Crematogaster osakensis

This species inhabits grasslands (Hosoishi et al., 2015) to forests, and nests under stone and in soil or leaf litter. Colonies are polygynous. Reproductive alates fly at evening in September and are attracted to light (Japanese Ant Database Group 2008, 2008). In Japan, it is known to be winter-active (Hosoishi et al., 2019).

Identification
Hosoishi and Ogata (2016) - In the worker this species can be distinguished from all other members of the Crematogaster biroi group by the distinct compound eyes, generally smooth dorsal surface of head, petiole tapering posteriorly in dorsal view, and erect setae on body tapering distally. This species is similar to Crematogaster vieti, but can be distinguished from it by the slender propodeal spines, petiole tapering posteriorly and subpostpetiolar process angulate.

Distribution based on Regional Taxon Lists
Palaearctic Region: China, Democratic Peoples Republic of Korea, Japan, Republic of Korea.

Biology
Hosoishi et al. (2019) examined winter foraging activity in western Japan. They found foraging activity was generally low, except during relatively warm periods when the surface ground temperature was above 6–7°C or soil temperature was above 4–5°C. Tetramorium tsushimae, Messor aciculatus and Pheidole nodus were the most abundant in the open land type, whereas Nylanderia flavipes, Pheidole nodus and Crematogaster osakensis were the most abundant in the forest type.

Myrmecophilous beetles

 * Komatsu & Maruyama (2008) - Nomura (Staphylinidae) are known from the nests in Tsushima Island, Japan.
 * Ikeshita et al. (2017) - (Staphylinidae). In Kagawa Prefecture, Shikoku Island, western Japan, we often observed D. sparsa adults were walking in the vicinity of foraging workers of the myrmicine ant C. osakensis. The body color of the beetle is similar to C. osakensis and were collected only in sites where C. osakensis ants occurred. When the beetles encountered the ant workers, they bent the abdominal tip toward the ants. The ants licked the abdominal tip, and then the beetles usually walked away. Drusilla sparsa preferred to feed on dead workers of C. osakensis even when other ants were available as food.

Nomenclature

 *  osakensis. Crematogaster sordidula var. osakensis Forel, 1900e: 269 (w.) JAPAN. Wheeler, W.M. 1928d: 111 (q.m.). Combination in C. (Orthocrema): Emery, 1922e: 131. Subspecies of sordidula: Emery, 1912e: 671; Wheeler, W.M. 1928d: 111. Raised to species: Collingwood, 1976: 303; Kupyanskaya, 1990: 129; Wu & Wang, 1995: 63. Senior synonym of japonica: Brown, 1949e: 37.
 * japonica. Crematogaster sordidula var. japonica Forel, 1912l: 339 (w.) JAPAN. Junior synonym of osakensis: Brown, 1949e: 37.

Worker
Hosoishi and Ogata (2016) - (n=8) HW 0.47–0.57; HL 0.49–0.59; CI 92–100; SL 0.38–0.44; SI 77–84; EL 0.11–0.13; PW 0.27–0.35; WL 0.54–0.68; PSL 0.08–0.11; PtL 0.16–0.19; PtW 0.15–0.18; PtH 0.12–0.15; PpL 0.11–0.12; PpW 0.14–0.18; PtHI 68–81; PtWI 83–94; PpWI 117–150; WI 88–100.

Workers monomorphic. Head subquadratic in full-face view. Mandibles with four teeth arranged at an equal distance, apical and subapical teeth large, basal two teeth smaller. Anterior clypeal margin slightly concave in medial portion. Compound eyes distinctly projecting beyond lateral margins of head in full-face view. Scapes reaching posterolateral corners of head.

Pronotal collar with almost straight anterior margin in dorsal view, distinctly lower than pronotum in lateral view. Pronotal dorsum with ridges laterally. Mesonotal dorsum with lateral ridges posteriorly that irregularly extend posteriad to tips of propodeal spines. Pronotum and mesonotum in lateral view forming convex, continuous dorsal outline. Metanotal groove in dorsal view transverse, almost straight in median portion, forming deep concavity that is laterally margined by lamellate ridges. Propodeal spiracles oval, situated at posterolateral corners of propodeum, touching metapleural gland bullae. Propodeal spines developed, longer than diameter of propodeal spiracles, in dorsal view directed posteriad.

Petiole in dorsal view with subparallel sides and weakly angulate shoulders anteriorly, longer than wide. Posterior portion of petiole with short process that is slightly higher than posterior margin of petiole disc in lateral view. Subpetiolar process developed as acute process. Postpetiole in lateral view with weakly convex dorsum, as high as petiole, in dorsal view as wide as petiole, globular, not bilobed. Subpostpetiolar process developed as blunt process.

Integument essentially smooth and shining. Dorsal surface of head generally smooth and shining, but with rugulae on surrounding region of antennal sockets. Mandibles with feeble rugulae and smooth interspaces. Clypeus generally smooth and shining, but with two distinct pairs of longitudinal rugulae; rugulae not extending to posterior clypeal margin. Anterolateral shoulders of pronotum with rugulae. Dorsal surface of pronotum with rugulae. Lateral surface of pronotum smooth and shining. Mesopleura weakly sculptured, but relatively smooth on central areas. Rugula on higher portion of mesopleura extending to small pit of mesothoracic spiracles. Dorsal surface of propodeum with reticulated rugulae. Dorsal surface of petiole smooth and shining. Lateral surface of petiole weakly sculptured. Dorsal surface of postpetiole smooth and shining. Lateral surface of postpetiole weakly sculptured posteriorly.

Standing pilosity sparse. Dorsal face of head with three pairs of erect and stout long setae, and short and appressed setae sparsely. Clypeus with two pairs of long setae in anterior portion, one directed upward and the other downward. Anterior clypeal margin with one single long setae medially and one pairs of long setae laterally, and some pairs (three to four) of short setae laterally. Scapes with suberect to decumbent setae. Mesosoma with three pairs of long erect and stout setae (ps1PN, psaMN, and pspMN) that are much longer than other setae and one pair of shorter setae (ps1PN). Posterolateral tubercles of petiole posteriorly with one pair of stout long setae. Postpetiole with three pairs of stout long setae on disc anterodorsally, anterolaterally and posteriorly. Fourth abdominal tergite with suberect to decumbent stout setae sparsely.

Body yellow. All flagellar segments yellow.