Monomorium disetigerum

Monomorium disetigerum is nocturnally active, and I have collected this species in early evening on tree trunks and low vegetation (Heterick 2001).

Identification
Heterick (2001) - This species may be easily be confused with Monomorium micula and pale morphs of Monomorium sydneyense. However, M. disetigerum can normally be distinguished from these species by the presence of a pair of erect promesonotal setae. Where this feature is lacking (due to breakage) or obscured, the presence of long setae on the nodes will always separate M. disetigerum from M. micula. The large eye and more rounded propodeum of M. disetigerum will also serve to distinguish that species from pale M. sydneyense specimens. In a few workers from the Coburg Peninsula the erect setae appear to be lacking in intact specimens, though these resemble M. disetigerum in all other respects. Not enough material is available for me to determine whether the setae may have been present originally, or whether they are naturally absent in workers belonging to this population.

Heterick (2009) - One of a trio (also Monomorium micula, Monomorium silaceum) of small, yellow species from drier regions that can often be hard to distinguish from pale Monomorium sydneyense.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

North of the Tropic of Capricorn, Monomorium disetigerum probably rivals the very similar Monomorium laeve in abundance (Heterick 2001).

Nomenclature

 *  disetigerum. Monomorium disetigerum Heterick, 2001: 399, figs. 37, 121, 129 (w.q.) AUSTRALIA.

Worker
Holotype. HML 1.18; HL 0.47; HW 0.39; Cei 83; SL 0.32; SI 82; PW 0.24. Others. HML 1.01-1.34; HL 0.41-0.55; HW 0.33-0.43; CeI 77-85; SL 0.30-0.40; SI 84-95; PW 0.13-0.27 (24 measured).

As for worker of Monomorium fieldi, but with the following apomorphies.

Head. Frons of head capsule smooth and shining with combination of appressed setulae and incurved decumbent and subdecumbent setulae. (Viewed from front) compound eyes set in anterior half of head capsule. Eye large, eye width greater than 1.5x greatest width of antennal scape. Posteromedial clypeal margin extending slightly posteriad of posterior surface of antennal fossae. Venter of head capsule without elongate, basket-shaped setae. Mandibles (viewed from front) triangular and smooth, with piliferous punctures.

Alitrunk. Erect and suberect promesonotal setae less than five. Propodeal sculpture absent; propodeum smooth and shining; dorsal propodeal face sloping posteriad, with wedge-shaped flattening or shallow depression that is widest between propodeal angles; lobes present as blunt flanges. Declivitous face of propodeum smoothly convex. Erect and suberect propodeal setae >5; propodeal setulae appressed. Propodeal spiracle lateral and nearer metanotal groove than declivitous face of propodeum.

Petiole and postpetiole. Petiolar node conical, dorsally rounded. Ventral lobe present in some individuals. Height ratio of petiole to postpetiole near 4:3.

Gaster. Pilosity of first gastral tergite consisting mainly of decumbent and subdecumbent setulae, or consisting entirely of well-spaced erect and suberect setae.

General characters. Colour light to tawny yellow, legs lighter in colour than body. Worker caste monomorphic.

Queen
HML 2.18; HL 0.62; HW 0.57; Cei 93; SL 0.47; SI 82; PW 0.48 (1 measured).

As for the queen of Monomorium fieldi, but with the following apomorphies.

Head. Compound eyes ovoid.

Alitrunk. Mesoscutum in profile convex anteriad; thereafter flattened. Mesoscutal pilosity consisting of dense incurved setulae and setae; length-width ratio of mesoscutum and scutellum combined near 2: 1. Axillae separated by distance less than half greatest width of scutellum. Dorsal propodeal face flattened. Propodeal setulae decumbent and subdecumbent. Propodeal spiracle lateral and nearer metanotal groove than declivitous face of propodeum.

Wing. Queen dealate.

Petiole and postpetiole. Petiolar node conical, dorsally rounded. Ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) near 4:3. Anteroventral process a tapering carina. Height-length ratio of postpetiole near 3: 1. Sculpture present in form of microreticulation; ventral process present and distinct, formed by extension of anterior process of postpetiole.

General characters. Colour tawny yellow; gaster brown. Brachypterous alates not seen. Ergatoid or worker-female intercastes not seen.

Etymology
Latin: “bearing two setae”.