems, can be very fragile and susceptible to threats from exotic species of wildlife.

Millipedes, especially Arkansas’s species, are essentially harmless to humans, as they lack structures to bite, pinch, or sting. However, when handled, millipedes often emit defensive secretions. These secretions are generally harmless, but some people can develop skin and eye problems after contact. Millipedes that emit defensive secretions often exhibit aposematic coloration (colors warning about defensive secretions).

In general, millipedes can be found wherever the habitat is moist. They play an integral ecological role, as they are important in nutrient recycling of plant material. They fragment this plant material (detritus), increasing microbial breakdown and soil nutrient cycling. They are more commonly seen in the spring and fall months of the year when the temperatures are not too hot or too cold and there is ample moisture in the ground. During certain times of the year (such as in the fall months when temperatures and moisture levels align), large numbers of millipedes can be seen moving about. Although Arkansas is home to some rare and even endemic millipedes, many species are easily encountered just by flipping logs, rocks, or other debris.

For additional information:
Hopkin, S. P., and H. J. Read. The Biology of Millipedes. Oxford, UK: Oxford University Press, 1992.

McAllister, C. T., C. S. Harris, R. M. Shelley, and J. T. McAllister III. “Millipeds (Arthropoda: Diplopoda) of the Ark-La-Tex. I. New Distributional and State Records for Seven Counties of the West Gulf Coastal Plain of Arkansas.” Journal of the Arkansas Academy of Science 56 (2002): 91–94. Online at http://libinfo.uark.edu/aas/issues/2002v56/v56a13.pdf (accessed September 13, 2021).

McAllister, C. T., and H. W. Robison. “Millipeds (Arthropoda: Diplopoda) of the Ark-La-Tex. V. New Distribution Records for Select Taxa in Arkansas, Oklahoma, and Texas.” Southwestern Naturalist 56, no. 3 (2011): 422–426.

McAllister, C. T., H. W. Robison, M. B. Connior, and L. C. Thompson. “Millipeds (Arthropoda: Diplopoda) of the Ark-La-Tex. VI. New Geographic Distributional Records from Select Counties of Arkansas.” Journal of the Arkansas Academy of Science 67 (2013): 87–93. Online at http://libinfo.uark.edu/aas/issues/2013v67/v67a14.pdf (accessed September 13, 2021).

McAllister, C. T., R. M. Shelley, and J. T. McAllister III. “Millipeds (Arthropoda: Diplopoda) of the Ark-La-Tex. II. Distributional Records for Some Species of Western and Central Arkansas and Eastern and Southeastern Oklahoma.” Journal of the Arkansas Academy of Science 56 (2002): 95–98. Online at http://libinfo.uark.edu/aas/issues/2002v56/v56a14.pdf (accessed September 13, 2021).

———.“Millipeds (Arthropoda: Diplopoda) of the Ark-La-Tex. III. Additional Records from Arkansas.” Journal of the Arkansas Academy of Science 57 (2003): 115–121. Online at http://libinfo.uark.edu/aas/issues/2003v57/v57a15.pdf (accessed September 13, 2021).

Robison, H., C. McAllister, C. Carlton, and G. Tucker. “The Arkansas Endemic Biota: An Update with Additions and Deletions.” Journal of the Arkansas Academy of Science 62 (2008): 84–96. Online at http://libinfo.uark.edu/aas/issues/2008v62/v62a12.pdf (accessed September 13, 2021).

Matthew B. Connior
South Arkansas Community College