Sceletolepisma Wygodzinsky, 1955 sensu Irish, 1987

Molero-Baltanás, Rafael, Gaju-Ricart, Miquel & Smith, Graeme B., 2024, New insights in the taxonomy of Lepismatidae (Insecta, Zygentoma) with an updated key to genera and future challenges, European Journal of Taxonomy 943 (1), pp. 80-126 : 108-111

Genus Sceletolepisma Wygodzinsky, 1955 sensu Irish, 1987 , stat. nov.

Ctenolepisma (Sceletolepisma) Wygodzinsky, 1955: 154 .

Type species: Ctenolepisma (Sceletolepisma) arenicola Wygodzinsky, 1955 by original designation [created as monotypic subgenus of Ctenolepisma Escherich for C. (S.) arenicola Wygodzinsky, 1955 on the basis of the 2+2 combs on urotergite I].

Ctenolepisma (Sceletolepisma) – Irish 1987: 149 [redefined subgenus to include all species with at least one pair of medial urosternal combs irrespective of the arrangement of urotergal combs].

Diagnosis

Silverfish with fusiform or subcylindrical shape, bearing feathered macrochaetae and lacking pronged sensilla on apical article of maxillary palp. Frontal area with a bare gap between two big lateral tufts of macrochaetae; clypeus and labrum usually with 1+1 tufts. Apical article of labial palp with (2)3–5 papillae arranged in one row. Pronotum with setal collar; all thoracic nota with several lateral combs and usually with 1+1 posterolateral combs. Anterior trichobothrial areas of pronotum and mesonotum associated with antepenultimate lateral combs (N−2); sometimes those of pronotum in anterior to antepenultimate (N−3) lateral combs. Thoracic sternites variable in shape, usually cordiform with convex hind margin, prosternum usually smaller than meso- and metasternum. Coxae and femora covered with rounded orbicular scales similar to those of body; tibiae and tarsi without scales, only covered by setae; remaining appendages without scales. Several urotergites (usually II–V, II–VI, II–VII or II–VIII) with 3+3 combs of macrochaetae. Urotergite I with 1+1 or 2+2 combs. Tenth urotergite trapezoidal, with straight hind margin, sometimes slightly concave or convex. Several urosternites (at least four) with one median comb, usually present on abdominal sternites I–VI, I–VII, II–VI or II–VII); 1+1 lateral combs on some urosternites (usually on II–VIII or III–VIII). Coxites IX with or without a transverse comb. One to three pairs of abdominal styli. Males without paramera. Ovipositor of primary type or apically sclerotized.

Remarks

This taxon was erected to include only one species, Ctenolepisma arenicola Wygodzinsky, 1955 , which has 2+2 combs on urotergite I; this character was exclusive to this species at that moment within Ctenolepisma , but several additional species sharing this character were discovered afterwards. Irish (1987) redefined Sceletolepisma using a different criterion for distinguishing this subgenus from the remaining species of Ctenolepisma , a group that includes the type species of the genus Ctenolepisma , C. lineatum . The new criterion was the possession of at least one median comb of macrochaetae on some urosternites, a character also present in C. arenicola .The subgenus Ctenolepisma s. str. includes, according to Irish, all species without median bristle-combs, only having 1+1 lateral combs of macrochaetae on most urosternites (usually III–VIII), but he suggested that this group could be heterogeneous. At the moment, the group with median urosternal bristle combs includes about 80 species. We consider that Sceletolepisma , in the sense established by Irish (1987), should be considered as an independent genus because all species examined share several characters apart from the presence of the median combs of macrochaetae; these characters are constant in Sceletolepisma , while in Ctenolepisma s. str. they are variable. Although the presence or absence of median combs is diagnostic in most cases, some exceptions have occasionally been observed in some Ctenolepisma s. str.

The additional characters that need to be considered include the following:

a) The scales of the appendages of all species of Sceletolepisma have a similar shape and distribution, where all appendages are covered with setae (not scales) with the exception of the scapus of the antennae and coxae of legs, which are covered with rounded scales similar to those covering the body. This distribution is, however, not exclusive to Sceletolepisma since it can also be found in Ctenolepisma (C.) ciliatum (Dufour, 1831) and related Palaearctic species of Ctenolepisma s. str. with a trapezoidal tenth urotergite (Molero-Baltanás et al. 2010), but different to other species of the genus, such as C. (C.) lineatum . In this species, for example, scales are also present on the femora as described in Molero Baltanás et al. (2012).

b) The arrangement of their trichobothrial areas is similar (contiguous on the metanotum, i.e., associated with the two last lateral combs; separated by one lateral comb on the mesonotum, i.e., associated with the last (N) and antepenultimate lateral comb (N−2) and separated by one or two lateral combs on the pronotum (associated with N and N−2 or N−3 combs)). This arrangement is also not exclusive to Sceletolepisma , but present in several genera of Ctenolepismatinae and in some species of Ctenolepisma s. str., but in this latter genus several types of arrangement of trichobothrial areas have been detected, suggesting that it represents a more heterogeneous group than Sceletolepisma .

c) Within Ctenolepisma s. lat., species having transverse bristle-combs on the inner process of the coxite IX belong only to Sceletolepisma , suggesting that only the lineage with median urosternal combs has developed this character (perhaps more than once). Probably the same happens with the occurrence of 2+2 pairs of combs on urotergite I and other apomorphies that are shared only by several species from South Africa, such as S. arenicola .

According to Irish (1987), the species Ctenolepisma unipectinatum Mendes, 1982 and Ctenolepisma howa Escherich, 1910 could be included inside Sceletolepisma , as bearing one and two median urosternal combs respectively. Nevertheless, types of both species have been re-examined (loaned by MUHNAC and MFN), concluding that the first one should be excluded from Sceletolepisma and the second has several additional median combs and not only two, fitting into the present diagnosis of the genus. A slide mounted paratype of Ctenolepisma unipectinatum , loaned by Luis F. Mendes, described as bearing only one small median comb on urosternite II, was checked and the occurrence of scales on the tibiae and narrow truncate or emarginate scales on the femora has been confirmed, so it should be excluded from Sceletolepisma . It is probably related to African species of Ctenolepisma s. str. showing similar scales on the legs. We have also examined a specimen collected in the Atlas Mountains in Morocco, related to C. brauni or C. lineatum (probably belonging to an undescribed species), that also has one small median comb on one abdominal sternite, so the occurrence of urosternal combs, at least if they are small and occurring on only one urosternite, is not a diagnostic character to separate Sceletolepisma from Ctenolepisma , but a combination of characteristics is necessary.

Although Ctenolepisma arenicola Wygodzinsky, 1955 is the type species of the genus, since it was the first to be included in the genus Sceletolepisma , it is not the first species to be described that can be assigned to this genus as defined here (sensu Irish, 1987), which would be Lepisma villosum Fabricius, 1775 .

Kaplin (1993), considering urotergal chaetotaxy as the key character for subdividing Ctenolepisma , raised the subgenus Sceletolepisma to the generic level but maintaining the criterion of Wygodzinsky (op. cit.); he also created three new subgenera within Ctenolepisma ( Allolepisma , Escherichisma and Silvestrellisma ). Irish (1994) rejected Kaplin’s arrangement as probably not reflecting phylogenetic history, retaining his earlier definitions when describing new species from southern Africa. Here the criterion of Irish is followed. Although a more extensive revision is required, Kaplin’s criteria make neither phylogenetic nor biogeographic sense. The character separating Sceletolepisma sensu Wygodzinsky from the remaining Ctenolepisma taxa, i.e., those bearing 2+2 combs on urotergite I instead of 1+1 combs, appears in several South African species that are closely related with those that do not have this character. However, species of Ctenolepisma s. lat. bearing median urosternal combs can be considered as a more uniform group sharing the characters indicated in the present diagnosis, and species lacking several median combs on abdominal sternites (in the present arrangement, the only representatives of the genus Ctenolepisma ) form a more heterogeneous group which requires a thorough revision, but not using the urotergal chaetotaxy as the unique character for splitting into subgroups.

Distribution

Representatives of Sceletolepisma are absent from America, while species belonging to Ctenolepisma s. str. are widespread on both sides of the Atlantic Ocean. This suggests that Sceletolepisma first appeared after the opening of this ocean, but we cannot discard a previous origin and a later extinction event in South America after its separation from Africa.