A third species of the rare ant genus Rotastruma (Hymenoptera, Formicidae) from Cambodia, with illustrated generic diagnoses and key to species

The myrmicine genus Rotastruma is a rarely collected arboreal ant group and only two species, R. recava Bolton and R. stenoceps Bolton, are known from the Oriental Region. A new species, R. epispina sp. nov., is described from Cambodia based on the worker and queen castes. The new species is distinguished from R. recava and R. stenoceps by having the propodeal spine directed upwards and elongated peduncle of the petiole. Herein, we provide an update to the identification key and diagnostic characters for the genus Rotastruma.


Introduction
The genus Rotastruma Bolton, 1991 was established with the type-species R. recava Bolton, 1991 and was assigned to the tribe Crematogastrini by Ward et al. (2015). It is a rarely collected arboreal ant group (Luo and Guénard 2016). Bolton (1991) provided a diagnosis of the worker caste and a key to species, based on the worker caste. Two species are currently recognised: R. recava from Singapore and Borneo and R. stenoceps Bolton, 1991 from China (Bolton 1991(Bolton , 2020). Luo and Guénard (2016) discovered a single queen in Hong Kong, which is less than 200 kilometres southeast from the type locality of R. stenoceps and identified it as being conspecific to R. stenoceps.
Rotastruma is known from China, Vietnam, Cambodia, Singapore and Malaysia (Borneo) (Bolton 1991 Thailand (Khachonpisitsak et al. 2020), the nest series including sexual forms has not been recorded from the Indochinese Peninsula, probably due to the rarity of the genus. During a survey of the ant fauna of Cambodia (Hosoishi et al. 2015), a nest series of an undetermined species of this genus was collected in a regrowth forest in Kampong Thom.
In this paper, we describe the undetermined species as the third Rotastruma species, based on the nest series including the queen and propose a minor definition change to the genus, with illustrations of diagnostic characters for the genus. A revised key to the species, based on the worker caste, is also provided.

Observations
Most observations were made on a Leica M205C (Leica Microsystems, Wetzlar, Germany) stereomicroscope. Images were taken using a Canon EOS 50D with a Canon MP-E 65 mm 1-5× Macro lens, then processed using Combine ZM (Hadley 2010). The scanning electron micrographs (SEM) were prepared with a JSM-5600LV (JEOL, Tokyo, Japan) scanning electron microscope. The samples were sputter-coated with gold (Ion Sputter JFC-1100, JEOL, Tokyo, Japan).

Measurements and indices
Measurements were made under a Leica M205C stereomicroscope using micrometers. All measurements are expressed in millimetres, recorded to the second decimal place. The measurements and indices follow Fischer et al. (2014) and Luo and Guénard (2016).
Head Width (HW): Maximum width of head in fullface view, excluding the eyes.
Head Length (HL): Maximum length of head from the anterior median clypeal margin to the median posterior margin of the cephalic capsule measured along the midline as a straight line.
Mandible length (MaL): Maximum length of mandible from the anterior margin of clypeus at outer side of mandibular insertion to mandibular apex.
Scape Length (SL): Maximum length of scape, from the proximal point of scape shaft, not including the condyle, to the distal end of scape.
Eye Length (EL): Maximum diameter of eye measured in lateral view.
Total length (TL): Maximum length of specimen measured from the tip of the mandible to the tip of the last abdominal segment, not including sting. Due to the position of the specimen, total length was measured as the sum of head length + thorax, petiole and postpetiole length + gaster length. Weber

Diagnosis of worker caste of Rotastruma
Bolton (1991) provided 18 diagnostic characters for the genus Rotastruma, which were based on the only two species known from the genus at that time: R. recava Bolton, 1991 and R. stenoceps Bolton, 1991. The diagnosis provided for the genus is morphologically restrictive due to the limited material. The addition of a new species to Rotastruma allows for a slight modification. The following definition is taken from his study. The character [15] in the diagnosis is slightly modified, based on the present study as below (in italics). The number in brackets (Figs 1-11) corresponds to each of the diagnoses presented by Bolton (1991) (Appendix 1).  [7,8,9,18]; 5, Mesosoma in profile view [10,11,12,18]; 6, Mesosoma in dorsal view [10,18]. Figures in brackets correspond to diagnostic characters (Appendix 1). [15]. Petiole nodiform to pedunculate, with an anterior peduncle; petiolar spiracle located at the mid-length of the peduncle (Fig. 10) Remarks. Bolton (2003) mentioned that "a curved transverse crest is present on the stipes of the maxilla in all but Rotastruma" in his Romblonella genus group (Poecilomyrma, Romblonella, Rotastruma, Vombisidris). Our observation supports his view; the curved transverse crest was also not found in R. epispina sp. nov. (Fig. 1).
In R. epispina sp. nov., the term 'peduncle of petiole' is used to refer to an anteriorly narrow region of the petiole that extends from the anterior proximity to the curving point of the ventral outline (Fig. 10 [13]; 8, Middle tibia [14]; 9, Hind tibia [14]; 10, Petiole and postpetiole in profile view [15,16]; 11, Sting in profile view [17]. With mesosoma in dorsal view, lateral margins converging posteriad; anterolateal corner of pronotum rounded; mesonotum and propodeum laterally marginate; metanotal groove indistinct. In lateral view, pronotum, mesonotum and anterior propodeum forming a slightly convex dorsal outline. Mesopleuron marginated anteriorly in lower half, demarcated from pronotum and metapleuron. Propodeum with steeply sloping declivity that is laterally marginated with carinae; spine long, directed diagonally upwards, with apical 2/3 slightly down-curved; spiracle very close to apex of metapleural gland bulla. In dorsal view, petiole rectangular with almost parallel sides. Petiole pedunculate in profile view; peduncle longer than high, dorsally almost continuous to anterior slope of node. Petiolar spiracle located at mid-length of peduncle (measured from anterior extremity of peduncle). Subpetiolar process small and triangular. Postpetiole globular and distinctly wider than petiole in dorsal view, as high as petiolar node in lateral view (PTH 0.22-0.24; PPH 0.22-0.23).
Frons with two to three pairs of longitudinal rugulae; irregular rugulae in several specimens. Dorsum of head between frontal carina and eye reticulate-rugose. Clypeus with median longitudinal rugula and one or two rugulae laterally. Pronotum dorsally reticulate with smooth interspaces, laterally with rugulae running anteroposteriorly. Mesonotum sparse with irregular rugae interrupted by transverse rugulae posteriorly. Mesopleuron, metapleuron and lateral surface of propodeum shiny, with sparse anteroposteriorly running rugulae. Dorsum of propodeum Description of queen. Head rectangular in full-face view, longer than broad (CI 81-83), with posterior margin almost straight (slightly concave medially), lateral margin slightly convex and posterolateral corner rounded. Eye large, in the longest dimension with 12-13 ommatidia. Median ocellus at the level with posterior margin of eye. Frontal carinae extending to anterior margin of lateral ocellus and diverging towards posterior margin of head. Mandible with 6 teeth; two apical teeth large. Antenna 12-segmented with 3-segmented club; scape not reaching posterolateral corner of head.
In dorsal view, mesosoma longer than wide, wider anteriorly, gradually narrowed posteriad; pronotum demarcated from mesonotum, with evenly convex anterior margin and weakly convex lateral margin; metanotal groove distinct. In lateral view, dorsum of mesonotum slightly convex. Propodeal spiracle round and close to anterior margin of metapleural gland bulla. Wing scars clearly visible. Propodeal spines directed diagonally upwards, almost straight, apically blunt.
In dorsal view, petiole rectangular with almost parallel lateral margins. In lateral view, petiole pedunculate, with an ill-defined node; peduncle longer than high, with dorsal surface almost flat; dorsum of node convex. Subpetiolar process small and triangular, located near propodeum. Petiolar spiracle located at mid-length of peduncle. Postpetiole globular and distinctly wider than petiole, as high as petiolar node in lateral view.
Frons with two to three pairs of longitudinal rugulae; irregular rugulae in several specimens. Dorsum of head between frontal carina and eye reticulate-rugose. Clypeus with one median longitudinal rugula and one or two rugulae laterally. Pronotum dorsally reticulate with shiny interspaces, laterally with reticulate or longitudinal rugulae. Dorsum of mesoscutum and mesoscutellar disc with longitudinal rugulae. Mesopleuron, metapleuron and lateral propodeum with longitudinal rugae. Dorsum of propodeum with rugulae; declivity smooth and shiny. Dorsal surface of petiolar peduncle smooth and shiny; dorsal and lateral surfaces of node reticulate-rugose. Postpetiole reticulate-rugose. First gastral tergite smooth, with some short basigastral costulae.
Dorsal surface of head with suberect setae. Two standing setae present around posterolateral corner of head. Scape with suberect setae. Outer margin of mandible with decumbent setae. Pronotum and mesonotum with long suberect setae and short suberect to decumbent setae. Dorsal surface of propodeal spine with decumbent setae. Petiolar peduncle with decumbent setae; node with longer decumbent setae. Postpetiole with decumbent setae; subpostpetiolar process with suberect setae. Gaster with decumbent to suberect setae.
Colour yellowish-brown; first gastral tergite with broad brown band.
Etymology. The species name refers to the upwardly directed propodeal spine of the species.
Distribution and biology. This species is known only from the type locality of Cambodia. The nest series was collected from a dead twig on lower vegetation (Fig. 18). As Bolton (1991) suggested that R. recava appeared to be polygynous, the colony of R. epispina found inside the dead twig also involved two dealate queens.
Remarks. This species is similar to R. stenoceps, but can be distinguished by the propodeal spine directed upwards and the long peduncle of the petiole.  Remarks. This species is similar to R. epispina sp. nov., but can be distinguished by the posterially directed propodeal spines and short petiolar peduncle. Luo and Guénard (2016) mentioned that "In full face view, head rectangular in shape, slightly wider than long (CI 85)" in the description of a single queen. However, the CI index and their image ( fig. 10B) suggest that the queen has a longer head. The R. stenoceps queen is similar to the R. epispina sp. nov. queen, but can be distinguished by the downcurved propodeal spine and short petiolar peduncle.

Rotastruma recava
Updated key to Rotastruma species, based on the worker caste The key is constructed basically after Bolton (1991), but slightly modified and updated, based on our observation.