Allomerus

Fernández (2007) - Allomerus are tiny ants that inhabit internal cavities or plant structures in South American forests. Associations with domatia-producing plants Cordia or Hirtella, and the ability to build insect traps using debris (from the host plant) reinforced with living fungi, was studied by Davidson & McKey 1993, Yu & Pierce 1997, Dejean et al. 2001, Corbara 2005, Dejean et al. 2005, Debout et al. 2005.

Identification
Modified from Fernández (2007) - Monomorphic myrmicine worker with 6 to 11 segmented antennae, with a distinctive 3 segmented club in which each segment is constricted basally. Eyes well developed, slightly prominent, in full face view situated posterior to cephalic midline. Clypeus broad, shield-like, evenly rounded, anterior margin forming a distinct arc. Palpal formula 3,2. Propodeum unarmed to dentate.

The clypeal configuration, broadly inserted between the frontal antennal lobes, and the constriction in the proximal base of each antennal club are distinctive. A challenging aspect of the species level taxonomy of the genus is resolving the species, subspecies and varieties that have been described so far in Allomerus. Species number had changed from one (Wheeler 1942) to three (Bolton 1995) with various species and varieties that were not well defined. Apart from the small size and external monotony, two other factors have hindered the taxonomical resolution of the genus. One of them is the relatively limited amount of material deposited in museums, as a result of the difficulty in capturing these organisms, which live in plant cavities and are not normally collected by standard traps or methods. Another possible factor is that the flagelomere number (reflected in the genus name) has a high intraspecific variability. Quoting Forel, Wheeler (1942) mentions that there can be a different number of antennal segments between both antennae of a single specimen! Also Kempf (1975) states that the second funicular segment can be sometimes divided or fused. Although this variation is not common, it is still an important issue in the taxonomy of the genus. Because the antennal segment number is generally unreliable, it is recommended to associate antennal segment number with other characters, especially propodeal morphology. The variation of flagelomere number (due to segment fusion or division) can be broader than observed, so large amounts of material and genetic studies will be needed to understand the nature and limits of this variation.

Bolton (1987) - At genus-level Allomerus now seems fairly well defined and compact, its females and workers isolated within the group by their shield-like evenly rounded clypeal structure, lack of clypeal carinae and reduction of fringing setae on the anterior clypeal margin, sometimes until only the median seta is present. These clypeal structures plus the conspicuous apomorphic character of basally constricted antennal club segments, PF 3,2, and relatively widely separated antennal insertions, all serve to identify the genus.

The general habitus of Allomerus is very similar to that of the South African monotypic genus Diplomorium. The overall similarity may be the result of convergence as Diplomorium lacks basally constricted antennal club segments (but does have a modified club), has PF 2,2, and retains cross-vein m-cu in the forewing. Set against these differences, however, is the structural similarity of the clypeus and associated structures in both genera.

Biology
Fernández (2007) - Allomerus plant associations are of high biological interest. Table 1 shows the species list and associated plants based on literature and specimen labels. Surely, more field collections will be needed before establishing the exclusiveness of some ants to certain plants. Additional information and taxonomically well defined species will result in further symbiosis and coevolutionary studies, as those dealing with the Crematogaster and Macaranga association in Asia (Itino et al. 2001).

Dejean et al. (2001) studied predatory behavior in A. decemarticulatus. Dejean et al. (2005) described trap building behavior for the first time in A. decemarticulatus. These elaborate traps help ants catch prey, and were an unknown behavior in ants.

A recent revision pointed out challenges that stand in the way of a clear understanding of the genus (Fernández 2007): This paper is not intended to be an exhaustive revision of the genus, due to problems such as the scarcity of available material and because some type material was not available (except for A. octoarticulatus var. demerarae W.M. Wheeler, cotypes in LACM). The major justification for this study is to clarify the difficulty of species delimitation. It is fundamental to have more worker, queen and male material from the same nests, and with data regarding the plant in which the ants were collected, for further ant-plant association studies.

Castes
Flying queens are much larger than workers, with a bulky flight thorax. See photo for Allomerus decemarticulatus

Worker Morphology
These characters are collated in the table "WorkerMorphology". View table.

Nomenclature

 *  ALLOMERUS [Myrmicinae: Solenopsidini]
 * Allomerus Mayr, 1878: 873. Type-species: Allomerus decemarticulatus, by subsequent designation of Wheeler, W.M. 1911f: 158.

Worker
Fernández (2007) - Head slightly to clearly longer than wide, narrowing anteriad. Posterior border nearly flat to concave in middle, sides slightly convex. Occipital corners rounded. Mandibles with 5 teeth, generally with larger apical and subapical teeth. Clypeal anterior margin evenly rounded, forming distinct arc between mandibles, including anteclypeus or apron in most species. Frontal carinae short and widely spaced. Antennae 6 to 11 segmented, commonly 7 to 10 segmented; 3-segmented club, with each segment constricted basally. Scapes fail to reach posterior lateral corner of head, thickening apicad except in one species (Allomerus undecemarticulatus). Eyes well developed, slightly convex in full face view, with numerous facets.

Promesonotum slightly to clearly convex. Promesonotal suture absent dorsally, impressed laterally (very feebly impressed dorsally in one species, A. undecemarticulatus). Metanotal suture shallow and broad, propodeum unarmed to bidentate, dorsal face short to almost absent followed by declivity, sometimes angulate in middle. Propodeal spiracle distinct, circular, opening laterally. Bulla of metapleural gland conspicuous. Propodeal lobes small. Petiole with peduncle shorter or as long as petiolar node, node with rounded apex; petiolar spiracle near anterior side of petiole. Subpetiolar process produced as small spine or tooth followed by longitudinal carina. Postpetiole dorsally concave, lower than petiole. Postpetiole broader than long and broader than petiole. Basal portion of first tergum with shallow cuneate trench.

Body smooth and shining. Sides of mesopleura and propodeum with few longitudinal short and irregular carinulae.

Hairs scarce to abundant on body dorsum. Long, erect / suberect hairs on dorsum of head, near vertexal margin and decumbent, rows of several hairs on frons; several on promesonotum, none on propodeum, several on petiole, postpetiole and gaster. Short hairs on dorsum of mesosoma, few on propodeum. Anterior clypeal margin with setae generally short and inconspicuos, central seta sometimes feebly longer than others. Antennae and legs with appressed pubescence.

Concolorous brownish yellow to dark brown, hairs whitish.

Queen
Mandibles 9 to 11 segmented, with poorly defined 4 segmented club (Fernández 2007). Propodeum unarmed to slightly angulated. Wings as in Fig. 7D. Body hairy.

Male
(Kempf 1975:350-351). Mandibles well developed, with 5 teeth on masticatory border. Clypeus shield-like, disc moderately convex, lacking carinae or grooves. Frontal carinae absent. Eyes protruding. Posterior ocelli on vertex slightly protruding. Notauli and parapsidial sutures present. Scutellum not overhanging metanotum. Basal face of propodeum longer than declivous face, unarmed. Front wing as in Figure 7D. Petiole without node, gradually thickened caudad.