Zygentoma (Thysanura, Silverfish) (Insects)

The Zygentoma (Thysanura sen.su stricto) are medium-sized apterygote insects with a body length from 5 to 30 mm. Their body is flattened and the eyes are small or absent. There are no ocelli except for the Lepidothrichidae, which has three ocelli. The flagellate (whiplike) antennae can be short (e.g., in Ateluridae) or much longer than the body (Fig. 1). The mouthparts are ectognathous. The mandibles are dicondylic (have two rotation points) and the maxillary palps have five segments that are of normal length. Nearly all parts of the body and appendices bear bristles of different length and structure. Scales are present on the thoracic segments of Lepismatidae and Ateluridae, but lacking in the three other families.
Male (above) and female of Thermobia domestica during premating (passing-by behavior), vertical view. In this phase the antennae and the cerci are in contact; body length about 12 mm.
FIGURE 1 Male (above) and female of Thermobia domestica during premating (passing-by behavior), vertical view. In this phase the antennae and the cerci are in contact; body length about 12 mm.
All Zygentoma are able to run quickly, but they cannot jump as do the Archaeognatha. The coxae of all legs are large and flattened. The penis of males is situated on abdominal segment 9; on the hind end, three caudal appendages are present as in the Archaeognatha. Molting continues in adults. Some species of the family Lepismatidae (e.g., Lepisma saccharina, Thermobia domestica, Acrotelsa collaris) prefer to live in human houses.


FOSSIL RECORD, SYSTEMATICS, BIOGEOGRAPHY

The oldest fossils that are clearly Zygentoma were found in Mesozoic layers (Lower Cretaceous) of Brazil. From the Cenozoic on, there are many Zygentoma fossils, and most of them are amber inclusions. Nearly all of them are similar to extant taxa (e.g., Ctenolepisma, Nicoletia, Lepidothrichidae). However there are representatives of apterygote Hexapoda from the Paleozoic (e.g., Monura and Cercopodata) that cannot be included in extant orders.
The Zygentoma comprise 5 families and some 400 species in about 90 genera.
1. Lepismatidae This largest family includes the well-known sil-verfishes, some of which occupy human houses. This family is the richest in genera (more than 21) and species (>200).
2. Nicoletiidae Relatively small and slim, and without dark pigment, eyes, and scales, they live in the upper layers of soils within the humus layer or beneath stones. Some species or populations reproduce parthenogenetically. More than 10 genera and more than 30 species are known.
3. Ateluridae Small, blind, and of droplike form, apparently all species in this family are commensal with ants or termites. There are more than 10 genera and more than 30 species.
4. Lepidothrichidae There are two species: Lepidothrix pilifera, which is known from Baltic amber, and the extant Tricholepidion gertschi, which occurs in forests of northern California (Fig. 3 ).
5. Maindroniidae There is one genus and three species from the Middle East and Chile; their habitat is somewhat similar to that of the Nicoletiidae.
The Lepismatidae and Nicoletiidae are distributed worldwide. Some Lepismatidae are household pests: L. saccharina in northern Europe; A. collaris in the tropics; and T. domestica in the United States ( Figs. 1 and 2 ). In southern Europe, L. saccharina and other lepisma-tid species also live outside houses. In Africa, Lepismatidae occur in areas with sand dunes. Nicoletiidae are found up to 3000 m elevation in Colombia and enter the northern parts of Europe via transport of potted plants to greenhouses. The distribution of Ateluridae coincides fairly well with that of ants and termites. The distribution of Lepidothrichidae and Maindroniidae appears quite restricted.
Their relatively flat body form enables the Zygentoma to enter small fissures. The Nicoletiidae are especially well adapted to the upper layers of the soil, whereas Lepismatidae generally rest in fissures during the day and are active during the night. If the relative humidity is above 50-60%, Lepismatidae can absorb atmospheric water by means of the anal sac (part of the hindgut).


PHYSIOLOGY AND DEVELOPMENT

As in Archaeognatha and in other orders of apterygote insects, the cuticle of Zygentoma is mainly thin and flexible. The mandibles present an intermediate form between monocondylic and a typical dicondylic structure. The Lepismatidae eat plant material and
Female of T. domestica, dorsal view. The female is picking up the spermatophore with her ovipositor. The spermato-phore was deposited on secreted threads spun, a short time before, by the male.
FIGURE 2 Female of T. domestica, dorsal view. The female is picking up the spermatophore with her ovipositor. The spermato-phore was deposited on secreted threads spun, a short time before, by the male.
Male of T. gertschi (Lepidothrichidae), vertical view, body length about 14 mm. This species is probably a relic and the most archaic within the Zygentoma. It is found only in the forests of Northern California. The animal is seen here on rotting wood, its natural environment.
FIGURE 3 Male of T. gertschi (Lepidothrichidae), vertical view, body length about 14 mm. This species is probably a relic and the most archaic within the Zygentoma. It is found only in the forests of Northern California. The animal is seen here on rotting wood, its natural environment.
detritus; L. saccharina is able to digest cellulose by the cellulase produced in its midgut.
Development has been studied mainly on Lepismatidae (especially L. saccharina, T. domestica, and Ctenolepisma spp.). The eggs of T. domestica are of oval form with a largest diameter of about 1.2 mm, and are laid underground. At a temperature of 37°C, they need 12-16 days until eclosion. In L. saccharina, egg development lasts about 35 days at temperatures near 20°C. The first free-living developmental stage possesses an egg tooth on the front of the head, and the first three stages lack scales. The molting intervals are dependent on age, temperature, loss of scales and/or appendages, species, and even population. At temperatures of about 40°C, T. domestica molts in 9- to 11-day intervals. The life span of L. saccharina is up to 3 years, whereas that of Thermobia is about 2 years.

EVOLUTIONARY ASPECTS

With flattened form, the Zygentoma are adapted to small fissures in soil, detritus, or in houses. The Lepismatidae leave those places during night to find food, favorable conditions (e.g., high relative humidity), and members of the same species for mating. In contrast with the Archaeognatha, their food use is broad. Lepismatidae have been spread widely by humans. For example, of the 13 lepis-matid species in the United States, only 3 are believed to be native. In comparison to the Archaeognatha, the Zygentoma show a great adaptability in terms of the different habitats they live in and the astonishing adaptation of the Ateluridae, which live with ants and termites. They possess some morphological structures that correspond to the Pterygota: a special form of mandibles; maxillary palps smaller than thoracic legs; an abdomen with tracheal commissures and connections; and an ovipositor base with well-developed gonangulum. Apparently, however, they are not the ancestors of the Pterygota. Because of the crosswise joining of abdominal terga with correspondent coxites (a unique apomorphy within the Hexapoda), the possession of 11 abdominal segments, styli on abdominal segments, eversible vesicles, three abdominal appendages (filum ter-minale and two cerci), and the structure of the ovipositor, the during night to find food, favorable conditions (e.g., high relative humidity), and members of the same species for mating. In contrast with the Archaeognatha, their food use is broad. Lepismatidae have been spread widely by humans. For example, of the 13 lepis-matid species in the United States, only 3 are believed to be native. In comparison to the Archaeognatha, the Zygentoma show a great adaptability in terms of the different habitats they live in and the astonishing adaptation of the Ateluridae, which live with ants and termites. They possess some morphological structures that correspond to the Pterygota: a special form of mandibles; maxillary palps smaller than thoracic legs; an abdomen with tracheal commissures and connections; and an ovipositor base with well-developed gonangulum. Apparently, however, they are not the ancestors of the Pterygota. Because of the crosswise joining of abdominal terga with correspondent coxites (a unique apomorphy within the Hexapoda), the possession of 11 abdominal segments, styli on abdominal segments, eversible vesicles, three abdominal appendages (filum ter-minale and two cerci), and the structure of the ovipositor, the Archaeognatha and Zygentoma are obviously linked. However the “jump” from Apterygota to Pterygota probably was from a line separate from the Thysanura.

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