Monomorium silaceum

Monomorium silaceum does not appear to be as common as some of the other small, yellow Monomorium, and populations are mainly concentrated in the northern half of the Australian mainland. The species is known only from workers.

Identification
Heterick (2001) - The broad head capsule and very large eyes identify M. silaceum. Fresh worker specimens have a head capsule that is darker in full-face view than the promesonotum, but this feature seems to fade in older material.

Normally, the propodeum of M. silaceum is rather cuboidal and sculptured, with a large propodeal spiracle. In the case of a series of three paratype workers from Mt Whaleback, however, the general appearance of the propodeum is smoother and more gracile, resembling that of Monomorium laeve.

Heterick (2009) - One of a trio (also Monomorium disetigerum, Monomorium micula) of small, yellow species from drier regions that can often be hard to distinguish from pale Monomorium sydneyense. WA Monomorium silaceum populations in some areas of the north and north-east of the SWBP and the Pilbara departs from the typical broad-headed, punctate yellow form. These variants have a brown head capsule, are smoother and less sculptured, and are more gracile.

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Nomenclature

 *  silaceum. Monomorium silaceum Heterick, 2001: 412, figs. 33, 130 (w.) AUSTRALIA.

Worker
Holotype. HML 1.41; HL 0.53; HW 0.48; CeI 91; SI 0.40; SI 83; PW 0.26. Others. HML 1.34-1.62; HL 0.51-0.58; HW 0.44-0.57; CeI 86-98; SL 0.40-0.43; SI 73-84; PW 0.26-0.33 (12 workers).

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

Head. Eye large, eye width greater than 1.5x greatest width of antennal scape. Frontal lobes parallel, sinuate. 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; erect and suberect promesonotal setae 5-10; setulae appressed. Propodeal sculpture present as uniform microreticulation, with few or no striae or costulae; 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 ofpropodeum flat. Erect and suberect propodeal setae >5; propodeal setulae absent. Propodeal spiracle lateral and about midway between metanotal groove and declivitous face of propodeum.

Petiole and postpetiole. Petiolar node cuneate, dorsally rounded; 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. Anteroventral process always present as pronounced spur. Height ratio of petiole to postpetiole near 4:3.

General characters. Colour yellow. Worker caste monomorphic.

Etymology
Latin: “the color of yellow ochre”.