The traditional taxon Thysanura, or Apterygota, has been recognized as an unnatural group for decades because silverfish (Zygentoma) are more closely related to the winged insects (Pterygota) than to the bristletails (e.g., Snodgrass, 1935). In his monumental work on the phylogeny and classification of insects, Hennig (1953, 1969, 1981) proposed the name Dicondylia for the group uniting the silverfish with the winged insects. Recent molecular studies have also supported the Dicondylia as a lineage (e.g., Wheeler et al., 2001). The hallmark character of this group is the presence of a novel, secondary articulation to the mandible (i.e., the dicondylic mandible) (Figure 5.9). This second articulation results in the movement of the mandible being roughly confined to a single plane of motion rather than the rotating motion possible in Archaeognatha. It is homologous to the monocondylic joint in Archaeognatha and Entognatha, since the condyle for the new point of articulation is located on the head capsule with the acetabulum on the mandible itself. Another significant character defining Dicondylia is the development of the gonangulum in the ovipositor base

Silverfish are similar to bristletails in gestalt but are more flattened; they lack the distinctive hump of the latter group and, therefore, do not jump. Like bristletails, these primitively wingless insects have a long terminal filament between the cerci and a surface covering of scales (which are lost in Nicoletiidae and Maindroniidae) (Figures 5.1, 5.4). The biology of the order is superficially similar to that described for the Archaeognatha, including the occurrence of indirect sperm transfer via a spermatophore; however, Zygentoma are mostly diurnal and omnivorous with the notable exception of the family Nicoletiidae, which is principally subterranean and vegetarian. Though they are not capable of jumping, silverfish are very agile and run swiftly, as anyone who has chased one across a kitchen counter or sink knows. In contrast to the Archaeognatha, the compound eyes are reduced or absent in Zygentoma, and most families except Lepidotrichidae lack ocelli entirely. Defining features of the silverfish are not immediately apparent, though the enlargement and modification of the distalmost palpal segment of the labium and the dorsoventrally flattened and enlarged coxae may be significant. The enigmatic

5.4. A male and female of the common European silverfish, Thermobia domestica (Lepismatidae), as they initiate their courtship. Photo: H. Sturm.

Lepidotrichidae are at times excluded from the order. Otherwise, most differences between Zygentoma and Archaeog-natha lie in those traits the former shares with winged insects. Major recent references for the Zygentoma include Mendes (1991, 1994, 2002), Larink (1997a,b), and Sturm (1997, 2003b).

The order consists of five Recent families in two groups:

Lepidotrichidae, and the families Lepismatidae, Nicoletiidae, Ateluridae, and Maindroniidae. The apparently primitive family Lepidotrichidae today is represented by a single modern species living in northern California, Tricholepidion gertschi (Wygodzinsky, 1961) (Figure 5.5), but the family was originally described from a fossil species, Lepidothrix pilifera, in mid-Eocene Baltic amber. This family, however, may not be monophyletic, since Lepidothrix may be more closely related to the remainder of the Zygentoma (Euzygentoma). Tricholepidion possesses distinct ocelli while Lepidothrix and Euzygentoma lack ocelli. Regardless of potential paraphyly of Lepidotrichidae, the family possesses a number of putative primitive features relative to Euzygentoma, including the large abdominal sterna and large number of abdominal styli and eversible vesicles. However, eversible vesicles are only absent in Lepismatidae and Maindroniidae, and this character is shared (primitively?) with Nicoletiidae and Ateluridae. Indeed, Tricholepidion and Nicoletiidae share a unique modification of sensillar structures on the terminal filament (Wygodzinsky, 1961) and the former shares sperm conjugation with Lepismatidae (Wingstrand, 1973; Kristensen, 1997), whereby individual sperm cells pair in the vas deferens of the testes. Evidence for sperm conjugation in Nicoletiidae (Jamieson, 1987) is apparently not conclusive (Dallai et al., 2001). It is possible that Lepidotrichidae is the sister group to all other Zygentoma, with Nicoletiidae and Ateluridae being sister to a clade consisting of Lepismatidae and Maindroniidae. Interestingly, Tricholepidion has five-segmented tarsi, like the presumed primitive condition for Pterygota. Archaeognatha and the remaining silverfish families have two- or three-segmented tarsi, conditions typically interpreted as derived, independent reductions. Alternatively,

5.5. The relict silverfish, Tricholepidion gertschi (Lepidotrichidae), from California. The only other member of the family is found in Baltic amber, which together may comprise the sister group to the remainder of the order Zygentoma. Photo: H. Sturm.
Lepidothrix Pilifera

5.6. A silverfish from Brazil's Early Cretaceous Santana Formation. Although silverfish undoubtedly are of Devonian age, this relatively modern-looking species is among the earliest records of the order Zygentoma, leaving a presumed 280 my gap in their earliest fossil record. AMNH; body length 14 mm.

5.7. An immature silverfish (Zygentoma) in Miocene Dominican amber. Morone Collection; length 4 mm.

5.6. A silverfish from Brazil's Early Cretaceous Santana Formation. Although silverfish undoubtedly are of Devonian age, this relatively modern-looking species is among the earliest records of the order Zygentoma, leaving a presumed 280 my gap in their earliest fossil record. AMNH; body length 14 mm.

but an unlikely scenario, pentamerous tarsi developed independently in Tricholepidion and Pterygota, or these are sister groups (e.g., Wheeler et al., 2001).

The Euzygentoma have been defined by the reduction of the abdominal sterna (although this also occurs in Archaeog-natha), the reduced number of abdominal styli, and the absence of ocelli. Reduction in the number of tarsomeres may also represent a defining feature of this lineage. The Maindroniidae is a rare family of three species restricted from Asia Minor and Chile. Little is known or understood of these presumed relics, but they may be derivatives of the Lepis-matidae (e.g., Remington, 1954). The Lepismatidae is the largest family of the order, with over 200 species worldwide and as such are the group most frequently encountered. The families Nicoletiidae and Ateluridae are close relatives, both lacking eyes and at times having been included in a single family (e.g., Remington, 1954; Paclt, 1963, 1967). The Nicoletiidae are, like the lepismatids, cosmopolitan in distribution. Interestingly, some nicoletiids can reproduce parthenogenetically. Ateluridae are inquilines that live in subterranean ant and termite nests, though a few lepismatids are also inquilines.

Despite the apparent antiquity of Zygentoma as the sister group to all other dicondylic insects, there is a huge gap in their early fossil record. We would expect to find fossil Zygen-toma as early as the Devonian, but their fossil record is almost entirely restricted to Cretaceous and Tertiary resins (e.g., Mendes, 1997, 1998; Sturm and Mendes, 1998), although nice compressions of Lower Cretaceous silverfish have been recovered from the Sanatana Formation of Brazil (e.g., Sturm, 1998: Figures 5.6, 5.7). Cuticular fragments from the Devonian of Gilboa, New York, may represent a species of Zygentoma (Shear et al., 1984), but a conclusive assignment remains impossible to make. Similarly, Carbotriplura kukalo-vae from the Late Carboniferous of the Czech Republic is likely a silverfish (Kluge, 1996), and although its assignment to Zygentoma is tentative, its placement in a separate suborder of wingless insects is unjustified. The Pliocene fossil Ony-chomachilis fischeri was described as a bristletail (Pierce, 1951) but is actually a silverfish (Sturm and Machida, 2001).

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