Symphylans

Symphylans belong to the Phylum Arthropoda, Subphylum Labiata, Superclass Myriapoda, and Class Symphyla. About 200 species of symphylans are known worldwide, predominantly in the tropics. There are two families: Scutigerellidae with five genera and about 128 species, and Scolopendrellidae containing nine genera and approximately seventy-three species. There are few reports of symphylans in Arkansas, aside from a new endemic scutigerellid species described in 1992 in Polk County. Many taxa are yet to be discovered and described in the state.

The fossil record of symphylans is poorly known; only five species have been recorded, all placed within living genera. They are ancestral arthropods dating back to the early Silurian approximately 430 million years ago, although the only fossil symphylans are known from amber with an age of 99 million years. As a result, the Scutigerellidae and Scolopendrellidae are thought to have diverged before the end of the Mesozoic Era.

Morphological studies place symphylans as more closely related to the millipedes (Diplopoda) and pauropods (Pauropoda) than to the centipedes (Chilopoda), within the clade Progoneata (a group excluding centipedes). However, molecular studies have shown opposing results, with some supporting the Progoneata clade and others aligning symphylans with centipedes or other arthropods, although some are weakly supported.

In terms of anatomical structures, once hatched, juveniles possess six pairs of legs, but over a lifetime of several years they add an additional pair at each molt so the adult instar has twelve pairs of legs.  The body is soft and translucent and divided into two body regions: head and trunk. The head has long, segmented antennae, a post-antennal organ, and three pairs of mouthparts as follows: (1) mandibles, (2) the long first maxillae, and (3) the second pair of maxillae, which are fused to form the lower lip or labium of the mouth. Organs of Tömösváry are disc-like organs that may sense vibrations, humidity, or light, although evidence for their true function is inconsistent.  The genital openings are located on the fourth body segment.

The trunk is composed of fifteen to twenty-four segments, which are protected by an overlapping dorsal plate, and ten or twelve segments possess legs. The first segment is largest and is usually provided with a pair of legs, whereas the last segment is slender, lacking legs, but possessing a pair of cerci. Each pair of legs is associated with an eversible structure, called a “coxal sac,” that helps the organism absorb moisture, and a small stylus that may be sensory in function. Symphylans are eyeless, they are without pigment, and their long antennae serve as sense organs. Like the early insects, they share similar structures, such as a labium (fused second maxillae), an identical number of head segments, and certain features of their legs.

The body of symphylans usually ranges in length from 2 to 10 mm (0.08 to 0.4 in.). The largest symphylan in the world, Hanseniella magna, an endemic Tasmanian cave dweller, attains lengths of 25 to 30 mm (1.0 to 1.2 in.). It is one of only a few symphylan troglobites and one of only a few true symphylan cave inhabitants. Indeed, most species that have been reported from caves are epigean species that are not adapted for cavernicolous life. Symphylans molt throughout their life and usually live six to twelve months but have been reported as living up to four years.

Symphylans are soil-dwelling herbivores and detritivores living deep in the soil, under stones, in decaying wood, and in other moist places where they feed primarily on fungi and decayed plant material. Here, they help make nutrients available for new plant growth. A few species are found in trees and in caves. They consume decaying vegetation, but a few feed on living plants, where they can do considerable harm in an agricultural setting by consuming seeds, roots, and root hairs in cultivated soil. The garden symphylan, Scutigerella immaculata, can be a pest of crops. A species of Hanseniella has been reported as a pest of sugar cane and pineapples in Queensland, Australia, and of pineapples and flower crops in Colombia. In addition, a species of Symphylella has been shown to be predominately predaceous, and some species are saprophagous.

Symphylans respire through a pair of spiracles on the sides of their head. These are connected to a system of tracheae that branch through the head and the first three segments of the body only.

For reproduction, males do not copulate with the female. Rather, they deposit 150 to 450 sperm packets (spermatophores) on top of short stalks of silk on the ground. The females pick up the sperm packets in their mouths. Most are swallowed and digested, but some are stored in special sacs in the female’s mouth that contain pouches for storing the sperm. She then removes each egg from her reproductive organs and fertilizes it in her mouth. A female deposits four to twenty-five pearly-white eggs in a mass at various depths in the soil depending on soil temperature, moisture, and structure, and to the sides of crevices or to moss or lichens with her mouth, smearing sperm over them as she does so. In about 40 days, eggs hatch and nymphs begin feeding on small roots. Symphylans have incomplete metamorphosis and develop through six growth stages, or instars. Growth from egg to adult requires about six to nine weeks, allowing for several generations per year.

In Arkansas, there are few reports of symphylans. A new scutigerellid species, Hanseniella ouachiticha, was described in 1992 from Rich Mountain (Polk County). It is an endemic species in the state. A great deal of work remains to be done on the symphylans of Arkansas, with many taxa yet to be discovered and described.

For additional information:
Adis, Joachim, and Ulf Scheller. “On the Natural History and Ecology of Hanseniella arborea (Myriapoda, Symphyla, Scutigerellidae), a Migrating Symphylan from an Amazonian Black-Water Inundation Forest.” Pedobiologia 27 (1984): 35–41.

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 December 12, 2020).

———. “A New Species of Hanseniella (Symphyla: Scutigerellidae) from the Interior Highlands of Arkansas.” Entomological News 103 (1992): 169‒174.

Clark, S., and P. Greenslade. “Review of Tasmanian Hanseniella Bagnall (Symphyla: Scutigerellidae).” Invertebrate Taxonomy 10 (1996): 189–212.

Gai, Yonghua, Daxiang Song, Hongying Sun, Qun Yang, and Kaiya Zhou. “The Complete Mitochondrial Genome of Symphylella sp. (Myriapoda: Symphyla): Extensive Gene Order Rearrangement and Evidence in Favor of Progoneata.” Molecular Phylogenetics and Evolution 49 (2008): 574–585.

Hilton, W. A. “Symphyla from North America.” Annals of the Entomological Society of America 24 (1931): 537‒553.

Moritz, Leif, and Thomas Wesener. “Symphylella patrickmuelleri sp. nov. (Myriapoda: Symphyla): The Oldest Known Symphyla and First Fossil Record of Scolopendrellidae from Cretaceous Burmese Amber.” Cretaceous Research 84 (2017): 258–263.

Podsiadlowski, L., H. Kohlhagen, and M. Koch. “The Complete Mitochondrial Genome of Scutigerella causeyae (Myriapoda: Symphyla) and the Phylogenetic Position of Symphyla.” Molecular Phylogenetics and Evolution 45 (2007): 251‒260.

Poinar, George O., and C. A. Edwards. “First Description of a Fossil Symphylan, Scutigerella dominicana sp. n. (Scutigerellidae, Symphyla), in Dominican Amber.” Experientia 51 (1995): 391‒393.

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.

Walter, D. E., J. C. Moore, and S. Loring (1989). “Symphylella sp. (Symphyla: Scolopendrellidae Predators of Arthropods and Nematodes in Grassland Soils.” Pedobiologia 33 (1989): 113–116.

Chris T. McAllister
Eastern Oklahoma State College