Tetramorium shilohense species group

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Based on Bolton 1980

Species

The species in the shilohense group are arranged into four complexes:

Identification

Diagnosis

Antennae with 12 segments. Sting appendage usually triangular or pennant-shaped but may be blunted apically. Anterior clypeal margin with or without a median notch or impression, the mandibles usually sculptured if only feebly so. Frontal carinae varying from strongly developed to absent, the antennal scrobes from moderately developed to absent. Eyes small to minute, commonly with only 1-5 facets in all, but always with 5 or fewer ommatidia across the greatest diameter. Maximum diameter of eye always less than 0.17 x HW and always less than the maximum width of the antennal scape. Propodeum armed with a pair of spines or teeth. Pilosity of head and body usually of numerous fine, short hairs which are erect-suberect, very dense in one species (Tetramorium intonsum), bizarre in the T. somniculosum-complex. Dorsal (outer) surfaces of middle and hind tibiae with decumbent to appressed fine pubescence except in T. intonsum.

species complexes

The complete set of the group's species form a morphocline that links the species together. This morphocline runs through the four complexes of closely related species into which the group can be divided. In general, the line from the first complex to the last shows a reduction in pigmentation, reduction in density and intensity of sculpture, reduction in length and strength of frontal carinae until they disappear, reduction in development of antennal scrobes until they disappear, reduction in scape length, and a reduction in eye size. The first complex contains the species Tetramorium diomandei and Tetramorium somniculosum, known from Ivory Coast and Mozambique respectively. These are brown or reddish brown, strongly sculptured species in which the frontal carinae run back almost to the occipital margin and are strongly developed, being surmounted by a strong raised rim or flange. The antennal scrobes are broad and shallow but conspicuous and the eyes are of moderate size, being about 0.08-0.10 (0.10-0.13 x HW) with a relatively broad head, CI range 91-95. Other peculiarities shared by these two species include the presence of bizarre pilosity, with spatulate or scale-like flattened hairs being conspicuous on the gaster, and the development of a very irregular outline to the dorsal alitrunk, more obvious in T. diomandei than in T. somniculosum.

The second complex includes four species which are closely related to the above but which lack the bizarre pilosity. These are Tetramorium intonsum, Tetramorium jugatum, Tetramorium shilohense and Tetramorium termitobium, in which all body hairs are fine and erect to subdecumbent as indeed is the case throughout the remaining complexes of the group. In these four species sculpture is still pronounced and very distinct although the colour of the ants is yellow or yellowish brown. The frontal carinae are shorter and less strongly defined, without a strong flange or rim and fading out behind the eyes, not reaching the occipital region. The antennal scrobes are weak or vestigial and the eyes small, with 3-5 ommatidia across the greatest diameter (maximum diameter range 0-06—0-10, about 0-11-0-15 x HW). Scapes are relatively long, SI 78-99. Tetramorium intonsum and T. jugatum are West African species, known from Ivory Coast, Ghana and Nigeria (the latter also occurring in Angola) whilst T. termitobium is a central African species of Gabon and Zaire, and T. shilohense represents the complex in the southern and eastern parts of the continent, being known from Malawi, Zambia and Rhodesia.

The third complex contains only the single Nigerian species Tetramorium dysderke, which combines a mixture of characters of both the shilohense-complex and the subcoecum-complex, below. In T. dysderke the sculpture is distinct and strongly developed as in T. shilohense and its allies, but the eyes consist of only a single facet and the frontal carinae are very feeble and end at the level of the eyes, characters which are present in Tetramorium subcoecum and its relatives. Besides these features, the scrobes are absent in T. dysderke, body colour is yellow and the scapes are intermediate in length between the second and fourth complexes, with SI 80.

Finally, the subcoecum-complex, contains the species Tetramorium amaurum, T. subcoecum, Tetramorium traegaordhi and Tetramorium warreni of southern and eastern Africa, and the Ivory Coast savannah species Tetramorium typhlops. In these, colour is yellow, sculpture is very much reduced or absent (dorsal alitrunk unsculptured), the eyes are minute and consist of only 1-5 ommatidia in total. The frontal carinae are vestigial, disappearing before the level of the eyes, or are completely absent. The antennal scapes are relatively short (SI 68-80) and antennal scrobes are absent. Within the complex some gradation of characters can be seen. In T. amaurum frontal carinae are feeble but present, whereas they tend to vanish in smaller workers of T. subcoecum and are not at all represented in T. warreni. Eye size also decreases within the complex, with 3-5 facets being present in T. amaurum, 2 in T. traegaordhi, 1 in T. subcoecum and T. warreni. The limit of this reduction in the eye is seen in T. typhlops where it is represented not by an ommatidium but by a discoloured patch on the side of the head.

The species-group most closely related to the shilohense-group appears to be that of inglebyi from India (Bolton, 1977). The three species in the inglebyi-group share most of the characters noted above but have the base of the first gastral tergite modified so that it forms an anterolateral pair of horns or points, a feature not developed in the allies of T. shilohense.

Notes

At first glance, taking into account the variability of the clypeal margin, frontal carinae, scrobes, pilosity etc., this collection of species appears to be only a convenience group, linked merely by their possession of reduced or vestigial eyes. In particular the relatively large, strongly sculptured, darkly coloured forms such as Tetramorium diomandei seem far removed from the minute, depigmented smooth species like Tetramorium warreni and Tetramorium typhlops. However, when all the species of the group are placed together and compared, a morphocline becomes visible which links all the species of the group. Arranging the species according to the variable characters that form this morphocline, it is possible to divide the species into four complexes. Each set of species within each complex are more similar in the set of characters that vary, and form the morphocline, from one complex to the next.

Additional Resources

References