The Zygentoma are part of the old Thysanura (or three pronged bristle-tails) along with the Microcoryphia. They are an ancient lineage linking the Apterygota (primarily wingless insects) and the Pterygota (primarily winged insects).
The old Thysanura contains some of the most primitive of insects known to man i.e., insects that are most like what we think the first insects looked like.
The Protura, Diplura and Collembola represent other branches from a shared arthropodal ancestor, which along with the Archeognatha (here part of the Thysanura) are considered part of the ‘Hexapoda’, but not part of the ‘Insecta’.
The Hexapoda is a separate class of arthropods, including the smaller group the ‘Insecta’- which contains the rest of what we think of as ‘The Insects’ in classification schemes.
Etymology of Zygentoma:- from the Greek zygón, in context meaning “yoke” or “bridge”; and entoma, “insects”. So bridge insects – to symbolize their taxonomic postion.
They get their common name of Silverfish because of the silvery glitter of the scales that covers the bodies of the most conspicuous species (family Lepismatidae). Furthermore some people think their movement is somewhat “fish-like” as if they were swimming across the ground.
There are 544 known species of Zygentoma according to the online database Catalogue of Life.
Biology of the Zygentoma
The Zygentoma are flightless insects with ectognathous mouth parts (externally visible, as compared to the entognathous mouth parts of the Diplura, Protura and Collembola – which are sunk into the head and thus not immediately visible). They have long filiform antennae with as many as, or more than, 30 segments.
The Zygentoma are characterised by the possession of a dorsoventrally depressed body, dicondylar jaws, coxas without styles, respiratory stigmata present in abdominal segments I-VIII. They have compound eyes, but these are usually small or absent. They usually lack ocelli, the only known exception is Tricholepidion gertschi.
A distinctive feature of Silverfish is the presence of three long (except in the family Nicoletiidae where they are short), tail-like filaments extending from their last body segment. The central filament is usually longer than the two side filaments. The two lateral ‘tails’ are called cerci and the central one is called an epiproct.
They spend most of their life concealed within the leaf litter or beneath stones, wood, houses, the soil etc.. They can also be found in the nest of other animals. Mostly Zygentomans feed on dead plant material, fungal hyphae and other inanimate organic matter, however in domestic habitats they will eat any food they can find.
Their most common natural enemies are spiders, although centipedes and carabid beetles are also know to eat them. Internally they may be parasitized by Sporozoans and externally they can have a problem with erythraeid mites. Some species of Zygentoma are also parasitized by Strepsipterans.
Some members of the Zygentoma, such as the Silver Fishes (Lepisma saccharina & Ctenolepisma longicauda) and the Firebrat (Lepismodes inquilinus) can be a nuisance in homes and offices, where they will eat both paper and cotton. They possess their own cellulase enzymes and are not dependent on bacteria to digest such products, in the way Termites are for instance.
Fertilization is indirect, meaning the males leave packets of sperm, called spermatophores, attached to the substrate for the females to find. Therefore there is no contact between the sexes for mating and no courtship. They lay their eggs (normally between 5 and 20) in small crevices, or loose in the substrate – although some members of the family Nicoletiidae lay their eggs singly, one here, one there.
Their eggs are colored orange, brown or grey when laid but quickly darken. The young possess an egg tooth in their first instar, this helps them escape the egg. They look like miniature (but pale) adults. They do not gain the scales that give them their color until the 3rd instar.
The time taken to reach maturity varies considerably. In some species they become sexually active after the 9th instar, but in others not until the 14th or 15th. They are slow growing and live relatively long lives, up to 4 or more years, and continue to moult regularly as adults.
You can see some lovely paintings of some Zygentoma done by A. T. Hollick in the 1860’s in John Lubbock’s 1871 monograph on the Collembola and Thysanura.
Some Strange Behaviour
In the past people observed aggregation and arrestment behaviour observed in Zygentoma. It was originally thought that this was caused by an unknown pherome. However it was later learned that this aggregation behaviour is not triggered by pheromones, but by an endosymbiotic fungus, Mycotypha microspora (Mycotyphaceae), and an endosymbiotic bacterium, Enterobacter cloacae (Enterobacteriaceae), both of which have been found present in the faeces of the firebrat, Thermobia domestica. It was also shown that firebrats could detect the presence of E. cloacae based on the presence of certain polysaccharides.. Mycotypha microspora is only detected by firebrats in the presence of cellulose. A later study showed that gray silverfish, Ctenolepisma longicaudatum, also responded to Mycotypha microspora, but that the common silverfish Lepisma saccharinum did not.
- Genus Tricholepidion
- Family Lepismatidae
- Genus Acrotelsa
- Genus Allacrotelsa
- Genus Anallacrotelsa
- Genus Ctenolepisma
- Genus Heterolepisma
- Genus Lepisma
- Genus Leucolepisma
- Genus Mirolepisma
- Genus Prolepismina
- Genus Stylifera
- Genus Thermobia
- Family Nicoletiidae
- Genus Allonicoletia
- Genus Anelpistina
- Genus Grassiella
- Genus Nicoletia
- Genus Texoreddellia
The other part of the old Thysanura are the Bristletails, they are now found in the order Microcoryphia.
Image Credits:- Cover image – Lepisma saccharina by Christian Fischer, – License CC BY-SA 3.0; Thermobia domestica by Jscottkelley – License CC BY 3.0; Ctenolepisma longicaudatum by Bj.schoenmakers – License CC0