Mystrium shadow

Identification
Key to Malagasy Mystrium Species

The worker of Mystrium shadow is distinguished from other Mystrium species by the combination of the following characters: a central longitudinal furrow on the pronotal dorsum; projected anteromedial portion of the clypeus; blunter angle between the dorsal and posterior faces of the vertex on the median line of the head; the straight posterior declivity of the propodeum. The workers of M. shadow are quite similar to those of Mystrium mirror and Mystrium voeltzkowi, and the differences among the workers of the three species are even slighter in small-sized individuals. When workers in the same body size range are compared, M. shadow can be distinguished from M. mirror by the central pair of conical setae that is larger in size than the adjacent pair, and from M. voeltzkowi by a straight declivity of the propodeum. The queen of M. shadow can be distinguished from the other Mystrium queens by a combination of the mesosoma without developed wing sclerites but with vestigial wing appendages (ergatoid), posterior face of the vertex forming a blunt angle with its dorsal face on the median line of the head, spatulate setae on dorsum of head and pronotum, and the metapleural gland bulla that is moderately developed and not expanding dorsally to the propodeal spiracle. The ergatoid queens similar to those of M. shadow are those of M. voeltzkowi and M. mirror; however, a less-developed metapleural gland bulla distinguishes M. shadow from M. voeltzkowi, and spatulate setae on the pronotal dorsum and the presence of the genal tooth of the head distinguish M. shadow from M. mirror. The male of M. shadow is not that similar to that of M. voeltzkowi or M. mirror. The male of M. shadow can be easily separated from the latter two males by the small ocelli and small eye. On the other hand, the male of M. shadow is similar to that of Mystrium janovitzi. The shape of the vertex is the character that separates these two similar males: the posterior face of the vertex in posterior view is not clearly differentiated from its dorsal face in M. shadow, while the posterior face of the vertex is clearly divided from its dorsal face on the median line of the head in M. janovitzi. (Yoshimura and Fisher 2014)

Distribution based on type material
Madagascar.

Nomenclature

 *  shadow. Mystrium shadow Yoshimura & Fisher, 2014: 88, figs. 19A, 21B, 25B, 25D, 28D, 30B, 31D, 32A, 32C, 37G, 38G, 39G, 40G, 41G, 50E, 51E, 52E, 53E, 54E, 55E, 56E (w.eq.m.) MADAGASCAR.

Worker
Measurements: holotype. HL 2.11, HW 2.15, SL 1.60, ML 2.52, HD 1.43, WL 2.42, PnW 1.09, PpW 0.99, PtW 0.90, PtL 0.59, CI 101.9, SI 74.4, MI 116.9, PpI 91.1, PtI 152.6.

HL 1.36–2.25, HW 1.37–2.36, SL 1.00–1.78, ML 1.40–2.89, HD 0.90–1.49, WL 1.55–2.67, PnW 0.74–1.16, PpW 0.67–1.03, PtW 0.72–1.04, PtL 0.40–0.66, CI 97.0–104.9, SI 70.5–76.8, MI 102.3–125.5, PpI 85.6–93.6, PtI 147.3–179.4 (10 specimens measured).

Posterolateral corner of head strongly expanding posteriorly. Posterior face of vertex forming blunt angle with dorsal face on median line of head, so that declivity of vertex on lateral part distinctly steeper than on median part. Ventral half of vertex sculptured. Eye moderately small. Anterior margin of clypeus strongly convex with long conical setae. Genal tooth of head moderately developed, reaching to about half of lateral lobe of clypeus. Masticatory surface of mandible in full-face view slightly visible on basal half and invisible on distal half, width of dorsal surface of mandible almost identical from mandibular shaft to distal portion. Second maxillary palpomere longer than third. First flagellomere (third antennal segment) about 1.0-1.3× length of pedicel (second antennal segment). Pronotal dorsum covered with strong longitudinal striae, which are waved and irregular, but median stria always deeply, straightly and clearly impressed. Strong, deep, and irregular longitudinal striae impressed on lateral surface of pronotum. Mesonotum differentiated from propodeum in dorsal view, length shorter than that of propodeum. Metanotal groove shallowly and gently impressed, mesonotum higher than pronotum in lateral view. Metapleural gland bulla moderately developed, and propodeal declivity in lateral view almost straight but sometimes weakly rounded. Petiole relatively short (PtI ), gently narrowing from anterior 1/3 in dorsal view, anterior margin straight to gently rounded and not edged by striae.

Body color reddish brown to black. Four distal segments of antennal club brighter.

Queen
Ergatoid Measurements: HL 1.30–1.67, HW 1.31–1.70, SL 1.00–1.42, ML 1.30–1.81, HD 0.93–1.20, WL 1.74–2.33, PnW 0.76–1.00, PpW 0.76–0.98, PtW 0.76–0.96, PtL 0.45–0.61, CI 98.7–105.3, SI 76.2–85.5, MI 97.1– 108.2, PpI 95.5–101.7, PtI 157.7–178.6 (10 specimens measured).

Wings vestigial and reduced to small but distinct appendages. Wing sclerites undeveloped. Posterolateral corner of head weakly expanding posteriorly, expansion distinctly weaker than that in workers. Posterior face of vertex forming blunt angle with dorsal face on median line of head, so that declivity of vertex on lateral part distinctly steeper than on median part. Ventral half of vertex sculptured. Eye small but distinct. Ocelli absent. Anterior margin of clypeus widely convex with short conical setae, sometimes with median notch. Genal tooth of head with short spine. Masticatory margin of mandible almost invisible in full-face view, dorsal surface on distal portion as wide as that on mandibular shaft. Spatulate seta present on basal side of each basal denticle on masticatory margin of mandible. First flagellomere (third antennal segment) moderately long, about 1.5–1.2 × length of pedicel (second antennal segment). Setae on pronotum widened distally with sharp apex. Metapleural gland bulla moderately developed, not expanding to dorsum of propodeal spiracle nor propodeal declivity margin in lateral view, so that posterior margin of propodeum in lateral view almost straight. Petiole relatively long in dorsal view, about 0.5–0.8× length of abdominal segment III.

Body color reddish to blackish brown.

Male
Measurements: HL 1.12–1.18, HW 1.53–1.54, SL 0.41– 0.49, EL 0.56–0.60, WL 2.32–2.69, MnW 1.44–1.53, CI 130.6–136.6, SI 26.8– 31.8, EI 47.4–53.6, MnI 93.9–99.7 (2 specimens measured).

Eye moderately large, occupying 0.6× head length. Ocelli relatively distant from dorsal margin of head in full-face view. Dorsal margin of head in full-face view weakly rounded. Both anterior and lateral ocelli small. Lateral ocellus relatively small and distant from eye: distance between these more than 2.5× maximum diameter of lateral ocellus. Posterior face of vertex not clearly differentiated from dorsal face, so that margins almost continuously rounded. Palpal formula 4,3. First maxillary palpomere flattened and distinctly wider than second. Second maxillary palpomere longer than third. Notauli shallowly and weakly impressed on mesoscutum, but often unclear. Petiole in dorsal view thin, length 0.6× that of abdominal tergite III. Petiolar dorsum covered with shallow, irregular punctures. Abdominal tergum VIII without deep punctures, almost smooth.

Distal portion of abdominal sternum IX smooth and not punctured. Basal ring moderately long, gently expanding basally. Telomere distinctly extending distally farther than digitus. Basoventral expansion of aedeagus less developed basoventrally, as long as dorsal extension. Ventral margin of aedeagus almost straight in lateral view. Aedeagus distinctly narrowing distally on distal half, but distal portion rounded.

On forewing, cu-a located far basal from junction of Media (M) and Cubitus (Cu).

Body color reddish brown to black.

Type Material
Holotype. Worker: CASENT0318933, BLF09661, MADAGASCAR, Antsiranana, Forêt de Binara, 9.1 km 233° SW Daraina (-13.26333°, 49.60333°), 650-800 m alt., 3.xii.2003, B.L. Fisher leg. . Paratypes. 4 workers: CASENT0077646 ], CASENT0077648, CASENT0318920 , CASENT0318931 ; 3 ergatoid queens: CASENT0077647 CASC, CASENT0318932 BMNH, CASENT0318934 MHNG, with same data as holotype.

Etymology
This species name is the English word shadow, inspired by the difficulty of recognizing this new species, which is distributed like Mystrium voeltzkowi but hidden in its shadow. The species epithet is a noun and thus invariant.