Monomorium castaneum

Known from three collections, two of them single workers, from three Australian states Heterick (2001).

Identification
Heterick (2001) - A member of the monomorium group. This species stands in the same relation to M. fieldi as Monomorium megalops does to Monomorium sordidum. The worker is virtually identical to a typical M. fieldi worker, except for the conformation of the compound eye, and sculpture on the katepisternum. (Some M. fieldi workers have vestigial microreticulation on this sclerite.) The queen is similar to the M. fieldi queen, though with a mat or shagreenate propodeum. Future research may well sink M. castaneum under M. fieldi, though they are treated as separate species in this work. Monomorium castaneum can be distinguished from the similar Monomorium nanum, by the rounded propodeum and small metapleural lobes.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  castaneum. Monomorium castaneum Heterick, 2001: 398, fig. 36 (w.q.) AUSTRALIA.

Worker
HML 1.10; HL 0.41; HW 0.37; CeI 89; SL 0.33; SI 89; PW 0.22.

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

Head. Frons of head capsule smooth and shining with evenly spaced, appressed setulae, or longitudinally striate with well-spaced, appressed setulae. Compound eyes reniform, with posterior surface of eye emarginate; (viewed from front) compound eyes set in anterior half of head capsule; (viewed laterally) compound eyes set posterior of midline of head capsule; eye large, eye width greater than 1.5 x greatest width of antenna! scape. Venter of head capsule without elongate, basket-shaped setae.

Alitrunk. Promesonotal sculpture present in form of microreticulation and striolae on and around katepisternum, otherwise promesonotum smooth and shining; setulae appressed. Propodeal sculpture present as faint microreticulation with few striae, mainly on lower lateral surface; dorsal propodeal face strongly convex. Declivitous face of propodeum smoothly convex. Erect and suberect propodeal setae >5. Propodeal spiracle lateral and nearer metanotal groove than declivitous face of propodeum.

Petiole and postpetiole. Petiolar node conical, dorsally rounded, or conical, sharply tapered; sculpture absent, petiolar node smooth and shining. Ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) near 4:3. Ventral lobe present in some individuals. Height ratio of petiole to postpetiole near 4:3; height-length ratio of postpetiole near 2:1.

General characters. Colour alitrunk, petiole and postpetiole tawny orange, head, gaster, and appendages brown. Worker caste monomorphic.

Queen
HML 1.98-2.03; HL 0.57; HW 0.54-0.56; CeI 94-98; SL 0.41-0.44; SI 73-81; PW 0.41-0.46 (2 measured).

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

Alitrunk. Mesoscutum in profile convex anteriad; thereafter flattened. Mesoscutal pilosity consisting of dense incurved setulae and setae; dorsal appearance of mesoscutum smooth and shining; length-width ratio of mesoscutum and scutellum combined near 2:1. Axillae separated by distance less than half greatest width of scutellum. Propodeal sculpture present as uniform microreticulation, with few or no striae or costulae; dorsal propodeal face flattened. Metapleurallobes present as blunt flanges. Erect and suberect propodeal setae 5-10. Propodeal spiracle lateral and nearer metanotal groove than declivitous face of propodeum.

Petiole and postpetiole. Petiolar spiracle lateral and in anterior sector of petiolar node. Petiolar node conical, dorsally rounded; sculpture present in form of microreticulation. Ratio of greatest node breadth (viewed from front) to greatest node width (viewed in profile) near 1:1.

General characters. Colour of gaster chocolate; other body parts tan. Brachypterous alates seen and examined. Ergatoid or worker-female intercastes seen and examined.

Etymology
Latin: “brown, the color of chestnuts”.

References based on Global Ant Biodiversity Informatics

 * Heterick B. E. 2001. Revision of the Australian ants of the genus Monomorium (Hymenoptera: Formicidae). Invertebrate Taxonomy 15: 353-459.