Thysanura

(invertebrate zoology) The silverfish, machilids, and allies, an order of primarily wingless insects with soft, fusiform bodies.

(Silverfish and fire brats)

Class: Insecta

Order: Thysanura

Number of families: 4

Evolution and systematics

Fossils of thysanurans are known from the upper Carboniferous period, including Ramsdelepidion schusteri, a very large silverfish 2.36 in (60 mm) long found in the state of Illinois in the United States.

The order Thysanura is considered the sister group of the Pterygota (winged insects), with which it shares a double articulation between mandibles and head and a well-sclerotized thoracic pleuron.

The Thysanura, or Zygentoma, includes four extant families: Lepidotrichidae, with only one extant species; Nicoletiidae, with soil, subterranean, and myrmecophilous (associated with ants) species; Lepismatidae, including the common domestic species, and Maindroniidae, including species of restricted distribution that live under rocks.

Physical characteristics

Thysanurans are primitively wingless insects. Most species are covered with overlapping silvery-gray scales although some lack scales. Members of the order are usually 0.4–0.8 in (10–20 mm) long, but may range from 0.04–1.9 in (1–50 mm); the bodies are dorsoventrally flattened. The compound eyes are small or absent; there are long filiform antennae; and the external mouthparts include mandibles with two points of articulation to the head. Thysanurans have short styli on abdominal segments two through seven, and two cerci and a median caudal filament at the tip of the abdomen. Females have a jointed ovipositor. The eggs are elliptical, about 0.04 in (1 mm) long. They are soft and white when first laid, but after several hours turn yellow and eventually brown. The larvae resemble small adults.

Distribution

There are about 370 species of thysanurans worldwide.

Habitat

Thysanurans are found in humid locations; under bark, rocks, rotting logs, and leaf litter; in caves; in ant and termite nests; and in synanthropic situations (those associated with human habitation). A few species live in sandy deserts.

Behavior

Silverfish hide under stones or leaves during the day and emerge after dark to search for food. All are fast running. They are nocturnal, even the early larval instars.

Feeding ecology and diet

Thysanurans are omnivorous, feeding on decaying or dried vegetable material and animal remains. Domestic species feed on starchy material such as paper, binding, and artificial silk. Some species associated with ants are cleptobiotic, robbing food from the ants. Species that live in deserts are able to absorb water through the rectum.

Reproductive biology

Thysanurans have ametabolous development; the larvae of silverfish resemble the adults, but are generally smaller in size. Silverfish continue to molt throughout their lives and individuals may live for up to six years. Most silverfish reproduce sexually, with the male depositing a sperm packet on the substrate beneath a silken thread, which is picked up by the female. Some species are parthenogenetic. Females use their ovipositor to insert the oval, whitish eggs into cracks and soil litter.

Conservation status

No species in Thysanura is listed by the IUCN.

Significance to humans

Domestic species of thysanurans are common household pests, causing extensive damage to household goods by feeding on wallpaper paste, book bindings, cardboard, and other paper products, and starch sizing of some textiles.

Species accounts

Resources

Quintero, D., and A. Aiello, eds. Insects of Panama and Mesoamerica. New York: Oxford University Press, 1992.

Smith, G. B., and J. A. L. Watson. "Thysanura." In The Insects of Australia. A Textbook for Students and Research Workers, Vol. 2 (CSIRO), 2nd edition. Carlton, Australia: Melbourne University Press, 1991.

Wygodzinsky, P. "Order Thysanura" In Immature Insects, Vol. 2, edited by F. W. Stehr. Dubuque, IA: Kendall/Hunt Publishing, 1987.

Sturm, H. "The Mating Behaviour of Tricholepidion gertschi Wygodzinski, 1961 (Lepidotrichidae, Zygentoma) and Its Comparison with the Behaviour of Other 'Apterygota.'" Pedobiologia 41, nos. 1–3 (1997): 44–49.

Wygodzinsky, P. "On the Surviving Representative of the Lepidothrichidae (Thysanura)." Annals of the Entomological Society of America 54 (1961): 621–627. ——. "A Review of the Silverfish (Lepismatidae, Thysanura) of the United States and Caribbean Area." American Museum Novitates 2,481 (1972): 1–26.

[Article by: Natalia von Ellenrieder, PhD]

Primarily wingless insects, with soft, fusiform bodies from 0.12 to 0.80 in. (3 to 20 mm) long, often covered with scales forming diverse patterns (see illustration). These insects are considered by some authors to constitute two orders: the true Thysanura (silverfish and allies) and the Microcoryphia (machilids). The mouthparts are free. The mandibles are monocondylous in the machilids, and scraping in function; they are dicondylous—as in all winged insects—in the silverfish, in which they are of the chewing type. Machilids have large compound eyes, but silverfish have simple ommatidia, or lack eyes altogether. Many machilids have coxae with styliform appendages. Tarsi are from two- to five-segmented; there are two or three claws. Abdominal ventral plates are entire, or subdivided into a central sternum and posterolateral coxites. Abdominal styli are present in varied numbers. The females have well-developed ovipositors, and the males have a penis and often one or rarely two pairs of parameres. There are cerci and a median caudal filament.

Machilinus sp. (Microcoryphia).

These primitive insects, fossils of which are known as far back as the Devonian (primitive machilids, the Monura), are worldwide in distribution. Machilids are more numerous in temperate climates, the silverfish in the tropics and subtropics. See also Apterygota; Insecta.

Thysanura Fossil range: Late Carboniferous–Recent PreЄ Є O S D C P T J K Pg N [1]
A silverfish, Lepisma saccharina
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Subclass:	Apterygota
Order:	Thysanura
Families
Lepismatidae Nicoletiidae Ateluridae Lepidotrichidae Maindroniidae

Thysanura is an order of insects, encompassing silverfish and firebrats, known for their three long caudal filaments.

The families Machilidae and Meinertellidae of the jumping bristletails were once included with Thysanura. Occasionally, the correspondingly restricted order Thysanura is referred to as Zygentoma.

Contents 1 Description and ecology 2 Families 3 Reproduction 4 References 5 External links

Description and ecology

Silverfish are so called due to the silvery glitter of the scales covering their bodies. Their movement is "fish-like" and makes it look as if they are swimming. They are less than 1 centimetre (0.39 in) long, and found in damp corners or amongst books and paper in houses.

Silverfish have flattened bodies and may be elongated or oval in shape. They have flexible antennae and small or absent compound eyes. They have short mandibles and relatively unspecialised mouthparts. Many species also have a number of short appendages on their abdominal segments, but the most distinctive feature of the group is the presence of three long, tail-like filaments extending from their last segment. The two lateral filaments are formed from the abdominal cerci.^[2]

They may be found in moist, humid environments or dry conditions, both as free-living organisms or nest-associates.

Silverfish feed on cereals, paste, paper, starch in clothes, rayon fabrics and dried meats.^[3] Silverfish can sometimes be found in bathtubs or sinks at night, as they have difficulty moving on smooth surfaces and so become trapped. Wild species often are found in habitats such as caves, and some are commensals living in association with ant colonies, e.g. Trichatelura manni ^[4] .

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.

Families

Lepismatidae is the largest family, widespread with more than 200 species, many living in human habitations. The Nicoletiidae are small and live in soil litter, humus and under stones. The Lepidothrichidae are represented by two species Lepidothrix pilifera from Baltic Amber and Tricholepidion gertschi from forests of northern California. Three species of Maindroniidae are found in the Middle East and in Chile. The Ateluridae live in nests of ants and termites and are small and blind.^[5]

Reproduction

Silverfish have an elaborate courtship ritual to ensure exchange 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, and then she ejects the empty spermatophore and eats it.

Silverfish continue to moult throughout their life, with several sexually mature instars, unlike more advanced insects. They are relatively slow growing, and lifespans of up to four years have been recorded.^[2]

References

1. ^ Hoell, H.V., Doyen, J.T. & Purcell, A.H. (1998). Introduction to Insect Biology and Diversity, 2nd ed.. Oxford University Press. p. 320. ISBN 0-19-510033-6. 
2. ^ ^{a b} Hoell, H.V., Doyen, J.T. & Purcell, A.H. (1998). Introduction to Insect Biology and Diversity, 2nd ed.. Oxford University Press. p. 343. ISBN 0-19-510033-6. 
3. ^ Silverfish
4. ^ Reproductive Morphology and Behavior of a Thysanuran, Trichatelura manni, Associated with Army Ants, R. Torgerson, R. D. Akre 1969. Annals of the Entomological Society of America, 62, pp. 1367-1374
5. ^ Helmut Sturm Zygentoma in Resh VH, Cardé RT. (eds.) (2003) Encyclopaedia of Insects. Academic Press. pp. 1203–1205

• Grimaldi, D. and Engel, M.S. (2005). Evolution of the Insects. Cambridge University Press. ISBN 0-521-82149-5. 
• 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, ISBN 1-55297-612-2, 2002

External links

• Thysanura – Tree of Life Web Project
• Virginia Extension Service Article
• Harvard University Fact Sheet on silverfish and firebrats

This article incorporates text from the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.

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