PHALACROMYRMECINI: defending the defensible

Barry Bolton

     Baroni Urbani & de Andrade give a number of arguments that appear to destabilize the concept of a tribe Phalacromyrmecini. I would like to present a few notes of counter-argument in support of retention of the tribe.

     Mandibular structure. A main feature of phalacromyrmecine mandible morphology is the (presumably plesiomorphic) absence of a basimandibular process such as is universally present in Dacetini, except in a few documented cases of obvious secondary reduction or modification of function of the process (Bolton, 2000). This dacetine apomorphy is not mentioned by Baroni Urbani & de Andrade, who concentrate solely on aspects of dentition.

     The characteristic phalacromyrmecine dentition of alternating large and small teeth, with the main teeth increasing in size towards the base, are not exactly duplicated in any dacetine or basicerotine species. However, accepting that the three tribes together form a monophyletic unit, and given the enormous number of species in these tribes and their universally predatory natures, it would be strange indeed if there were no convergences in any degree among the dentitions of the many species groups. Of the specific examples which Baroni Urbani & de Andrade have chosen as destabilizing the tribal dentition characters I offer the following comments.

     Pyramica bunki. This species has a typically “smithistrumiform” dentition (rostrata group, with a large basal lamella). It so happens that in this species the basal row of five enlarged teeth alternate longer/shorter. They are however followed distally by two much smaller teeth, four minute denticles and an apical tooth. Other members of the rostrata group have different relative tooth lengths in the basal row of five and throughout the group tooth 3 from the base tends to be the longest. The dentition of bunki is not phalacromyrmecine and certainly appears to be an independent evolution within its species group.

     Pyramica kichijo. Contrary to the statement by Baroni Urbani & de Andrade, the authors of this species (Terayama, Lin & Wu, 1996: 335) did not describe it as having alternating large and small teeth. In fact they said that it had “principal dental row with 4 relatively acute and large teeth followed by 2 smaller teeth and 4 minute denticles.” They also mentioned the presence of a “basal lobe”. Again this is a “normal smithistrumiform” dentition.

     Octostruma balzani, betschi and relatives. These species, as well as O. stenognatha, inca and rugifera, have only the 2 – 3 larger teeth near the midlength of the masticatory margin subtended by slightly offset intercalary denticles. But the main teeth decrease in size towards a much-expanded basal angle and the apical tooth is relatively large. The overall dentition does not appear phalacromyrmecine. In fact, the presence of the denticles may indicate the origin of the double-ranked teeth seen in several species of Eurhopalothrix (E. apharogonia, bolaui, floridana, gravis, heliscata, procera, speciosa, spectabilis, szentivanyi). It is interesting to note that in all of these species there are three such pairs of double-ranked teeth (as in the Octostruma species mentioned) except for speciosa, in which 5 are present.

     Therefore Baroni Urbani & de Andrade’s statement that “at least four closely related but non-phalacromyrmecine species exhibit the same morphology”, is just not so.

Katepisternal structure. First, Baroni Urbani & de Andrade’s rhetorical question about what should be done with a Camponotus or Pheidole species in which a katepisternal groove is present, is irrelevant as no such species has ever been discovered and I suspect never will be. This is because the structure is linked to the presence of a mesopleural gland and such a gland has not been detected in either of these genera. However, many similar pointless questions immediately spring to mind, so let me ask a few. What if a Strumigenys species with a Tetramorium sting appendage is found? What if a Formica species with Myrmecia venation is discovered? What if a Dorylus with a Pachycondyla metapleural gland structure is found? And so on.

     I admit that my usage of a katepisternal oblique groove was too heavily influenced by its appearance in Ishakidris, where it is most strongly developed, but to say that there is no equivalent structure in the other two genera is excessive. In his original description of Pilotrochus Brown’s (1978: 223) illustration distinctly shows a groove or channel that extends posteroventrally from the mesopleural gland. It appears very short by comparison with Ishakidris because the mesopleural gland is hypertrophied in Pilotrochus. In Phalacromyrmex the structure is apparently represented by oblique katepisternal rugulae that follow the same track – posteroventrally from the mesopleural gland. A scan through the dacetine and basicerotine material available to me, and through the SEM photographs in Bolton (1994, 2000), shows no equivalent structure in either tribe. Perhaps I should have said that in Phalacromyrmecini there is some katepisternal system that appears to channel the products of the mesopleural gland posteroventrally.

     Baroni Urbani & de Andrade’s selectively edited quote from Bolton (1984) reads in full: “The apparently glandular mesopleural organ seen in Ishakidris is much more strikingly developed in Pilotrochus and is roughly circular, though apparently not subtended by the open groove seen in Ishakidris.” This is primarily a comment on the mesopleural gland and of course the groove is not as strong in Pilotrochus, but to judge from Brown’s illustration the channel is present but shorter and not as sharply defined at its sides.

      Baroni Urbani & de Andrade’s final paragraph is insulting and unnecessary. It is apparently designed to show how clever these authors are compared to other mere mortals.

Reference additional to those supplied by Baroni Urbani & de Andrade

Brown, W.L. Jr. 1978. An aberrant new genus of myrmicine ant from Madagascar. Psyche 84 (1977): 218-224.