Diplurans

The primitive insects known as diplurans belong to the phylum Arthropoda, subphylum Hexapoda, class Insecta and order Diplura. They are considered to have three lineages: the Campodeoidea, Japygoidea, and Projapygoidea. These superfamilies are defined morphologically by three different types of cerci (paired appendages) found across all the dipluran families. There are ten families in this cosmopolitan order distributed from the tropics to the temperate zones. Diplurans belong to one of the four groups of Hexapoda, with other primitive apterygote insects, including springtails (Collembola) and coneheads (Protura). There are about 800 described species, of which around seventy (9%) occur in North America, twelve (2%) in the United Kingdom, and two (0.3%) in Australia. In 2016, species of diplurans were reported from Alaska for the first time. Not much is known about diplurans in Arkansas, although several specimens have been collected from the state.

The phylogenetic relationships are not well resolved among the four groups of hexapods, but the results of most recent research reject a monophyletic Entognatha (wingless ametabolous arthropods). The fossil record of diplurans is extremely poor. For example, japygid diplurans are extremely rare in the fossil record, with only three previously reported occurrences. The oldest known japygid, Testajapyx thomasi, from the Carbondale Formation of Illinois, dates from the Carboniferous period (360 to 299 million years ago). This early dipluran possessed well-developed compound eyes, long maxillary and labial palps, and multi-articulated abdominal leglets that more closely resembled those of true insects versus those of modern diplurans. Other japygid fossil specimens are from Eocene Baltic amber, Miocene Dominican amber, or Pliocene onyx marble from Arizona. In addition, the first Mesozoic japygid, Ferrojapyx vivax, was described from the Lower Cretaceous (Aptian) Crato Formation of northeastern Brazil.

Diplurans can be found in the top layer of moist soil, in forest leaf litter or humus, under stones and logs, or under bark in rotting wood. Diplurans are also often found in caves or abandoned mines as well as inhabiting the nests of ants and termites. They are rarely observed because of their small size and subterranean lifestyle. Diplurans serve an important role in the soil community of decomposers by breaking down dead organic matter and recycling organic nutrients. They also have biting mouthparts and ingest a variety of live prey. For example, members of Japygoidea are mainly predatory hunters that use their pincer-shaped cerci to capture and hold down prey, including springtails, isopods, small myriapods (centipedes and millipedes), insect larvae, and even other diplurans. Members of the Campodeoidea and Projapygoidea are generalist feeders on soil fungi, mites, springtails, and other small soil invertebrates, as well as detritus. Any species with long cerci can be expected to be herbivorous.

Diplurans are primarily small, ranging in size from 2–5 mm (0.08–0.20 in.) long, although some species in the genus Japyx may be up to 50 mm (2.0 in.) long. Among their derived characteristics, they have no compound eyes or ocelli, and, apart from the darkened cerci in some species, they are white or unpigmented. They have long, slender antennae with ten or more bead-like segments projecting forward from the head, and a pair of cerci projecting backward from the last of the eleven abdominal somites. These cerci may be long and filamentous or short and pincer- or forcep-like. Some diplurans have the ability to shed (autotomize) their cerci if necessary, and of all terrestrial arthropods, only diplurans have the power to regenerate these lost appendages over a series of molts. Up to thirty molts may occur throughout their short lifespan, which is estimated to be up to one year. The abdomen of diplurans possesses eversible vesicles, which appear to help with water balance by absorbing moisture from the environment. A tracheal respiratory system is present, but Malpighian tubules are vestigial or absent.

In terms of reproduction, dipluran sexes are separate, and, as in other primitive wingless non-insect hexapods, fertilization is external. The males produce up to 200 packets of sperm (spermatophores) a week and randomly deposit them on various substrates. The spermatophores are typically held off the ground by a short stalk. In the most commonly encountered campodeid diplurans, the spermatophore resembles a small globule, supported above the substrate by a short stalk. Females pick up the spermatophores with their genital opening and fertilize their eggs. Individuals lay eggs in small clusters in crevices or soil cavities in the substrate. Female campodeid diplurans desert their eggs, but japygid diplurans are known to remain in the brood chamber with the egg cluster, providing some parental care by protecting the eggs and newborn larvae. Larval development is slow and direct by molting, and the hatchlings do not undergo metamorphosis but, apart from their smaller size, resemble the adults, although they have an incomplete number of body setae and lack reproductive organs. Diplurans continue to molt throughout their life, slowly increasing in size, and over the course of several molts are able to regenerate lost antennae, abdominal cerci, and legs, a unique trait among hexapods.

Little is known about the diplurans of Arkansas. However, most of what is known was based on research conducted by Robert T. “Tommy” Allen (1939‒2016). He joined the faculty of the University of Arkansas (UA) in Fayetteville (Washington County) in 1968 and became curator of the UA Arthropod Museum. While at UA, Allen worked on the systematics of diplurans and was a world-renowned expert on that group. In 1988, a new species of Japygidae, Occasjapyx carltoni, was described by Allen from Indian Creek (Newton County) in the Ozark Mountains of the Interior Highlands of the state. This was the first record of Occasjapyx in North America outside of California; the genus is also known from China and Japan. A second specimen was collected by C. T. McAllister in 2004 along Powers Creek from Blevins Cave (Independence County). This additional collection site is approximately 167 km (104 mi.) southeast of the type locality. To date, no other specimens of O. carltoni have been reported from Arkansas. Other diplurans reported from the state include Catajapyx ewingi (Japygidae) from Howard County; Clivocampa solus from Mount Magazine (type locality, Logan County) and Cleburne, Franklin, Fulton, and Johnson counties; Metriocampa aspinosa from Mount Magazine; Parajapyx isabellae from Johnson County; and Podocampa inveterata from various counties in the Interior Highlands. More work is needed on this interesting group of primitive insects.

For additional information:
Allen, Robert T. “Additions to the Known Endemic Flora and Fauna of Arkansas.” Proceedings of the Arkansas Academy of Science 42 (1988): 18–21. Online at https://scholarworks.uark.edu/cgi/viewcontent.cgi?article=2316&context=jaas (accessed September 8, 2020).

———. “Description of a New Genus and Species of Campodeidae from the Interior Highlands of North America (Diplura: Campodeidae).” Annals of the Entomological Society of America 87 (1994): 270–276.

———. “Four New Species of Epigean Litocampa Silvestri, from the Southeastern United States (Insecta: Diplura: Campodeidae).” Proceedings of the Academy of Natural Sciences of Philadelphia 155(2006):106‒116.

———. “New Species of Occasjapyx from the Interior Highlands (Insecta: Diplura: Japygidae).” Journal of the Arkansas Academy of Science 42 (1988): 22‒23. Online: https://scholarworks.uark.edu/cgi/viewcontent.cgi?article=2317&context=jaas (accessed September 8, 2020).

———. “A New Species of Podocampa from North America (Diplura: Campodeidae: Campodeinae).” Journal of the Kansas Entomological Society 66 (1993): 328‒337.

———. “A Synopsis of the Diplura of North America: Key to Higher Taxa, Systematics, Distribution and Description of New Taxa (Arthropoda: Insecta).” Transactions of the Entomological Society 128 (2002): 403‒466.

———. “Two New Species of Epigean Litocampa (Insecta: Diplura: Campodeidae) from the Southeastern Appalachians.” Transactions of the American Entomological Society 129 (2003): 4549–559.

Ferguson, L. M. “A New Litocampa (Diplura: Campodeidae) from an Epigean Environment in the Great Smoky Mountains and the Biogeography of Cavernicolous Diplurans in the Appalachians.” In Proceedings of the Appalachian Biogeography Symposium edited by Ralph P. Eckerlin. Virginia Museum of Natural History, Special Publication 2 (1999): 109–120.

Hilton, W. A. “Campodea from the United States.” Journal of Entomology and Zoology 28 (1936): 5‒10.

Koch, M. “Monophyly and Phylogenetic Position of the Diplura (Hexapoda). Pedobiologia 41 (1997): 9‒12.

Kukalová-Peck, J. “New Carboniferous Diplura, Monura, and Thysanura, the Hexapod Ground Plan, and the Role of Thoracic Side Lobes in the Origin of Wings (Insecta). Canadian Journal of Zoology 65 (1987): 2327‒2345.

Lagerlöf, J., and O. Andrén. “Abundance and Activity of Collembola, Protura and Diplura (Insecta, Apterygota) in Four Cropping Systems.” Pedobiologia 35 (1991): 337–350.

Luan, Y., R. Xie, and W. Yin. “Preliminary Study on Phylogeny of Diplura.” Zoological Research 23 (2002): 149‒155.

McAllister, Chris T., and Christopher Carlton. “Second Record of the Dipluran, Occasjapyx carltoni Allen, 1988 (Japygidae) from Arkansas.” Journal of the Arkansas Academy of Science 59 (2005): 203‒204. Online: https://scholarworks.uark.edu/cgi/viewcontent.cgi?article=1554&context=jaas (accessed September 8, 2020).

McDaniel, V. Rick, Kenneth N. Paige, and C. Renn Tumlison. “Cave Fauna of Arkansas: Additional Invertebrate and Vertebrate Records.” Journal of the Arkansas Academy of Science 33 (1979): 84‒85. Online: https://scholarworks.uark.edu/cgi/viewcontent.cgi?article=2720&context=jaas (accessed September 8, 2020).

Poinar, George O., Jr. Life in Amber. Stanford: Stanford University Press, 1992.

Reddell, James R. “A Checklist and Bibliography of the Japygoidea (Insecta: Diplura) of North America, Central America, and the West Indies.” The Pearce-Sellars Series, Texas Memorial Museum 37 (1983): 1‒41.

Smith, L. M. “The Japygidae (Diplura) of North America, 3. Occasjapyx Silvestri and Hecajapyx n. gen.” Annals of the Entomological Society of America 52 (1959): 363‒368.

———. “Japygidae of North America,7. A new genus of Provalljapyginae from Missouri.” Proceedings of the Biological Society of Washington 73 (1960): 261‒262.

Chris T. McAllister
Eastern Oklahoma State College

Henry W. Robison
Sherwood, Arkansas