Diagnosis. Myrmicine ants with the following combination of characters:
Mandibles with 4 to 6 teeth in the masticatory margin. Internal side of mandibles with a row of 2-3 to 5-6 hairs modified as lamelliform setae. Clypeus with raised median longitudinal plate, ridge or strip. Palpal formula 2,2 or less. Frontal lobes not extending posteriorly as frontal carinae. Antennal scrobes absent. Antennae 12 segmented, with club 2-segmented. Propodeum angulated or armed with teeth. U-shaped sulcus in the basalmost portion of the first abdominal tergum. Monomorphic.
Discussion. Two features noted above and setation of the clypeus require some discussion. The most interesting character, evidently an autapomorphy for the group, is the presence of modified setae on the internal face of the mandibles. Most ants exhibit setae on the internal face of the mandibles, mainly in form of a parallel row near the masticatory margin (Figs. 1, 2, 3). This configuration is more or less general in ants, with some few setae scattered or diffuse ( Ponerinae , Pseudomyrmecinae , Formicinae , Myrmicinae ). For Adelomyrmex genus group, there is an important difference: these setae are lamelliform, transparent, varying in length and shape (Figs. 4, 5), from narrow and long to triangular or subtriangular. These lamelliform setae are very small and transparent, which makes their observation difficult with traditional magnification and light. Moreover, they need to be viewed from certain angles with particular light incidence. Also, in the ants normally mounted with mandibles closed, the internal setae are not visible. Only with the SEM pictures are the lamelliform setae easily visible (Figs. 4, 5) as a result of the gold coating.
The size and position of the lamellae suggest that these are modified setae, as stated below. These modified setae are very flexible, because in specimens with closed mandibles these structures bend on the integument. When a previously dry specimen is dampened and is mounted with open mandibles, the setae return to their erect position without breaking. The function of these structures is unknown, although they could be implicated in some type of sensorial specialization. Longino (1997) speculates about some kind of special prey for Adelomyrmex , based on the clypeomandibular configuration (lateral clypeal teeth opposed to teeth in basal margin of mandible, Fig. 9). Unfortunately we do not know what these ants eat.
In some Adelomyrmex species the modified setae are parallel to the occlusal border, with few or none near the basal tooth of the basal margin. In other species, seemingly, the setae are located more toward the basal margin of the mandible or are limited to two on the internal face of the basal margin, between the tooth of basal margin and the basal tooth of occlusal margin.
Although the U-shaped sulcus in the most basal portion of the first tergum (Fig. 6) may be a synapomorphy for the group (Bolton, pers. comm.), this structure is only shallowly impressed in some species. In workers of some other myrmicine genera (e.g. Myrmicaria sp., Meranoplus mucronata ) a similar structure is evident, and in the females of Solenopsidini this sulcus is very deep. However, most myrmicines examined lack this sulcus(e.g. Myrmica , Hylomyrma , Monomorium , Megalomyrmex , Solenopsis , Ochetomyrmex , Tranopelta , Pheidologeton , Atopomyrmex , Colobostruma , Daceton , Pheidole , Aphaenogaster ). I postulate that this trait has evolved more than once in Myrmicinae .
The genus Adelomyrmex exhibits an apical median seta on the clypeus, distinguished from the other clypeal setae, which I postulate as an apomorphic condition, but ancestral for the group. Nonetheless, the other genus of the group, Baracidris , lacks this feature (at least the seta was not apparent at 200 X), which I believe to be a loss. Its absence could be a consequence of the reduction of the median clypeal fringe. However, if the clypeal configuration in Baracidris and some Adelomyrmex ( A. biroi and A. hirsutus ) is postulated to be ancestral, that is to say, the median portion of the clypeus represented merely by a narrow fringe, and the configuration in the species of the genus Adelomyrmex is the derived condition, it can be thought that a central seta is an apomorphy for this genus alone, rather than for the Adelomyrmex genus-group.
The Solenopsis genus group possesses an apical clypeal seta projected forward (Bolton, 1987). However, this group exhibits other traits (number of antennomeres in antennae and antennal club) that excludes the possibility of the inclusion of Adelomyrmex (and Baracidris ) in the group. The median clypeal seta is also observed sporadically in some Stenammini . Because the nearest tribe to Solenopsidini seems to be Pheidologetonini (Bolton, pers. comm.), which lacks the central clypeal seta, I postulate that the presence of a central seta could be a convergence among Solenopsidini , Adelomyrmex genus group and some Stenammini .
How could a central seta have evolved in Adelomyrmex ? In the ancestral situation we can imagine a continuous, flat clypeus with a row of apical setae of similar size (as in Myrmica or Tetramorium , Fig. 1). When the median bulge of the clypeus developed, some of these setae "migrated" to different positions, leaving one of them as central, accompanied at sides by paracarinal setae (Ettershank, 1966). In Solenopsidini the clypeus does not present the degree of modifications as in Adelomyrmex genus group; so the presence of a central seta must have an independent origin in this group. The same may be stated for those few Stenammini that possess a central clypeal seta.
Geographical distribution. The range of the Adelomyrmex genus-group includes the Neotropical and Afrotropical Regions, New Guinea, toward the eastern end of the Oriental Region, and the Samoan and Fijian archipelagoes in Oceania. See "Biogeographical Considerations ", below.
Key to genera of Adelomyrmex genus-group
1 Basal border of mandible with tooth (as in Fig. 54); apical clypeal seta usually present (as in Fig. 9) and hypostomal bridge with median tooth as in (Fig. 29) or petiolar node low, poorly differentiated (Figs. 64, 65); Neotropics, New Guinea, Fiji, Samoa .......... ...................................................................................................... Adelomyrmex Emery
- Basal border of mandible unarmed (Fig. 69); apical clypeal seta absent or not noticeable(Fig. 69); hypostomal bridge simple; node of petiole high (Fig. 69); West and Central Africa.................................................................................... Baracidris Bolton
No one has provided updates yet.