Zygentoma
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Zygentoma are an order in the class
Insecta Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs o ...
, and consist of about 550 known species. The Zygentoma include the so-called
silverfish The silverfish (''Lepisma saccharinum'') is a species of small, primitive, wingless insect in the order Zygentoma (formerly Thysanura). Its common name derives from the insect's silvery light grey colour, combined with the fish-like appearance ...
or fishmoths, and the
firebrat The firebrat (''Thermobia domestica'') is a small hexapod (typically 1–1.5 cm), in the order Zygentoma. Habitat Firebrats prefer higher temperatures and require some humidity, and can be found in bakeries and near boilers or f ...
s. A conspicuous feature of the order are the three long
caudal filament Insect morphology is the study and description of the physical form of insects. The terminology used to describe insects is similar to that used for other arthropods due to their shared evolutionary history. Three physical features separate insec ...
s. The two lateral filaments are cerci, and the medial one is an
epiproct This glossary of entomology describes terms used in the formal study of insect species by entomologists. A–C A synthetic chlorinated hydrocarbon insecticide, toxic to vertebrates. Though ...
or ''appendix dorsalis''. In this they resemble the
Archaeognatha 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 sa ...
, although the cerci of Zygentoma, unlike in the latter order, are nearly as long as the epiproct. Until the late twentieth century the Zygentoma were regarded as a suborder of the
Thysanura 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 (jumping bristletails) and the Zygentoma (silverfish and fireb ...
, until it was recognized that the order Thysanura was
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
, thus raising the two suborders to the status of independent monophyletic orders, with Archaeognatha as
sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...
to the
Dicondylia The Dicondylia are a taxonomic group (taxon) that includes all insects except the jumping bristletails (Archaeognatha). Dicondylia have a mandible attached with two hinges to the head capsule (dicondyl), in contrast to a hypothetical ancestral m ...
, including the Zygentoma.


Etymology

The name "Zygentoma" is derived from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
('), in context meaning "yoke" or "bridge"; and ('), "insects", literally "cut into" because of the segmented anatomy of typical insects. page 488 The idea behind the name was that the
taxon In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
formed a notional link between the
Pterygota The Pterygota ( grc, πτερυγωτός, pterugōtós, winged) are a subclass of insects that includes the winged insects. It also includes insect orders that are secondarily wingless (that is, insect groups whose ancestors once had wings b ...
and the
Apterygota The name Apterygota is sometimes applied to a 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 ...
. This view is now totally obsolete, but the
phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
of the
Insecta Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs o ...
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 Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number w ...
covering the bodies of the most conspicuous species (family
Lepismatidae 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 (''Lepisma saccharinum'') and the firebrat (''Thermobia domestica'' ...
). Their movement has been described as "fish-like" as if they were swimming. Most
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...
species have a body length less than long, though Carboniferous fossils about 6 cm long are known. Zygentoma have
dorsiventral A dorsiventral (Lat. ''dorsum'', "the back", ''venter'', "the belly") Organ (anatomy), organ is one that has two surfaces differing from each other in appearance and structure, as an ordinary leaf. This term has also been used as a synonym for dorso ...
ly flattened bodies, generally elongated or oval in outline. Their antennae are slender and mobile. The
compound eye A compound eye is a visual organ found in arthropods such as insects and crustaceans. It may consist of thousands of ommatidia, which are tiny independent photoreception units that consist of a cornea, lens, and photoreceptor cells which disti ...
s tend to be small, and the twe two families
Nicoletiidae 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 ' ...
and
Protrinemuridae Protrinemuridae is a family of primitive insects belonging to the order Zygentoma. The family was previously classified as a subfamily of the Nicoletiidae Nicoletiidae is a family of primitive insects belonging to the order Zygentoma. These i ...
, and some
troglobitic A troglobite (or, formally, troglobiont) is an animal species, or population of a species, strictly bound to underground habitats, such as caves. These are separate from species that mainly live in above-ground habitats but are also able to live u ...
species, lack eyes entirely. The Lepismatidae have compound eyes composed of 12 ommatidia on each side of the head.
Ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...
are absent in all species except for '' Tricholepidion gertschi'', the only member of the family
Lepidotrichidae Lepidotrichidae is a family of basal insects belonging to the order Zygentoma ( silverfish and allies) The family contains the extinct '' Lepidotrix'', known from specimens preserved in Eocene The Eocene ( ) Epoch is a geological epoch tha ...
. The
mandibles In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
are short, and the mouthparts unspecialised. Tricholepidion, Nicoletiidae and Protrinemuridae have eight pairs of short appendages called styli on their
abdominal The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
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. Or styli can be completely absent. 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 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 ' ...
, 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 This glossary of entomology describes terms used in the formal study of insect species by entomologists. A–C A synthetic chlorinated hydrocarbon insecticide, toxic to vertebrates. Though ...
. Silverfish may be found in moist, humid environments or dry conditions, both as free-living organisms or nest-associates. In domestic settings, they feed on cereals, paste, paper, starch in clothes,
rayon Rayon is a semi-synthetic fiber, made from natural sources of regenerated cellulose, such as wood and related agricultural products. It has the same molecular structure as cellulose. It is also called viscose. Many types and grades of viscose f ...
fabrics and dried meats. In nature, they will feed on organic detritus. 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
commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
s living in association with
ant Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total of 22 ...
colonies, e.g., ''Trichatelura manni'' and ''Allotrichotriura saevissima'', which lives inside nests of
fire ant Fire ants are several species of ants in the genus ''Solenopsis'', which includes over 200 species. ''Solenopsis'' are stinging ants, and most of their common names reflect this, for example, ginger ants and tropical fire ants. Many of the nam ...
s in
Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
. There are no current species formally considered to be at conservation risk, though several are
troglobite A troglobite (or, formally, troglobiont) is an animal species, or population of a species, strictly bound to underground habitats, such as caves. These are separate from species that mainly live in above-ground habitats but are also able to live u ...
s 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 A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
was assumed to be responsible for the aggregation and arrestment behaviour observed in Zygentoma. 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 ''Enterobacter cloacae'' is a clinically significant Gram-negative, facultatively-anaerobic, rod-shaped bacterium. Microbiology In microbiology labs, ''E. cloacae'' is frequently grown at 30 °C on nutrient agar or at 35 °C in tr ...
'' (
Enterobacteriaceae Enterobacteriaceae is a large family (biology), family of Gram-negative bacteria. It was first proposed by Rahn in 1936, and now includes over 30 genera and more than 100 species. Its classification above the level of family is still a subject ...
), both present in the faeces of the
firebrat The firebrat (''Thermobia domestica'') is a small hexapod (typically 1–1.5 cm), in the order Zygentoma. Habitat Firebrats prefer higher temperatures and require some humidity, and can be found in bakeries and near boilers or f ...
, ''Thermobia domestica''. It was also shown that firebrats detect the presence of ''E. cloacae'' based on its external
glycocalyx The glycocalyx, also known as the pericellular matrix, is a glycoprotein and glycolipid covering that surrounds the cell membranes of bacteria, epithelial cells, and other cells. In 1970, Martinez-Palomo discovered the cell coating in animal cells ...
of
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
s, most likely based on its D-
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...
component. ''Mycotypha microspora'' is only detected by firebrats in the presence of
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
, suggesting that metabolites of the enzymatic cellulose digestion by ''M. microspora'' (such as D-glucose) serve as the aggregation/arrestment cue. A follow-up study showed that gray silverfish, ''
Ctenolepisma longicaudatum ''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 190 ...
'', also respond with arrestment to ''Mycotypha microspora'', but not so the common silverfish ''
Lepisma saccharinum The silverfish (''Lepisma saccharinum'') is a species of small, primitive, wingless insect in the order Zygentoma (formerly Thysanura). Its common name derives from the insect's silvery light grey colour, combined with the fish-like appearanc ...
''. Furthermore,
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
-powered low-level
electromagnetic coil An electromagnetic coil is an electrical Electrical conductivity, conductor such as a wire in the shape of a wiktionary:coil, coil (spiral or helix). Electromagnetic coils are used in electrical engineering, in applications where electric curre ...
s with static electromagnetic fields were found to induce attraction or arrestment behaviour in ''Lepisma saccharinum'' and ''Thermobia domestica''. This behavioural trait has potential application in traps for Zygentoma, and a respective patent has been issued.


Taxonomy

* Order Zygentoma Börner 1904 ** Suborder Archizygentoma Engel 2006 *** Family Tricholepidiidae Engel 2006 ** Suborder Neozygentoma Engel 2006 *** Infraorder † Parazygentoma Engel 2006 **** Family †
Lepidotrichidae Lepidotrichidae is a family of basal insects belonging to the order Zygentoma ( silverfish and allies) The family contains the extinct '' Lepidotrix'', known from specimens preserved in Eocene The Eocene ( ) Epoch is a geological epoch tha ...
Silvestri 1913 *** Infraorder Euzygentoma Grimaldi & Engel 2005 **** Family
Maindroniidae Maindroniidae is a very small family of silverfish, basal insects belonging to the order Zygentoma. It contains just a single genus, ''Maindronia'', and a handful of species. Four species of these insects are found in some of the driest deserts ...
Escherich 1905 **** Family
Lepismatidae 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 (''Lepisma saccharinum'') and the firebrat (''Thermobia domestica'' ...
Latreille 1802 **** Family
Protrinemuridae Protrinemuridae is a family of primitive insects belonging to the order Zygentoma. The family was previously classified as a subfamily of the Nicoletiidae Nicoletiidae is a family of primitive insects belonging to the order Zygentoma. These i ...
Mendes 1988 **** Family
Nicoletiidae 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 ' ...
Escherich 1905 The Tricholepidiidae are represented by '' Tricholepidion gertschi'' from forests of northern California. The
Lepidotrichidae Lepidotrichidae is a family of basal insects belonging to the order Zygentoma ( silverfish and allies) The family contains the extinct '' Lepidotrix'', known from specimens preserved in Eocene The Eocene ( ) Epoch is a geological epoch tha ...
are represented by the extinct '' Lepidotrix pilifera'', known from
Baltic amber The Baltic region is home to the largest known deposit of amber, called Baltic amber or succinite. It was produced sometime during the Eocene epoch, but exactly when is controversial. It has been estimated that these forests created more than 1 ...
. The
Lepismatidae 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 (''Lepisma saccharinum'') and the firebrat (''Thermobia domestica'' ...
is the largest family and they include the physically largest specimens. The family is
cosmopolitan Cosmopolitan may refer to: Food and drink * Cosmopolitan (cocktail), also known as a "Cosmo" History * Rootless cosmopolitan, a Soviet derogatory epithet during Joseph Stalin's anti-Semitic campaign of 1949–1953 Hotels and resorts * Cosmopoli ...
with more than 200 species. Many are
anthropophilic In parasitology, anthropophilia, from the Greek ἅνθρωπος (anthrōpos, "human being") and φιλία (philia, "friendship" or "love"), is a preference of a parasite or dermatophyte for humans over other animals.Braun-Falco, Otto (2000). ...
, living in human habitations. Some species are
inquiline In zoology, an inquiline (from Latin ''inquilinus'', "lodger" or "tenant") is an animal that lives commensally in the nest, burrow, or dwelling place of an animal of another species. For example, some organisms such as insects may live in the h ...
s in ant colonies. The
Nicoletiidae 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 ' ...
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 In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end subfamily botanical names with "-oideae", and zoologi ...
. The
Maindroniidae Maindroniidae is a very small family of silverfish, basal insects belonging to the order Zygentoma. It contains just a single genus, ''Maindronia'', and a handful of species. Four species of these insects are found in some of the driest deserts ...
comprise three species, found in the Middle East and in Chile. The
Protrinemuridae Protrinemuridae is a family of primitive insects belonging to the order Zygentoma. The family was previously classified as a subfamily of the Nicoletiidae Nicoletiidae is a family of primitive insects belonging to the order Zygentoma. These i ...
comprise four genera. Like Nicoletiidae species living in caves, they lack eyes. Some molecular phylogenies have found Tricholepidiidae to form an independent, more basal branch of insects unrelated to other zygentomans.


Evolutionary history

The fossil record for Zygentoma is poor, though they must have diverged from all other insects during the
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
. The oldest fossils of the order are indeterminate specimens of Lepismatidae from the
Santana Formation The Santana Group is a geologic group, formerly included as the middle part of the Araripe Group, in the Araripe Basin of northeastern Brazil. The group comprises the Crato, Ipubi and Romualdo Formations and is dated to the Aptian to Albian stag ...
of Brazil, dating to the
Aptian The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous Epoch or Series and encompasses the time from 121.4 ± 1.0 Ma to 113.0 ± 1.0 Ma (million years ago), a ...
stage of the Early
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
around 113 million years ago, with other specimens of Lepismatidae known from the
Burmese amber Burmese amber, also known as Burmite or Kachin amber, is amber from the Hukawng Valley in northern Myanmar. The amber is dated to around 100 million years ago, during the latest Albian to earliest Cenomanian ages of the mid-Cretaceous period. The ...
of Myanmar, dating to around 100 million years ago.J .F. Mendes, J. Wunderlich New data on thysanurans preserved in Burmese amber (Microcoryphia and Zygentoma Insecta) Soil Org., 85 (2013), pp. 11-22 Fossils of Nicoletiidae are known from
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
aged
Dominican amber Dominican amber is amber from the Dominican Republic derived from resin of the extinct tree ''Hymenaea protera''. Dominican amber differentiates itself from Baltic amber by being nearly always transparent, and it has a higher number of fossil incl ...
.


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 A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially salamanders and arthropods, and transferred in entirety to the female's ovipore during reproduction. Spermatophores ...
) 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 The name Apterygota is sometimes applied to a 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 ...
insects, silverfish continue to moult throughout their lives, with several sexually mature
instar An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of arthropods, such as insects, between each moult (''ecdysis''), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or ass ...
s, unlike the pterygote insects. They are relatively slow growing, and lifespans of four to up to eight years have been recorded.


Research for biofuel production

Since silverfish consume
lignocellulose Lignocellulose refers to plant dry matter (biomass), so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth for the production of biofuels. It is composed of two kinds of carbohydrate polymers, cellulose a ...
found in wood, they are one type of insect (along with
termites Termites are small insects that live in colonies and have distinct castes (eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blattode ...
, wood-feeding
roach Roach may refer to: Animals * Cockroach, various insect species of the order Blattodea * Common roach (''Rutilus rutilus''), a fresh and brackish water fish of the family Cyprinidae ** ''Rutilus'' or roaches, a genus of fishes * California roach ...
es,
wood wasp Sawflies are the insects of the suborder Symphyta within the order Hymenoptera, alongside ants, bees, and wasps. The common name comes from the saw-like appearance of the ovipositor, which the females use to cut into the plants where they lay ...
s, and others) currently being researched for use in the production of
biofuel Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. According to the United States Energy Information Administration (E ...
. The guts of these insects act as natural bioreactors in which chemical processes break down
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
. They have been studied in the hope of developing commercially cost-effective biofuel production processes.


References


Further reading

* * Charles A. Triplehorn and Norman F. Johnson, ''Borror and DeLong's Introduction to the Study of Insects'', 7th edition (Thomas Brooks/Cole, 2005), pp. 177–180 *''Firefly Encyclopedia of Insects and Spiders'', edited by Christopher O'Toole, , 2002


External links


Thysanura
– Tree of Life Web Project

{{Taxonbar, from=Q30111 Insect orders Extant Pennsylvanian first appearances Taxa named by Carl Julius Bernhard Börner Dicondylia