Zygentoma

Habitus of Trinemura callawae, from Smith et al. (2012).

Belongs within: Panhexapoda.
Contains: Lepismatidae, Nicoletiidae.

The Zygentoma, silverfish, are primitively wingless insects with an elongate terminal filament between the cerci and usually a covering of scales (Grimaldi & Engel 2005). Certain members of the group are familiar as inhabitants of human houses. Living members of the order, excluding the Baltic amber species Lepidothrix pilifera), form a clade Euzygentoma characterised by reductions of the abdominal sterna, number of abdominal styli, and number of tarsal segments (Grimaldi & Engel 2005).

The other silver fish
Published 23 December 2018

A couple of years ago, I presented a post about the Lepismatidae, the family including the familiar household silverfishes. In that post, I made an offhand reference to other, less well known families of the wingless insect order Zygentoma. The time has come to look at those families.

Squamatinia algharbica, a subterranean nicoletiid from Portugal, copyright S. Reboleira.

The Zygentoma are divided between five or six living families, depending on how you count them. The largest of these other than the Lepismatidae is the Nicoletiidae, representatives of which may be found in most parts of the world if you know where to look. And therein lies the rub: the eyeless nicoletiids are usually to be found in subterranean habitats, burrowed into soil or within caves. They are pale in coloration, usually white or golden. Nicoletiids have less flattened bodies than lepismatids and often lack the covering of scales found in the latter. One subfamily of nicoletiids, the Atelurinae, is sometimes treated as a separate family (hence the uncertainty above): not only do they have the covering of scales most other nicoletiids lack, they are generally less elongate and are oval or teardrop-shaped in form. Atelurines are inquilines of social insects, making their living in the nests of ants or termites. While some observations have been made of atelurines taking food directly from their hosts, it seems that they mostly live as scavengers on items dropped within the nest. They avoid capture and/or eviction by their hosts through their slipperiness and speed (Smith 2017).

An atelurine silverfish from Tasmania, copyright Zosterops.

The other three families are more localised in their distributions. The Protrinemuridae are found in scattered, disjunct locations including the Middle East, eastern Asia and Chile. They are similar in appearance to the Nicoletiidae, being eyeless, scaleless and subcylindrical, and were classified with that family until fairly recently. Differences between the Nicoletiidae and Protrinemuridae include the nature of the cuticular plates making up the underside of the abdomen. In nicoletiids, these are divided between median sternites and lateral coxites; in protrinemurids, a single undivided plate covers the underside of each abdominal segment. Maindronia is a genus of silverfish placed in its own family that resembles Lepismatidae in possessing eyes and a covering of scales. This genus also has a disjunct distribution, being known from Egypt, Saudi Arabia, Afghanistan and Chile.

Tricholepidion gertschi, copyright Samuel DeGrey.

The fifth family of Zygentoma includes a single living species Tricholepidion gertschi known from the coastal region of northern California, where it is found under decaying tree bark. Tricholepidion retains a number of plesiomorphic features for Zygentoma and is universally accepted as the most divergent member of the order: it lacks scales, it possesses ocelli as well as the compound eyes, it has five rather than four or fewer segments in the tarsi, and it possesses a greater number of ventral styli on the abdomen. Tricholepidion is also hypognathous (that is, it has the head oriented so that its mouth is directed downwards) whereas other Zygentoma are generally prognathous, with the mouthparts directed forwards (Engel 2006). Tricholepidion has usually been included in the Lepidotrichidae, a family originally described for a fossil species Lepidothrix pilifera from the Eocene Baltic amber, but Engel (2006) argued that Lepidothrix was in some ways more derived than Tricholepidion (specifically, it lacks ocelli) and that Tricholepidion should be placed in its own family. Indeed, Tricholepidion is divergent enough that some have even suggested it be excluded from the Zygentoma and regarded as the sister taxon to the broader clade uniting silverfish with the winged insects. This is certainly a minority view, however: most authors continue to regard it as a true, albeit highly unusual, zygentoman.

Systematics of Zygentoma

Characters (from Grimaldi & Engel 2005): Compound eyes reduced or absent; ocelli usually absent. Distalmost palpal segment of labium enlarged and modified. Coxae dorsoventrally flattened, enlarged.

<==Zygentoma [Lepismatida, Lepismatoidea]
    |  i. s.: Battigrassiella Paclt 1963TW05, F90 [Noticoliidae]
    |           `--B. wheeleriTW05 [=Grassiella wheeleriF90]
    |         Carbotriplura kukalovaeGE05
    |         Ramsdelepidion Kukalová-Peck 1987A99, K-P87
    |           `--*R. schusteri Kukalová-Peck 1987K-P87
    |--Lepidothrix [Lepidothricidae, Lepidotrichidae]GE05
    |    `--L. piliferaGE05
    `--EuzygentomaGE05
         |--+--Maindronia Bouvier 1897C92 [MaindroniidaeGE05, Maindroniinae]
         |  `--LepismatidaeGE05
         `--NicoletiidaeGE05

*Type species of generic name indicated

References

[A99] Ax, P. 1999. Das System der Metazoa II. Ein Lehrbuch der phylogenetischen Systematik. Gustav Fisher Verlag: Stuttgart (translated: 2000. Multicellular Animals: The phylogenetic system of the Metazoa vol. 2. Springer).

[C92] Crawford, R. L. 1992. Catalogue of the genera and type species of the harvestman superfamily Phalangioidea (Arachnida). Burke Museum Contributions in Anthropology and Natural History 8: 1–60.

Engel, M. S. 2006. A note on the relic silverfish Tricholepidion gertschi (Zygentoma). Transactions of the Kansas Academy of Science 109 (3–4): 236–238.

[F90] Ferguson, L. M. 1990. Insecta: Microcoryphia and Thysanura. In: Dindal, D. L. (ed.) Soil Biology Guide pp. 935–949. John Wiley & Sones: New York.

[GE05] Grimaldi, D., & M. S. Engel. 2005. Evolution of the Insects. Cambridge University Press: New York.

[K-P87] Kukalová-Peck, J. 1987. 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: 2327–2345.

Smith, G. B. 2017. The Australian silverfish fauna (order Zygentoma)—abundant, diverse, ancient and largely ignored. Gen. Appl. Ent. 45: 9–58.

[TW05] Terry, M. D., & M. F. Whiting. 2005. Mantophasmatodea and phylogeny of the lower neopterous insects. Cladistics 21: 240–257.

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