Collembollans

Springtails (collembolans) belong to the phylum Arthropoda and subphylum Hexapoda. They form the largest (about thirty-five families and 9,000 different species) of the three lineages of modern hexapods that are no longer considered to be included in the class Insecta (the other two are the proturan and dipluran apterygotes). Since each has internal mouthparts, the three are sometimes grouped together into a class called Entognatha. However, they do not appear to be any more closely related to one another than they all are to insects, which have external mouthparts. Indeed, they do share some features of insects, such as a body divided into three parts, a head with antennae, a three-segmented thorax, and each segment having a pair of jointed legs and an abdomen. Some molecular studies suggest that collembolans represent a separate evolutionary line from other hexapods and are closely related to crustaceans, but others disagree. Apparently, this appeared to be the result of widely divergent patterns of molecular evolution among the Arthropoda. As of 2019, thirty-five species have been reported from Arkansas caves, including ten cave-limited species. In addition, three cave species are considered Arkansas endemics, and one species is known only from the Ozark Plateaus Ecoregion of Arkansas and adjacent Missouri and Oklahoma.

Fossil collembolans are rare, and most are found in preserved in amber. The best deposits are from the early Eocene of Canada and Europe, Miocene of Central America, and the mid-Cretaceous of Burma and Canada. The fossil springtail, Rhyniella praecursor, from the Early Devonian (400 million years ago), is the oldest terrestrial arthropod and was found in Aberdeenshire, Scotland, in the famous Rhynie chert.

Members of the Collembola are minute wingless entognathas that are normally 1 to 5 mm (0.04 to 0.2 in.) long. For this reason they are rarely observed, in addition to the fact that they inhabit hidden situations. Members of the Poduromorpha and Entomobryomorpha have an elongated body, whereas the Symphypleona and Neelipleona have a globular body. Collembolans can be mottled or patterned, with varied coloration from white, gray, yellow, and orange to metallic green, lavender, red, or some other color. They possess six or fewer abdominal segments, mouthparts (such as mandibulae and maxillae) located within a “gnathal pouch,” antennae, and a tubular appendage called the collophore (ventral tube) on the ventral side of the first abdominal segment. At the apex of the collophore is a bilobed eversible sticky vesicle. This structure was originally thought to help specimens cling to surfaces, but it has been shown that it may play a role in water uptake and electrolyte balance. Most also have an abdominal forked or tail-like appendage called a furcula that aids in jumping when threatened by predators. This structure arises from the ventral side of the fourth abdominal segment. When at rest, it is folded forward under the abdomen where it is held in place by a clasp-like structure on the third abdominal segment called the retinaculum (or tenaculum). When this structure is released, it snaps against the substrate, launching the springtail into the air. This behavior can take place in as few as 18 milliseconds. In those without a furcula, such as seashore springtails (Anurida spp.), the eversible vesicles of the collophore may adhere to the surface waterfilm on which they can walk and distort it in such a way that it is springloaded; once the waterfilm is released, the specimen is propelled into the air.

Most collembolans lack a tracheal respiration system, which forces them to respire through a porous cuticle. The notable exception is members of the Sminthuridae (globular springtails), which exhibit a rudimentary, although fully functional, tracheal system. As a group, springtails are highly susceptible to desiccation because of their tegumentary respiration. However, some species that possess thin, permeable cuticles have been shown to resist severe drought by regulating the osmotic pressure of their body fluid.

Collembolans are cryptozoa (small animals that live in darkness and under conditions of high relative humidity) that prefer moist conditions, living in soil and litter or under bark, leaf mold, or decaying logs, and in fungi. Some epigean species prefer the surface of freshwater pools, ponds, and lakes (such as the hygrophilous marsh springtail, Isotomurus palustris), and others are found along the intertidal zones of seashores and on vegetation. Some species such as the water springtail (Sminthurides aquaticus) and Malmgren’s springtail (S. malmgreni) are able to move at the surface of water. A unique representative of the family Poduridae is another water springtail (Podura aquatica). This species is one of the first springtails to have been described by Carl von Linné (a.k.a. Carolus Linnaeus) (1707‒1778) in Systema Naturae (1758), and it spends its entire life at the surface of water.

Specialized habitats frequented by other species include termite and ant nests, caves, or snow fields of glaciers. In addition, a few species routinely climb trees and form a dominant component of canopy faunas, where they can be collected by beating limbs or fogging with insecticides. In temperate zones, a few species (e.g., Anurophorus spp., Entomobrya albocincta, Hypogastrura arborea, and Xenylla xavieri) are almost exclusively arboreal. In a mature temperate deciduous woodland, leaf litter and vegetation typically support thirty to forty species of springtails compared to over 100 species in the tropics. In these various microhabitats, they contribute to the fragmentation of organic matter as detritivores and microbivores and to the control of soil microbes as free-living omnivores.

Many of the soil inhabitants feed on decaying plant matter, fungal hyphae/spores, and bacteria, whereas others feed on a great variety of items, including fecal material from arthropods, algae, pollen, animal remains, minerals, and colloidal materials. Predators of collembolans include spiders, centipedes, mites, and pseudoscorpions.

A few collembolans have been reported to cause damage to gardens, greenhouses, or mushroom cellars, and they can be agricultural pests of some crops. For example, the lucerne flea (Sminthurus viridis) is considered a pest in Australia and has been shown to cause severe damage to agricultural crops, particularly alfalfa. Members of the family Onychiuridae are also known to feed on tubers and to damage them to some degree. However, most springtails are beneficial due to their ability to carry spores of mycorrhizal fungi and mycorrhiza helper bacteria on their tegument, which increases host root colonization. In addition, soil springtails play a constructive role in the establishment of plant-fungal symbioses and, thus, are actually beneficial to agriculture. They also contribute to controlling plant pathogens through their active consumption of mycelia and spores of damping-off and pathogenic fungi. (Damping–off is caused by several soil-borne fungi that kill seedlings that are just germinating and seedlings that have been growing for several days.)

Sexual reproduction occurs in springtails through the clustered or scattered deposition of spermatophores by adult males. The stimulation of spermatophore deposition by female pheromones has been demonstrated in the slender collembolan, Sinella curviseta. In mating behavior among Symphypleona, males of some in the family Sminthuridae use a clasping organ located on their antenna. Many collembolan species, mostly those living in deeper soils or subterranean environments, are parthenogenetic. Parthenogenesis (also called thelytoky) is under the control of symbiotic gram-negative bacteria of the genus Wolbachia, which live, reproduce, and are carried in the reproductive organs and eggs of female collembolans.

In absolute numbers, collembolans are supposed to be one of the most abundant of all macroscopic animals, with estimates of 100,000 individuals per square meter (10.8 sq. ft.) of substrate. These tend to be the larger (>2 mm [0.08 in.]) species, mainly slender springtails in the genera Entomobrya and girdled springtails, Orchesella, though the densities on a per square meterbasis are typically one to two orders of magnitude lower than soil populations of the same species. In the tropics, a single square meterof canopy habitat can support many species of Collembola.

Because most springtails are small, they are rather difficult to view simply by chance observation. However, one, the snow flea (Hypogastrura nivicola), can be observed on warm Canadian winter days when it is jumping about on the snow surface, as its dark blue color contrasts sharply with a background of snow. Interestingly, it has been found to contain an antifreeze-like glycine-rich protein that allows it to function in sub-zero environments by inhibiting growth of ice.

Sensitivity to drought varies from species to species and increases during ecdysis (shedding of an outer layer). Given that springtails are repeatedly molting during their entire life, they spend much time in concealed microhabitats where they can find protection against desiccation and predation during ecdysis, a benefit reinforced by synchronized molting. The high-humidity environment of many caves also favors springtails, and, as such, there are numerous cave-adapted subterranean species, including one, Plutomurus ortobalaganensis, living at 1,980 m (6,500 ft.) underground in the second-deepest cave on Earth, the Krubera Cave of the Western Caucasus. As such, this springtail is the deepest terrestrial animal ever found on Earth.

There have been unconfirmed reports that suggest that some springtails may parasitize humans; however, this is entirely inconsistent with their natural history, and no such phenomenon has ever been confirmed scientifically and documented in any refereed scientific journal. It has been reported that the scales or hairs from collembolans can cause a contact dermatologic irritation or allergy when rubbed onto the human skin. More often, claims of persistent human skin infection by springtails may indicate a neurological problem, such as delusional parasitosis, a psychological rather than entomological problem.

For millennia, insects and other bugs have been incorporated into human conflict, with the goals of destroying food, inflicting pain, and transmitting pathogens. Although firmly and repeatedly denied by the federal government, it has been reported that springtails were used by the U.S. Army as a “six-legged weapon” in biological warfare performed during the Korean War (1950‒1953). Two species of collembolans noted in allegations were Isotoma (Desoria) negishina (a local Arkansas species) and the white rat springtail (Folsomia candida). According to these unverified reports, isotomids were intended to serve as carriers of fatal diseases (anthrax, cholera, dysentery, and smallpox), dropped by aircraft above North Korean villages.

A study was done in the loblolly/shortleaf pine stands in southeastern Arkansas to examine whether springtail abundance, composition, and diversity were different between areas burned every two to three years over a twenty-year period versus areas not burned at all. Results indicated that springtail populations on the sites are influenced more by other environmental factors rather than by prescribed fire.

Most of the information available on collembolans was provided by Kenneth A. Christiansen (1924‒2017) of Grinnell College in Iowa. During his six decades at Grinnell, he published countless papers on cave and non-cave collembolans of North America, as well as other world localities. Remarkably, he described nearly fifty of the sixty known species of cave Collembola known from U.S. caves.

There are no detailed listings or checklists reporting collembolans from Arkansas. However, in 1958, Thomas C. Barr (1931‒2011) reported the earliest collections of Collembola from Arkansas caves. As of 2019, thirty-five species have been reported from caves, including ten troglobitic, or cave-limited, species. In addition, three cave species are considered Arkansas endemics, including Pygmarrhopalites buffaloensis from Walnut Cave (Newton County), P. youngsteadtii from Tom Barnes Cave (Newton County), and Typhlogastrura fousheensis from Foushee Cave (Independence County). One species, Pseudosinella dubia, is known only from the Ozark Plateaus Ecoregion of Arkansas and adjacent Missouri and Oklahoma.

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 http://scholarworks.uark.edu/jaas/vol42/iss1/8/ (accessed September 22, 2021).

Bellinger, P. F., Kenneth A. Christiansen, and Frans Janssens. Checklist of the Collembola of the World, 1996–2018. Online at http://www.collembola.org (accessed September 22, 2021).

Christiansen, Kenneth A. “The Genus Arrhopalites (Collembola: Sminthuridae) in the United States and Canada.” International Journal of Speleology 2 (1966): 43‒73.

———. “The Genus Pseudosinella (Collembola, Entomobryidae) in Caves of the United States.” Psyche 67 (1960): 1‒25.

———. “The Genus Sinella Brook (Collembola: Entomobryidae) in Nearctic Caves.” Annals of the Entomological Society of America 53:481-91.

———. “The Nearctic Members of the Genus Entomobrya (Collembola).” Bulletin of the Museum of Comparative Zoology 118 (1958): 440–545.

Christiansen, Kenneth A., and P. Bellinger. The Collembola of North America, North of the Rio Grande: A Taxonomic Analysis. Grinnell, IA: Grinnell College, 1998.

Christiansen, Kenneth A., and R. J. Snider. “Aquatic Collembola.” In An Introduction to the Aquatic Insects of North America, edited by R. W. Merritt, K. W. Cummins, and M. B. Berg. 4th ed. Dubuque, IA: Kendall/ Hunt, 2008.

Christiansen, Kenneth, and H. Wang. “A Revision of the Genus Typhlogastrura in North American Caves with Description of Five New Species.” Journal of Cave and Karst Studies 68 (2006): 85‒98.

DeWalt, R. Edward, V. H. Resh, and W. L. Hilsenhoff. “Diversity and Classification of Insects and Collembola.” In Ecology and Classification of North American Fresh Water Invertebrates, edited by J. H. Thorp and A. P. Covich. 3rd ed. New York: Elsevier, 2010.

D’Haese, C. A. D. “Morphological Appraisal of Collembola Phylogeny with Special Emphasis on Poduromorpha and a Test of the Aquatic Origin Hypothesis.” Zoologica Scripta 32 (2003): 563‒586.

Dunivan, James D., C. Renn Tumlinson, and V. Rick McDaniel. “Cave Fauna of Arkansas: Further Records.” Proceedings of the Arkansas Academy of Science 36 (1982): 87‒88. Online: https://scholarworks.uark.edu/jaas/vol36/iss1/28/ (accessed September 22, 2021).

Giribet, G., G. D. Edgecombe, and W. C. Wheeler. “Arthropod Phylogeny Based on Eight Molecular Loci and Morphology.” Nature 413 (2001): 157‒161.

Graening, G. O., Michael E. Slay, and Danté B. Fenolio. Cave Life of Oklahoma and Arkansas: Exploration and Conservation of Subterranean Biodiversity. Norman: University of Oklahoma Press, 2011.

Hirst, Stanley, and S. Maulik. “On Some Arthropod Remains from the Rhynie Chert (Old Red Sandstone).” Geological Magazine 63 (1926): 69–71.

Hopkin, S. P. Biology of the Springtails (Insecta: Collembola). London: Oxford University Press, 1997.

Lee, Linda A., and Barton, Harvey E. “Soil Traversing Arthropod Populations as Sampled by Pitfall Traps in Sunflower and Three Adjacent Habitats in Northeast Arkansas.” Proceedings of the Arkansas Academy of Science 36 (1982): 38‒42. Online at https://scholarworks.uark.edu/jaas/vol36/iss1/13/ (accessed September 22, 2021).

Lockwood, Jeffery A. Six-legged Soldiers: Using Insects as Weapons of War. London: Oxford University Press, 2009.

McAllister, Chris T., Henry W. Robison, and Michael E. Slay. “The Arkansas Endemic Fauna: An Update with Additions, Deletions, a Synthesis of New Distributional Records, and Changes in Nomenclature.” Texas Journal of Science 61 (2009): 203–218.

McDaniel, V. Rick, and Kenneth L. Smith. “Cave Fauna of Arkansas: Selected Invertebrate Taxa.” Proceedings of the Arkansas Academy of Science 30 (1976): 57‒60.

Renschin, Michele L., Lynne C. Thompson, and Michael G. Shelton. “Long-Term Prescribed Burning Regime Has Little Effect on Springtails in Pine Stands of Southern Arkansas.” Edited by Kristina F. Connor. Proceedings of the 12th Biennial Southern Silvicultural Research Conference. Gen. Tech. Rep. SRS–71. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station, 2004.

Robison, Henry W., and Robert T. Allen. Only in Arkansas: A Study of the Endemic Plants and Animals of the State. Fayetteville: University of Arkansas Press, 1995.

Robison, Henry W., and Chris T. McAllister. “The Arkansas Endemic Flora and Fauna: An Update with 13 Additional Species.” Journal of the Arkansas Academy of Science 69 (2015): 78–82. Online at http://scholarworks.uark.edu/jaas/vol69/iss1/16/ (accessed September 22, 2021).

Robison, Henry, Chris McAllister, Christopher Carlton, and Robert Tucker. “The Arkansas Endemic Biota: With Additions and Deletions.” Journal of the Arkansas Academy of Science 62 (2008): 84–96. Online http://scholarworks.uark.edu/jaas/vol62/iss1/14/ (accessed September 22, 2021).

Robison, Henry W., and Kenneth L. Smith. “The Endemic Flora and Fauna of Arkansas.” Proceedings of the Arkansas Academy of Science 36 (1982): 52–57. Online at http://scholarworks.uark.edu/jaas/vol36/iss1/17/ (accessed September 22, 2021).

Slay, Michael E., and G. O. Graening “Recent Collections and Additional Records of Collembola from Arkansas Caves.” Journal of the Arkansas Academy of Science 63 (2009): 158‒162. Online at https://scholarworks.uark.edu/jaas/vol63/iss1/20/ (accessed September 22, 2021).

Slay, Michael E., G. O. Graening, and Danté B. Fenolio. “New State Record and Western Range Extension for Pseudosinella dubia Christiansen (Collembola: Entomobryidae) from Oklahoma, U.S.A.” Entomological News 120 (2009): 545.

Tebbe, Christoph C., Alice B. Czarnetzki, and Torsten Thimm. “Collembola as a Habitat for Microorganisms.” In Intestinal Microorganisms of Termites and Other Invertebrates, edited by Helmut König and Ajit Varma. Berlin: Springer, 2006.

Tedder, Sandra A. “A Faunistic Survey of the Collembola of Magazine Mountain, Logan County, Arkansas.” PhD diss., University of Arkansas, 1996.

Tedder, Sandra A., and Robert T. Allen. “A Preliminary Survey of the Collembola of Magazine Mountain, Logan County, Arkansas.” Proceedings of the Arkansas Academy of Science 43 (1989): 113‒114. Online at https://scholarworks.uark.edu/jaas/vol43/iss1/34/ (accessed September 22, 2021).

Chris T. McAllister
Eastern Oklahoma State College