Zygentoma

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Zygentoma
Temporal range: Aptian–Recent
Thermobia domestica1.jpg
The firebrat (Thermobia domestica) (Lepismatidae) typical of the order Zygentoma
Atelura formicaria 18180053.jpg
Atelura formicaria (Nicoletiidae)
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
(unranked): Dicondylia
Order: Zygentoma
Börner, 1904
Families

Zygentoma are an order in the class Insecta, and consist of about 550 known species. [1] The Zygentoma include the so-called silverfish or fishmoths, and the firebrats. A conspicuous feature of the order are the three long caudal filaments. The two lateral filaments are cerci, and the medial one is an epiproct or appendix dorsalis. In this they resemble the Archaeognatha, although the cerci of Zygentoma, unlike in the latter order, are nearly as long as the epiproct. [2]

Contents

Until the late twentieth century the Zygentoma were regarded as a suborder of the Thysanura, [3] until it was recognized that the order Thysanura was paraphyletic, thus raising the two suborders to the status of independent monophyletic orders, with Archaeognatha as sister group to the Dicondylia, including the Zygentoma. [4] [5] [ page needed ]

Lateral aspect of a silverfish typical of the Zygentoma Silverfish 2007-2.jpg
Lateral aspect of a silverfish typical of the Zygentoma

Etymology

The name "Zygentoma" is derived from the Greek ζυγόν (zygón), in context meaning "yoke" or "bridge"; and ἔντομα (entoma), "insects", literally "cut into" because of the segmented anatomy of typical insects. [6] The idea behind the name was that the taxon formed a notional link between the Pterygota and the Apterygota. [7] This view is now totally obsolete, but the phylogeny of the Insecta was in its infancy in the late 19th and early 20th century, and the name was firmly established by the time that more sophisticated views were developed.

Description and ecology

Silverfish are so-called because of the silvery glitter of the scales covering the bodies of the most conspicuous species (family Lepismatidae). Their movement has been described as "fish-like" as if they were swimming. Most extant species have a body length less than 2 centimetres (0.79 in) long, though Carboniferous fossils about 6 cm long are known. [5]

Zygentoma have dorsiventrally flattened bodies, generally elongated or oval in outline. Their antennae are slender and mobile. The compound eyes tend to be small, and the two families Nicoletiidae and Protrinemuridae, and some troglobitic species, lack eyes entirely. The Lepismatidae have compound eyes composed of 12 ommatidia on each side of the head. [8] Ocelli are absent in all species except for Tricholepidion gertschi , the only member of the family Lepidotrichidae. [9] The mandibles are short, and the mouthparts unspecialised. Tricholepidion, Nicoletiidae and Protrinemuridae have eight pairs of short appendages called styli on their abdominal segments 2 to 9, but in Lepismatidae styli are only found on segment 7 to 9 or 8 to 9, and sometimes just on the ninth segment. [10] [11] [12] Or styli can be completely absent. [13] A distinctive feature of the group is the presence of three long, tail-like filaments extending from their last segment. These three generally subequal, except in some members of the family Nicoletiidae, in which they are short, and the cerci are hard to detect. The two lateral filaments are the abdominal cerci and the medial one is the epiproct. [14]

Silverfish may be found in moist, humid environments or dry conditions, both as free-living organisms or nest-associates. [15] In domestic settings, they feed on cereals, paste, paper, starch in clothes, rayon fabrics and dried meats. [16] In nature, they will feed on organic detritus. [17] Silverfish can sometimes be found in bathtubs or sinks at night, because they have difficulty moving on smooth surfaces and so become trapped if they fall in.

Wild species often are found in dark, moist habitats such as caves or under rocks, and some are commensals living in association with ant colonies, e.g., Trichatelura manni [18] and Allotrichotriura saevissima, [15] which lives inside nests of fire ants in Brazil.

There are no current species formally considered to be at conservation risk, though several are troglobites limited to one or a few caves or cave systems, and these species run an exceptionally high risk of extinction.

Aggregation behaviour

In the past, a contact pheromone was assumed to be responsible for the aggregation and arrestment behaviour observed in Zygentoma. [19] It was later found out that the aggregation behaviour is not triggered by pheromones, but by an endosymbiotic fungus, Mycotypha microspora (Mycotyphaceae), and an endosymbiotic bacterium, Enterobacter cloacae (Enterobacteriaceae), both present in the faeces of the firebrat, Thermobia domestica. [20] It was also shown that firebrats detect the presence of E. cloacae based on its external glycocalyx of polysaccharides, most likely based on its D-glucose component. Mycotypha microspora is only detected by firebrats in the presence of cellulose, suggesting that metabolites of the enzymatic cellulose digestion by M. microspora (such as D-glucose) serve as the aggregation/arrestment cue. [21] A follow-up study showed that gray silverfish, Ctenolepisma longicaudatum , also respond with arrestment to Mycotypha microspora, but not so the common silverfish Lepisma saccharinum . [22]

Furthermore, direct current-powered low-level electromagnetic coils with static electromagnetic fields were found to induce attraction or arrestment behaviour in Lepisma saccharinum and Thermobia domestica. [23] This behavioural trait has potential application in traps for Zygentoma, and a respective patent has been issued. [24]

Taxonomy

The Tricholepidiidae are represented by Tricholepidion gertschi from forests of northern California.

The Lepidotrichidae are represented by the extinct Lepidotrix pilifera , known from Baltic amber.

The Lepismatidae is the largest family and they include the physically largest specimens. The family is cosmopolitan with more than 200 species. Many are anthropophilic, living in human habitations. Some species are inquilines in ant colonies.

The Nicoletiidae tend to be smaller, pale in colour, and often live in soil litter, humus, under stones, in caves (with reduced eyes) or as inquilines in ant or termite colonies. The family is subdivided into five subfamilies. [26]

The Maindroniidae comprise three species, found in the Middle East and in Chile.

The Protrinemuridae comprise four genera. [27] Like Nicoletiidae species living in caves, they lack eyes. [28]

Some molecular phylogenies have found Tricholepidiidae to form an independent, more basal branch of insects unrelated to other zygentomans. [29]

Evolutionary history

The fossil record for Zygentoma is poor, though they must have diverged from all other insects either during the Carboniferous, or the Devonian if Leverhulmia is an example of the group. The oldest fossils of the order are indeterminate specimens of Lepismatidae from the Santana Formation of Brazil, dating to the Aptian stage of the Early Cretaceous around 113 million years ago, with other specimens of Lepismatidae known from the Burmese amber of Myanmar, dating to around 100 million years ago. [30] [31] Fossils of Nicoletiidae are known from Miocene aged Dominican amber. [31]

Reproduction

Silverfish have an elaborate courtship ritual to ensure the transfer of sperm. The male spins a silken thread between the substrate and a vertical object. He deposits a sperm packet (spermatophore) beneath this thread and then coaxes a female to walk under the thread. When her cerci contact the silk thread, she picks up the spermatophore with her genital opening. Sperm are released into her reproductive system, after which she ejects the empty spermatophore and eats it.

As ametabolous insects, silverfish continue to moult throughout their lives, with several sexually mature instars, unlike the pterygote insects. They are relatively slow growing, and lifespans of four to up to eight years have been recorded. [14] [32]

Research for biofuel production

Since silverfish consume lignocellulose found in wood, they are one type of insect (along with termites, wood-feeding roaches, wood wasps, and others) currently being researched for use in the production of biofuel. The guts of these insects act as natural bioreactors in which chemical processes break down cellulose. They have been studied in the hope of developing commercially cost-effective biofuel production processes. [33]

Related Research Articles

<span class="mw-page-title-main">Thysanura</span> Deprecated order of insects

Thysanura is the now deprecated name of what was, for over a century, recognised as an order in the class Insecta. The two constituent groups within the former order, the Archaeognatha and the Zygentoma, share several characteristics, such as of having three long caudal filaments, the lateral ones being the cerci, while the one between (telson) is a medial cerciform appendage, specifically an epiproct. They are also both wingless, and have bodies covered with fine scales, rather like the scales of the practically unrelated Lepidoptera. In the late 20th century, it was recognized that the two suborders were not sister taxa, therefore Thysanura was paraphyletic, and the two suborders were each raised to the status of an independent monophyletic order, with Archaeognatha sister taxon to the Dicondylia, including the Zygentoma.

<i>Lepisma</i> Genus of silverfishes

Lepisma is a genus of primitive insects in the order Zygentoma and the family Lepismatidae.

<span class="mw-page-title-main">Firebrat</span> Insect similar to a silverfish

The firebrat is a small insect, in the order Zygentoma.

<span class="mw-page-title-main">Apterygota</span> Subclass of insects

The name Apterygota is sometimes applied to a former subclass of small, agile insects, distinguished from other insects by their lack of wings in the present and in their evolutionary history; notable examples are the silverfish, the firebrat, and the jumping bristletails. Their first known occurrence in the fossil record is during the Devonian period, 417–354 million years ago. The group Apterygota is not a clade; it is paraphyletic, and not recognized in modern classification schemes. As defined, the group contains two separate clades of wingless insects: Archaeognatha comprises jumping bristletails, while Zygentoma comprises silverfish and firebrats. The Zygentoma are in the clade Dicondylia with winged insects, a clade that includes all other insects, while Archaeognatha is sister to this lineage.

<span class="mw-page-title-main">Archaeognatha</span> Order of jumping bristletails

The Archaeognatha are an order of apterygotes, known by various common names such as jumping bristletails. Among extant insect taxa they are some of the most evolutionarily primitive; they appeared in the Middle Devonian period at about the same time as the arachnids. Specimens that closely resemble extant species have been found as both body and trace fossils in strata from the remainder of the Paleozoic Era and more recent periods. For historical reasons an alternative name for the order is Microcoryphia.

<span class="mw-page-title-main">Machilidae</span> Family of jumping bristletails

The Machilidae are a family of insects belonging to the order Archaeognatha. There are around 250 described species worldwide. These insects are wingless, elongated and more or less cylindrical with a distinctive humped thorax and covered with tiny, close-fitting scales. The colour is usually grey or brown, sometimes intricately patterned. There are three "tails" at the rear of the abdomen: two cerci and a long central epiproct. They have large compound eyes, often meeting at a central point. They resemble the silverfish and the firebrat, which are from a different order, Zygentoma.

<span class="mw-page-title-main">Lepismatidae</span> Family of silverfishes

Lepismatidae is a family of primitive wingless insects with about 190 described species. This family contains the two most familiar members of the order Zygentoma: the silverfish and the firebrat. It is one of five families in the order Zygentoma.

<i>Ctenolepisma</i> Genus of silverfishes

Ctenolepisma is a genus of primitive insects in the order Zygentoma, closely related to the silverfish and firebrat but less reliant on human habitation, some species being found both indoors and outdoors and some found exclusively outdoors. The genus is distributed nearly worldwide in warm regions. Australia lacks native Ctenolepisma, but is home to introduced species.

<i>Ctenolepisma lineatum</i> Species of silverfish

Ctenolepisma lineatum is a species of insect of the order Zygentoma. It is similar to the closely related silverfish but can be distinguished by being rather stouter and less shiny with all appendages noticeably longer. The abdomen is often marked with dark brown lines and the species is sometimes called four-lined silverfish.

A silverfish is a wingless insect in the order Zygentoma. The same name can be applied to many species in the order as a whole, which comprises the families Lepismatidae, Nicoletiidae, Lepidotrichidae, Maindroniidae and Protrinemuridae.

<span class="mw-page-title-main">Silverfish</span> Small land insect in the order Zygentoma

The silverfish is a species of small, primitive, wingless insect in the order Zygentoma. Its common name derives from the insect's silvery light grey colour, combined with the fish-like appearance of its movements. The scientific name indicates that the silverfish's diet consists of carbohydrates such as sugar or starches. While the common name silverfish is used throughout the global literature to refer to various species of Zygentoma, the Entomological Society of America restricts use of the term solely for Lepisma saccharinum.

<span class="mw-page-title-main">Nicoletiidae</span> Family of silverfishes

Nicoletiidae is a family of primitive insects belonging to the order Zygentoma. These insects live primarily underground, under detritus, or in caves. A few species are recorded as commensals inside nests of social insects, such as the species Allotrichotriura saevissima which lives inside fire ant nests. Nicoletiidae don't have eyes as other existing species and they lack pigment. They have medium size, with extended antennae and terminal filaments. Coletinia, a genus of this family, has many characters including a body length that ranges between 10 and 15mm. They may have yellowish pigmentation but most of them are transparent. They have bilayered and very large acrosome, a short nucleus including chromatin being really condensed with tubular invaginations and sperm length and head size classified as short.

Protrinemuridae is a family of primitive insects belonging to the order Zygentoma. The family was previously classified as a subfamily of the Nicoletiidae, but raised to family level in 2002.

Protrinemurella is a genus of silverfish in the family Protrinemuridae. It comprises only the type species Protrinemurella allacrotelsoidesMendes, 2002, known from Thailand's Krabi province.

<i>Tricholepidion</i> Species of silverfish

Tricholepidion is a genus of wingless insect belonging to Zygentoma, with only a single described species T. gertschi, native to the northern coast of California in Western North America. It lives under dead bark and in rotting wood of conifers in mesophytic forests. It is alternatively considered the only living member of the family Lepidotrichidae, which also includes Lepidotrix from Eocene aged European amber, or the only member of the family Tricholepidiidae. The taxonomic position of Tricholepidion is uncertain, in some molecular phylogenetics studies it has been recovered as less closely related to flying insects (Pterygota) than the rest of Zygentoma is, rendering Zygentoma paraphyletic. Each compound eye contains ~40 ommatidia, and they have three ocelli. Scales on the body are absent. Unlike Archaeognatha and the other families of Zygentoma, which have three- and sometimes two-segmented tarsi, they have five-segmented tarsi like many winged insects.

Thermobia aegyptiaca is a species of silverfish in the family Lepismatidae. The species was described by Hippolyte Lucas in 1840 based on specimens collected in Egypt. Thermobia aegyptiaca is distributed in Africa and the eastern Mediterranean Basin.

Ctenolepisma guadianicum is a species of silverfish in the family Lepismatidae. Like all silverfish, guadianicum prefers very humid environments, although some members of Thysanura have evolved minor adaptations to alter this. Guadianicum, similar to the vast majority of the near four hundred discovered silverfish, lives outdoors, under detritus, leaves, and other fallen forest debris. This silverfish is grey in appearance, less than an inch long, with an oblong body shape. All members of Ctenolepisma have lengthy, angled antennae that extend from their head and reach towards their abdomen. Guadianicum also lacks wings, has three pairs of legs, and has abdomen appendages appearing similar to antennae. Unlike the indoor dwelling silverfish species, guadianicum is not considered a pest, however, this means that little else is known about this particular species discovered in the 1990s.

<i>Ctenolepisma longicaudatum</i> Species of insect

Ctenolepisma longicaudatum, generally known as the gray silverfish, long-tailed silverfish or paper silverfish, is a species of Zygentoma in the family Lepismatidae. It was described by the German entomologist Karl Leopold Escherich in 1905 based on specimens collected in South Africa, but is found worldwide as synanthrope in human housings.

Mycotypha microspora, also known as Microtypha microspora, is a filamentous fungus in the division Zygomycota. It was discovered in a Citrus aurantium peel in 1932 by E. Aline Fenner, who proposed a new genus Mycotypha to accommodate it. Mycotypha africana, which is another species in the genus Mycotypha, is closely related to M. microspora. The fungus has subsequently been isolated from both outdoor and indoor settings around the world, and is typically found in soil and dung. The species rarely causes infections in humans, but has recently been involved in the clinical manifestation of the life-threatening disease mucormycosis.

Sceletolepisma is a genus of primitive insects closely related to the silverfish and firebrat but less reliant on human habitation, some species being found both indoors and outdoors and some found exclusively outdoors. The genus is distributed nearly worldwide in warm regions.

References

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Further reading