Platythyreini

Brown, W. L., 1975, Contributions toward a reclassification of the Formicidae. V. Ponerinae, tribes Platythyreini, Cerapachyini, Cylindromyrmecini, Acanthostichini, and Aenictogitini., Search: Agriculture; Cornell University Agricultural Experiment Station 15, pp. 1-115 : 4-6

Tribe Platythyreini

Since 1952, our concept of platythyreine generic classification has changed, and advances have been registered also at the species level, so a new section on the Platythyreini is offered here to bring this early part of the reclassification into line with the developing format of the work.

In 1952, I recognized 4 genera of Platythyreini :

Platythyrea Eubothroponera Probolomyrmex Escherichia

My suspicion that Escherichia was a synonym of Probolomyrmex has been confirmed by Taylor (1965) in his revision of the latter genus, and I have been able to check this point myself in the Forel Collection (unpublished notes). Meanwhile, my study of adults and the study of the larvae by G. C. and J. Wheeler (1971) have tended to bridge the differences by which Platythyrea and Eubothroponera have been thought to be distinguished. It does not now seem possible to regard the species of Eubothroponera as more than a primitive, endemic, Australian species-group of Platythyrea . After the required synonymy, tribe Platythyreini contains just 2 distinct genera: Platythyrea and Probolomyrmex .

The treatment here is limited to a brief diagnosis and discussion of the genera, with species lists, regional keys, and revisions of some species-level situations. Species taxonomy of Platythyrea is more than usually difficult, and the revisions offered here need to be followed up. Taylor's (1965) revision of Probolomyrmex is the key work on the genus; my summary is taken in large part from that paper, with the addition of 2 new species and a. few records.

DIAGNOSIS

Worker and queen: Typical Ponerinae with elongate body and well-developed exsertile sting. Head slightly to markedly longer than broad, with sides parallel or gently converging in front. Compound eyes large, moderate in size, or absent. Ocelli always absent in worker, and sometimes even in winged queens. Antennae 12 - segmented, with relatively long scape; funiculus very smooth in outline (without constrictions between segments), without a defined club, but sometimes gradually thickened apicad. Mandibles unremarkable, triangular or short and curvedhorizontal, serially toothed, denticulate or edentate. Maxillary palpi with 3 - 6 segments, labial palpi with 2 - 4 segments.

Promesonotal suture present and apparently flexible ( Platythyrea ) or absent ( Probolomyrmex ); other sutures or grooves on truncal dorsum indistinct or lacking. Propodeal angles variably armed with sharp or blunt margins, or even blunt, tooth-like processes; the declivity concave from side to side, and often from top to bottom as well. Petiole sessile, node more or less rounded anterodorsally, but with a usually truncate, excavate, or at least abruptly descending posterior face; posterodorsal border usually (but not always) marginate, or with 2 or 3 teeth or low, angular or rounded processes. Subpetiolar process present in some form. Gaster long-cylindrical, with a tapered apex; postpetiolar (third true abdominal) segment much larger than petiole and subequal to the succeeding segment, and therefore an integral part of the gaster, although it is separated from the rest of the gaster by a modest constriction, at least in side view. Stridulatory file present on the pretergite of the second gastric segment in Platythyrea but lacking in Probolomyrmex .

Legs with at least one pectinate spur on each tibial apex; tarsal claws with or without a submedian tooth.

Sculpture and pilosity fine, usually reduced to an opaque or subopaque pruinose condition that is particularly characteristic, with erect pilosity restricted at most to mandibles, clypeus, gastric apex, and the meatus of the metapleural gland; but in a few ( Platythyrea ) species, short, fine, erect or suberect hairs are on the body and appendage surfaces and may be rather abundant. The sculpture apparently consists of extremely fine, dense punctation, often too small to be resolved at magnification upwards of 100 X with the light microscope, and associated with this is often a more or less well-developed, but never obtrusive, fine, short, appressed pubescence. In addition, a coarser but still rather shallow sprinkling of larger round punctures or foveolae is superimposed on the fine pruinose integument; this coarser punctation is probably always present in some form, but is variably distinct in different parts of the body and in different species, and varies also in gauge and density.

Color ranging from ferruginous to black, sometimes with contrasting yellowish or reddish antennae and legs, but, even in the black species, the callow period seems to last a long time as far as pigmentation is concerned, and yellow, reddish, or brown forms of normally black species are common.

The reader may have noted that the important characters of antennal insertions, clypeal form, and frontal lobes and carinae were not mentioned in the preceding remarks. These structures differ strikingly in the 2 genera of Platythyreini , Platythyrea and Probolomyrmex , and it seems foolish to make a tribal definition that contains such wide differences as these or, for example, those in wing venation or larval form.

Male: Typically proponerine in form, differing rather modestly from the queens in size and other characters, as compared with the higher subfamilies or even in tribe Ponerini . Mandibles opposable or crossing over at full closure. Head broader than long, with much larger, more convex eyes than in workers. Antennae 13 - merous, smooth. Genitalia retractile; pygidium sometimes terminating in a point or in a downcurved spine like that of many Ponerini males. Genitalia including recognizable parameres, laciniae, and aedeagus, the last generally with at least some serration on the ventral edge.

distribution: Mainly tropical, with limited extensions into subtropical regions on all main continents, New Guinea, and Madagascar.

bionomics: Platythyrea is known only from scattered observations. Most of the species are known from stray workers or small colonies in rotten wood, or from beetle burrows, hollow twigs, and similar cavities in standing live or fallen dead trees. Many of the species can be found on logs or tree trunks at forest edges in the tropics, running very rapidly over bark or leaves of trees. I have found P. sinuata running with marvellous speed and grace over leaves in living tree foliage more than 15 meters above the ground, and several of the African and Indo-Australian species are so agile that they are extremely difficult to catch. The large African species — P. cribrinodis , P. lamellosa , P. arnoldi , and P. schultzei — are ground dwellers and forage on the ground or on trees and shrubs. I found P. cribrinodis nesting in the bases of large earthen termitaries in Rhodesia, and also in grassland under large stones.

Termites seem to make up a large part of the diet of a number of Platythyrea species the world around, but Arnold (1915: 29) found that P. arnoldi in Rhodesia feeds entirely on adult beetles, especially Tenebrionidae .

The sting of Platythyrea is more severe, in my experience, than that of most ponerines, size for size. A sting of P. sinuata (in Panama) on my hand caused a deep throbbing pain that lasted most of one day, and left lingering tenderness for another day. Even the little P. parallela has a lasting sting to be respected, and I would expect that of the larger African species to be very painful, though I have not experienced it.

There is not much information about nesting, food, or foraging of Probolomyrmex , but the species all appear to be rare and relatively cryptic (or nocturnal?) in foraging habits. Species have been collected mainly in forest litter or soil, usually in the tropics, and they nest in hollow twigs, burrows, or small pieces of rotten wood in or on the soil. Their food is unknown.

relationships: It is certainly hard to escape the impression that the Platythyreini are related to the Ponerini . Not only is the habitus in general similar in all three castes of the more primitive members of each tribe, but there are also particular characters, for example, the downcurved pygidial spine shared by the males of many species in both tribes and the detailed correspondence of the primitive wing venations, which do not seem readily ascribable to convergent evolution. Even the best, general, differential worker-queen character — the structure of the clypeus, frontal area and frontal carinae — is a matter of degree rather than a clear-cut distinction. Nevertheless, the Platythyreini have two primitive characters that mark them as closer to the basic stock of the Ponerinae than are the Ponerini :

1. The male mandibles in Platythyreini , so far as known, are triangular and opposable, or cross over each other at full closure, while in Ponerini they are always reduced and nonopposable.

2. Some worker-queen Platythyrea still possess the primitive formicid palpal segmentation of 6 maxillary, 4 labial palpomeres. The highest count in worker-queen Ponerini is only 4,4, so that 2 segments of the maxillary palpi have been lost in females of the latter tribe. Quite a few male Ponerini still carry the 6,4 palpi.

The larvae of Platythyreini are somewhat ambiguous, especially since Taylor (1965: 348) described the cast cuticle of the last-instar larva of Probolomyrmex angusticeps . This larva has low mammiform tubercles in rows on the body segments, but these tubercles are not especially like those of Ponerini , and there are no substantial hairs on the head or body. Owing to the nature of the material, we do not know exactly what the larval body shape is. However, if Taylor's figures 3 and 4 represent this accurately (he did observe living larvae in addition to the cuticles figured), it is quite different from the Platythyrea shape by being straight, tapered at both ends, and without a slender " neck " in the thoracic region. Like Platythyrea , the last somite does form a bluntly rounded, ventrally projecting " tail ", but in Probolomyrmex , the last somite also bears, on the dorsal side near its base, a prominent, stalked " suspensory process, " resembling a rubber sucker-disc, by means of which the larvae are attached to the ceiling or walls of the nest chamber.

The known larvae of Platythyrea (G. C. and J. Wheeler, 1952, 1971: 1198) are much more " proponerme " in appearance (G. C. and J. Wheeler, 1964: 459). They are nontuberculate, unless some irregular papillae and small welts on the ventral surface be counted as tubercles, and much of the cuticular surface is densely spinulose and covered with short, simple to short-multifid hairs. The body is stout, but the thoracic and first abdominal somites are drawn out into a slender neck, curving ventrad, much as in the Wheeler's generalized ectatommiform or pachycondyliform body shape. The mandibles (G. C. and J. Wheeler, 1964: 459, fig. 17, Dl) are narrowly subtriangular, the thin mesial blade with a few fine irregular denticles, or 2 preapical teeth and some denticles, in addition to the acute, mesially curved apex. In Probolomyrmex , the mandibles are still narrower and straighter in frontal view, and there are 2 minute preapical teeth on the mesial margin. The primitive larval mandible in Ponerinae (and in Formicidae ) has 3 strong teeth, an apical and 2 preapicals, so that the larval mandibles of Platythyrea and Probolomyrmex must both be considered as slightly modified (derived) from the primitive condition. The Platythyrea larva is more primitive generally and fits the proponerine pattern, despite the relatively minor divergences in mandibular form and in the presence of small ventral processes and welts. I guess that the Probolomyrmex larva is derived from the Platythyrea type, and that the tubercles of the former evolved by convergence with those of larvae of tribe Ponerini . Still, Platythyreini and Ponerini are probably so closely related that we should not rule out all possibility that the tubercles of Ponerini and Probolomyrmex were derived from a common ancestor with tuberculate larvae. If this were the case, the ventral papillae and welts of Platythyrea larvae just might be the vestiges of a more complete ancestral pattern of tuberculation.

In other adult female ponerine characters, such as toothed tarsal claws, bicalcarate mid- and hind-tibial apices, full venation of both sets of wings, and the presence of an anal lobe on the hind wing, both Platythyreini and Ponerini have many species that qualify across the board as primitive. In the male genital capsule, all the structures can be matched as homologous in Platythyreini and Ponerini , as well as the known species of the other tribes of Ponerinae .

The Platythyreini have followed a different course of evolution from the Ectatommini , yet the more primitive members of these two tribes share fundamental similarities, such as larval form and pilosity, male mandible type, and the female palpal formula of 6, 4, plus the characters mentioned in the preceding paragraph. No single ectatommine species carries all of these primitive character states at once, however, and one has to postulate an archetypical ectatommine that combines all the traits needed to produce something as completely primitive as certain Platythyrea species appear to be. Only in the more complete division of the worker trunk by sutures do some ectatommines (e. g. Ectatomma ) have a character more primitive than in any platythyreine. From these considerations, it seems that Ectatommini and Platythyreini are cognate tribes in the " Proponerinae , " and that Ponerini (including Odontomachini and possibly Thaumatomyrmex ) represent a line even more closely cognate with the Platythyreini . All 3 tribes go back to Baltic Amber times, and no doubt to even earlier in the Tertiary or beyond. Ectatommini represents an old, dominant lineage that spread world wide early; it is being replaced from the Afro-Asian heartland outward by Ponerini . Platythyreini is the old, slowly dwindling sister-group of the Ponerini , specialized in different ways in different zoogeographical realms, but nowhere threatening dominance.