The Other Silver Fish

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.

REFERENCES

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.

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

All that is Silver is not Fish

Common silverfish Lepisma saccharina, copyright Christian Fischer.

The insects are deservedly recognised as one of the most successful groups of organisms on the planet. Thanks in no small part to their unlocking the ability of flight, insects can be seen today in almost every part of the planet above sea level. But not all insects, of course, are flighted; many remain firmly on the ground. A large proportion of these are the descendents of flighted ancestors that returned to a terrestrial existence but there are also some whose ancestors never took to the skies. For most people, the most familiar of these original land-huggers are likely to be the silverfish of the family Lepismatidae.

Silverfish are long-bodied insects with a covering of reflective scales—hence the 'silver' part of their name. The 'fish' part probably refers to the manner of their movement; speaking from my own experience collecting them, these buggers move fast, slipping along the ground like a silver minnow. There are over 250 known species of Lepismatidae (Mendes 2002); probably many more remain to be described. They comprise over half the known species of the insect order Zygentoma (sometimes referred to as the Thysanura though most current entomologists tend to avoid that name due to its previous history referring to a now-obsolete grouping of the Zygentoma with the superficially similar Archaeognatha); the other families in the order are commonly subterranean and less commonly encountered by the average person. The highest diversity of silverfish occurs in tropical and subtropical parts of the world, particularly in arid or semi-arid regions. Adaptations of the rectal epithelium allow silverfish to absorb moisture straight from the atmosphere (or, to put it another way, they drink through their butt), making them ideally suited to tolerating the dryness of deserts. They are also suited to tolerating the relatively dry habitats offered by the interiors of human houses and several species have become our associates (in cooler parts of the world, these synathropic species are often the only lepismatids around). These include the common silverfish Lepisma saccharina and the giant silverfish Ctenolepisma longicaudata. The firebrat Thermobia domestica is a colourfully patterned human associate that likes it particularly warm; it is usually restricted to places like the backs of stoves or alongside hot-water cylinders where it can find the heat it craves. Being detritivores (that is, they feed on dust), human-associated silverfish are usually quite innocuous though they may cause problems if their numbers get too high or if they get into stored foodstuffs.

Firebrat Thermobia domestica, copyright David R. Madison.

In areas where they are native, silverfish may be quite diverse. Watson & Irish (1998) conducted a study of an area of the Namib Desert that was home to eight different species of silverfish. They found a tendency for the species to differ in their preferred microhabitat within the area: some were restricted to the upper parts of the sand dunes dominating the region, others were restricted to the rocky hollows separating the dunes. Those found in rocky lower zones resembled the familiar human-associated species (indeed, they included members of the same genus as the giant silverfish, Ctenolepisma) in being elongate and slender. In contrast, those species found higher in the dunes themselves were shorter and more flattened with well-developed spines covering the legs. These features allowed the dune silverfish to effectively 'swim' through the sand, using the spines on the legs to dig about and their flattened form to slip between grains.

REFERENCES
Mendes, L. F. 2002. Taxonomy of Zygentoma and Microcoryphia: historical overview, present status and goals for the new millennium. Pedobiologia 46: 225–233.

Watson, R. T., & J. Irish. 1998. An introduction to the Lepismatidae (Thysanura: Insecta) of the Namib Desert sand dunes. Madoqua 15 (4): 285–293.