Geostiba (Sibiota) impressula Casey 1906
Creators: name: Gusarov, Vladimir I.
Publisher: name: Zenodo
Year: 2002
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Coleoptera
Staphylinidae
Geostiba
Geostiba impressula

description: 13. Geostiba (Sibiota) impressula (Casey, 1906), sp. propr. (Figs. 306­320)
Sibiota impressula Casey, 1906: 350.
Sipalia fossata Casey, 1910: 167, syn. nov.
Sipalia (s. str.) impressula: Fenyes, 1920: 251 (as valid species).
Sipalia (s. str.) fossata: Fenyes, 1920: 251 (as valid species).
Sipalia (s. str.) impressula: Bernhauer & Scheerpeltz, 1926: 601 (as valid species).
Sipalia (s. str.) fossata: Bernhauer & Scheerpeltz, 1926: 601 (as valid species).
Sibiota impressula: Seevers, 1978: 262 (as valid species).
Sibiota fossata: Seevers, 1978: 262 (as valid species).
Geostiba impressula: Lohse & Smetana, 1988: 270 (as synonym of Geostiba circellaris (Gravenhorst); misidentification).
Geostiba fossata: Lohse & Smetana, 1988: 270 (as valid species).
Type material. Holotype of Sibiota impressula ,, “Portland, Oreg.[on]”, “ Sibiota impressula Csy. ”, “Casey bequest 1925”, “ TYPE USNM 39028” (NMNH).
Lectotype of Sipalia fossata (here designated),, “Lane Co., Or.[egon]”, “ fossata Csy. ”, “Casey bequest 1925”, “ TYPE USNM 39029” (NMNH). The purpose of the lectotype designation is to assure correct and consistent application of the name in the future.
Additional material. UNITED STATES: Oregon:, Portland [45.5°N 122.7°W] (Wickham) (CASC);, 2, without locality data (Koebele) (CASC); Lane Co.: 17 specimens, Triangle Lake [44°10’N 123°35’W, 200­250 m], leaf litter (I.M.Newell), 13.iv.1947 (AMNH, KSEM, SPSU).
Diagnosis. Geostiba impressula can be distinguished from other Nearctic species of Geostiba by having large eyes (temple length to eye length ratio 1.8­2.5), pronotal pubescence of type VI, reduced wings, short elytra (pronotum length to elytron length ratio 1.3), the presence of two short parallel carinae in the middle of male abdominal tergum 7 in front of posterior margin, the shape of the aedeagus (Figs. 310­315, 318­319) and the shape of the spermatheca (Figs. 316­317).
Geostiba impressula differs from G. silvigena, another western Nearctic species, in having pronotal pubescence of type VI, reduced wings, shorter elytra, elytral suture behind scutellum flat, strong medial pronotal impression in males, larger aedeagus with the broader apex of the median lobe (in ventral view) (Figs. 310­312, 315, 323­324).
Description. Length 2.2­2.7 mm. Body reddish brown, sometimes with darker head and abdomen, antennae, legs and mouthparts brownish yellow. Body parallel­sided.
Head as wide as long, surface on disk with fine isodiametric microsculpture, puncturation very fine and weak, distance between punctures equal to 2­4 times their diameter. Temple length to eye length ratio 1.8­2.5. Antennal article 2 longer than article 3, article 4 slightly transverse (width to length ratio 1.2), articles 5­10 transverse (ratio 1.5­1.6), last article as long as 9 and 10 combined (Fig. 16).
Pronotum slightly transverse, width 0.37­0.50 mm, width to length ratio 1.0­1.1, wider than head (pronotal width to head width ratio 1.2); microsculpture and puncturation as on head. Pronotal pubescence of type VI. Elytra measured from humeral angle shorter than pronotum (pronotal length to elytral length ratio 1.3), wider than long (1.5), glossy, with traces of weak and fine isodiametric microsculpture and fine asperate puncturation, distance between punctures equals 2­3 times their diameter. Elytral suture not raised behind scutellum. Wings reduced to short vestiges, shorter than elytra.
Abdominal terga with fine microsculpture of transverse meshes, with fine and sparse puncturation, puncturation becoming finer towards abdomen apex, on terga 3­5 distance between punctures equals 3­6 times their diameter. Tergum 7 without white edge.
Males with strong, wide and deep impression along midline of pronotum, the impression becomes deeper and wider posteriorly. In females pronotum flattened along midline. Male tergum 7 with two medial carinae in front of posterior margin. Male tergum 8 with four small carinae in front of posterior margin, posterior margin slightly convex, the carinae protruding posteriorly as four denticles (Fig. 306). Male sternum 8 with convex posterior margin (Fig. 307).
Posterior margin of female tergum 8 convex (Fig. 308). Posterior margin of female sternum 8 slightly concave medially (Fig. 309).
Aedeagus as in Figs. 310­315, 318­320. Apex of median lobe in lateral view bent ventrally (Figs. 312­313), in ventral view apex outline slightly emarginate basally (Figs. 311, 315).
Spermatheca as in Figs. 316­317.
Discussion. Lohse and Smetana (1988) examined the holotype of this species without studying the genitalia and placed G. impressula in synonymy with G. circellaris. My examination of the spermatheca of the holotype of G. impressula demonstrates that it has a very different shape in comparison to that of G. circellaris. The difference between the males of G. impressula and G. circellaris in the secondary characters and the shape of genitalia is even more significant. Geostiba impressula is therefore a valid species and it is removed from the synonymy with G. circellaris.
The type series of Sibiota impressula and Sipalia fossata are represented by a single specimen each, a female and a male, respectively. Because of the sexual dimorphism comparison between these two types alone would not allow to confirm their conspecificity. Fortunately, the big series of specimens from Triangle Lake includes both sexes and both types are identical to these specimens in external characters and genitalia. This allows to synonymize the two names.
Distribution. Known from Portland and the Cost Ranges in western Oregon (Fig. 336).
Natural History. Geostiba impressula was collected in forest litter at altitudes up to 250 m.
In June, 2002, I tried in vain to collect additional specimens of this species around Portland and in different localities throughout Oregon, from sea level to 1650 m. The dates and localities provided on the labels of examined specimens of G. impressula (April 13; 200­250 m) and G. silvigena (October 1; near sea level), along with the absence of Geostiba specimens in extensive collections made by different staphylinid specialists in summer months seem to indicate that the western Nearctic species of Geostiba are active only during the cooler and wetter time of the year.
description: Published as part of Gusarov, Vladimir I., 2002, A revision of Nearctic species of the genus Geostiba Thomson, 1858 (Coleoptera: Staphylinidae: Aleocharinae), pp. 1-88 in Zootaxa 81 on pages 68-72, DOI: 10.5281/zenodo.155701