Threat Level: Low

Tetramorium caldarium is a small reddish ant. This species has a monomorphic worker caste with 12-segmented antennae, three-segmented antennal club, antennal scrobes, short antennal scapes that do not surpass the posterior margin of the head, a gradually sloped mesosoma, and small propodeal spines. Like all myrmicines, T. caldarium has two waist segments and a gaster armed with a stinger.

There are five additional Tetramorium species that are spread across the Pacific Island region. Tetramorium caldarium is nearly indistinguishable from T. simillimum, and accurate separation of these two species may require a taxonomic specialist or a side by side comparison with previously determined specimens. The characters presented in the comparison chart are often difficult to observe. If the user cannot confidently differentiate between the two species, it is recommended to determine the specimen as "near T. simillimum." It is likely that many instances of T. caldarium are consistently misidentified as T. simillimum in the literature and museum collections.

Both T. simillimum and T. caldarium can be distinguished from other PIAkey Tetramorium by the combination of the following characters: (1) short, sparse and thick pilosity, (2) short propodeal spines, (3) square-shaped petiolar node, and (4) small size. See the comparison chart for additional characters.

Wasmannia auropunctata is easily confused with both T. simillimum and T. caldarium. See the videos of T. simillimum and W. auropunctata for an example of how similar they can appear in the field. Although a careful examination under the microscope is required, W. auropunctata can be separated by its 11-segmented antennae, two-segmented antennal clubs, and long propodeal spines.

There are many other myrmicine genera in PIAkey that appear similar to Tetramorium. Pheidole species can be distinguished by their polymorphic worker caste. The major workers have disproportionately large heads, and the minor workers have long antennal scapes that extended beyond the posterior margin of the head and lack antennal scrobes. The mesosoma of the majors and minors are more steeply sloped than those of Tetramorium. Cardiocondyla species lack antennal scrobes.

Tetramorium caldarium is believed to be native to Africa and is now widely distributed across the Pacific and other tropical regions. The effect this species has on native biodiversity is not well known.

Petiole with two waist segments (petiole & postpetiole)
Propodeal spines present, but short
Antennae 12-segmented Antennal club with three segments
Antennal scrobe present, but week (compare with T. simillimum)
Slope of mesosoma even & gradual
Antennal insertion surrounded by a distinct elevated ridge
Hairs short and sparse
Shape of petiole node square Petiole node & peduncle relatively narrow (compare with T. simillimum)
Anterior margin of clypeus with notch present
Posterior margin of head weakly indented in middle

Chart 1. Tetramorium caldarium vs. T. bicarinatum, T. insolens, T. pacificum, T. lanuginosum
Chart 2. Tetramorium caldarium vs. T. simillimum, T. tonganum, Wasmannia auropunctata

	T. caldarium	T. bicarinatum	T. insolens	T. pacificum	T. lanuginosum
Head
Profile
Hairs	short & sparse	long & abundant	long & abundant	long & abundant	long & very dense
Propodeal spines	short	long	long	long	long
Petiole node shape	square	square	wave-shaped	wave-shaped	rounded
	T. caldarium	T. bicarinatum	T. insolens	T. pacificum	T. lanuginosum

(top of page)

Chart 2. Tetramorium caldarium vs. T. simillimum, T. tonganum, Wasmannia auropunctata

	T. caldarium	T. simillimum	T. tonganum	W. auropunctata
Head
Profile
Total number of antennal segments	12	12	12	11
Antennal club segments	three	three	three	two
Propodeal spines	short	short	moderate	long
Hairs	short & sparse	short & sparse	long & abundant	long & sparse
Petiole node shape	square	square	rounded	square
Petiole peduncle	thick	thick	thin	(not applicable)
Posterior margin of head	indented	flat	(not applicable)	(not applicable)
Antennal scrobe	weak	strong	(not applicable)	(not applicable)
Petiole node & peduncle	narrower	broader	(not applicable)	(not applicable)
	T. caldarium	T. simillimum	T. tonganum	W. auropunctata

• Bolton, B. (1977) The ant tribe Tetramoriini (Hymenoptera: Formicidae). The genus Tetramorium Mayr in the Oreintal and Indo-Australian regions, and in Australia. Bulletin of the British Museum (Natatural History) Entomology, 36, 67-151.
  
  
• Bolton, B. (1995) A new general catalogue of the ants of the world. Harvard University Press, Cambridge, Massachusetts, 504 pp.
  
  
• Wilson, E.O. & Taylor, R.W. (1967) The ants of Polynesia (Hymenoptera: Formicidae). Pacific Insects Monograph, 14, 1-109.