Probolomyrmex

Taylor, R. W., 1965, A monographic revision of the rare tropicopolitan ant genus Probolomyrmex Mayr (Hymenoptera: Formicidae)., Transactions of the Royal Entomological Society of London 117, pp. 345-365 : 346-351

II. Genus Probolomyrmex Mayr

Probolomyrmex Mayr, 1901, Ann. naturh. Hofmus. Wien 16: 2 - 3. Type species: Probolomyrmex filiformis Mayr , 1901, t. c. 3, [[ worker ]] Type locality: Port Elizabeth, South Africa. Monobasic.

Escherichia Forel , 1910, Zool. Jahrb. Syst. 29: 245 - 6. Type species: Escherichia brevirostris Forel , 1910, t. c, pp. 246 - 7, [[ worker ]] Type locality: Ghinda, Ethiopia. Monobasic, syn. n.

(1) Synonymy

As predicted by W. L. Brown (1952), study of the holotype of Escherichia brevirostris shows it to be unquestionably referable to Probolomyrmex , thus establishing the above synonymy. Moreover, brevirostris is almost certainly a senior synonym of the Ugandan P. parvus Weber (see p. 355). The single known brevirostris worker is a perfectly typical Probolomyrmex , except for its well developed compound eyes. Separate generic status on the basis of this character would be completely unjustified.

(2) Characters of the genus

Worker

Known for all species except the South American P. boliviensis Mann .

Small sized monomorphic ponerine ants. Head longer than broad, its maximum width less than 0.5 mm. Clypeus and anterior part of frons produced forwards as a narrow subrectangular shelf bearing the exposed and closely approximated antennal insertions, which are separated by a thin, vertical lamella formed by fusion of the frontal carinae. Mandibles small, elongate-triangular, obscured in facial view by frontoclypeal process; each with an acute apical tooth followed by a series of small denticles, the anterior one of which may be enlarged. Labrum transverse, its anterior border with a deep median cleft. Palpal formula, maxillary 4: labial 2. The 3 basal maxillary palpomeres about subequal in size (1 - 1.5 times longer than broad), the apical one longer (3 - 5 times longer than broad). Labial palpomeres subequal in length, about 2.5 - 4 times longer than broad. Eyes lacking, except in the unique holotype of P. brevirostris (Forel) , in which they are well developed, with about 14 facets. Antennae 12 - segmented; apical portion of scape with the flexor surface more or less concave in cross-section, receiving the folded funiculus; the latter slightly incrassate but without a distinct segmental club, its second joint sometimes strongly transverse, apical joint about as long as the 3 preceding together.

Body and legs slender. Mesosomal 1 sutures virtually lacking, represented only by weak ventrolateral traces, as shown in the accompanying figures. Propleura inflated, projecting ventrally. All tibiae with a single pectinate spur; pretarsal claws simple, lacking a median tooth. Declivitous face of propodeum margined on each side by a low obtuse carina, which is usually bluntly dentate above. Petiolar node narrow, strongly arched above, higher behind than in front, with an evenly curved anterodorsal profile and an almost vertical posterior face. The latter usually quite strongly concave in side view and enclosed laterally and dorsally by a low carina. A moderate constriction between first and second post-petiolar segments. Second post-petiolar segment (abdominal IV) with its tergite and sternite fused laterally to form a tubular structure (as usual in ponerine ants). Sting well developed.

Sculpturation with 2 basic components: dense fine shagreening and associated large scattered punctures, latter often weakly incised and rarely lacking. Pilosity very reduced, limited to a few minute bristles on underside of frontoclypeal shelf, some long stout hairs on mandibles and a few fine ones about openings of metapleural glands. Pubescence very fine and short, essentially absent in some species, moderately abundant in others. Colour pale yellowish- or reddish-brown.

Because of the extreme structural reduction of Probolomyrmex , taxonomic discrimination of the species is almost entirely dependent on characters of dimensions and proportions, especially those of the head, antennae and node, and sculptural details.

Queen

This caste is known for only four Probolomyrmex species: parvus , greavesi sp. n., angusticeps M. R. Smith and boliviensis ; all are figured below. The general habitus is very standard, with interspecific differences parallel to those of the workers, which are known for all these species except boliviensis .

Size about as in conspecific workers. Structure and proportions of head capsule, frontoclypeal process, mandibles, labrum, labio-maxillary complex, oral palpi, antennae, legs, petiole and gaster almost exactly as in workers; the scapes proportionately a little shorter and the gaster slightly more voluminous. Compound eyes and ocelli well developed. Mesosoma structurally unreduced. Pronotum large; propleura as in worker. Mesoscutum lacking notauli; parapsidal lines fine but distinct. Profile of mesonotum not indented at trans-scutal suture, which is finely incised. Scutellum shield-shaped, its anterior border straight, dorsal outline (viewed from side) evenly convex. Metanotum moderately convex, not produced into a point like that of the male. Suturation lacking between metepisternum and propodeum; general form of latter as in worker.

Wings (known for two species only) long and narrow, with very reduced venation (figs. 1 and 2). Fore wing with a single closed (median) cell. Hind wing with a single longitudinal vein (probably radius + subcosta) and 3 subapical hamuli and with no trace of an anal lobe. Pilosity, pubescence, sculpturation and colour as in conspecific workers.

Male

The only known male of Probolomyrmex is a paratype of the Australian P. greavesi , which is described below.

General features as in figures 26 and 27. Head subglobose, frontoclypeal region and frontal carinae produced anteriorly as in the female castes. Antennae 13 - segmented; scapes relatively long, reaching back to the anterior ocellus; funiculus slightly incrassate, proportions of its segments as shown in figure 26. Mandibles large, triangular, with a single strong apical tooth; masticatory border rounding evenly into posterior one. Palpal formula, maxillary 4: labial 2; proportions of palpomeres as in worker.

Pronotum well developed. Mesonotum lacking notauli; parapsidal lines distinct. Scutellum moderately convex. Metanotum produced into an obtusely pointed median dorsal tooth. Metepisternum separated from propodeum by a strong suture, and itself divided obliquely into anepisternal and katepisternal areas. Legs each with a single pectinate tibial spur. Pretarsal claws simple. Wing structure and venation as in female. Petiole rounded above, with a low, simple subpetiolar process. Constriction between postpetiole and gaster barely marked. Second post-petiolar segment with its tergite and sternite fused laterally to form a tubular structure, which is slightly arched ventrally. Pygidium (tergite VIII) without a terminal spine, its apex broadly rounded. Cerci lacking. Subgenital plate (sternite IX) short, its apical margin transverse, with a very obtuse median point. Genital capsule simple. Basal ring entire; gonoforceps fairly narrowly digitate; volsellae well developed, cuspal heads somewhat expanded, and digitae simple; penis valves triangular, narrowed apically, with ventral edge finely serrate, teeth directed basally.

Larva (figs. 3 - 7)

Described from two cuticles of final instar larvae, which originally contained pharate pupae.

Body straight, elongate-subelliptical, with 13 differentiated somites, separated by rather indistinct intersegmental lines. Head anteroventral, almost orthocephalic. Prothorax not narrowed to form a neck. Abdomen stout, diameter greatest at its third and fourth segments. Leg vestiges present on all thoracic segments. Spiracles small, apparently lacking on prothorax and last 2 abdominal segments. Terminal somite forming a stout, blunt, posteroventrally directed tail; also with a median posterodorsal suspensory process of the form shown in figures 3 and 4; a low cone-shaped structure articulated to the terminal somite by a narrow neck (in life the flat base of this cone serves to attach the larva to the ceiling or walls of the nest). Anus ventral, at anterior base of tail. Sides of body longitudinally crinkled, as shown in figure 4 (this may not be a feature of the larval cuticle prior to pupal formation). Body beset with numerous low mammiform tubercles, 12 each on the thoracic and first 8 abdominal segments; arranged in 12 longitudinal rows: 2 mid-dorsal, 2 mid-ventral, a midlateral pair on either side, and single dorsolateral and ventrolateral series. The tubercles form a single transverse row on each somite, except the prothorax, where the ventrolaterals are displaced anteriorly, and the mesothorax, where the mid-ventrals are displaced slightly forwards to accommodate the leg vestiges. Mid-ventral prothoracic tubercles displaced laterally by the leg vestiges and a large median welt, which lies across anteroventral part of segment and is apparently not homologous with the tubercles. Each tubercle bears a single median nipple-like papilla, except the prothoracic ventrolaterals, which each carry 2 papillae, the anterior one with a pair of minute bristle-like sensilla. Tubercles and papillae vary in size and shape, as shown in the figures. Integument, apart from the surfaces of the tubercles, densely papilligerous; papillae 0.003 - 0.005 mm. high, arranged generally in transverse rows. Pilosity completely lacking.

Cranium large, subcircular in anterior view, slightly concave behind. Head naked, except for a few sensilla and some minute hairs. Antennae a pair of low flat subcircular elevations, each with 3 sensilla. Mouthparts only moderately prominent. Labrum small, semicircular, breadth at base slightly more than twice length; apical border entire, with a few small sensilla; posterior surface densely spinulose, the spinules arranged in arcuate rows. Mandibles long, narrow, moderately sclerotised, not greatly expanded at base; apex slightly curved posteriorly and drawn into a strong mesally inclined tooth, with 2 much smaller teeth on its inner border. Maxillae hemispherical. Palpi not peg-like, each represented by a group of 3 sensilla shaped as shown in figure 6. Galea closely adjacent to palpal sensilla, a relatively very small finger-like structure with a slender apical process. Labrum prominent. Palpi reduced similarly to those of maxillae, each represented by a group of 4 sensilla, shaped as shown in figure 7. Opening of sericteria small, slit-like. Hypopharynx spinulose, the spinules arranged in many short arcuate rows.

The Probolomyrmex larva is distinguished from those of all other known ponerine ants by the shape of the body and the unique posterodorsal suspensory organ, which is analogous (but clearly not homologous) with the dorsal " doorknob " tubercles found in some genera of the tribe Ponerini (see G. C. and J. Wheeler, 1952, 1964).

Pupa

This stage is known only for P. angusticeps , the pupae of which are unusual in that they lack cocoons. A very few other ponerine ants, including some species of Amblyopone , Discothyrea and Ponera , share this same negative characteristic. It is not a universal character in any of these genera and may not be in Probolomyrmex .

(3) Life History and Biology

Very little is known concerning the biology of Probolomyrmex . The few available ecological details indicate that most of the extra-Australian collections were made in rain-forest, or in islands of native forest in plantations. Nests in such situations are apparently located in leafmould or fragments of rotting wood on the forest floor. A shift in ecological preferences may have taken place in the evolution of the Australian P. greavesi ; both collections of this species were made in drier forest types (open Eucalyptus woodland and an exotic Pinus plantation), in which the nests were located in the soil under rocks.

Some features of the social biology of P. angusticeps are described below (see page 360). These are based on the only known observations of a live colony of Probolomyrmex ; unfortunately it is impossible to estimate whether certain features, particularly the peculiar aspects of larval and pupal life and such details as colony size and composition, are normal for the genus. Direct positive feeding records are not available, although the holotype worker of P. brevirostris was taken in a termite nest, where it may have been seeking prey. It is noteworthy that several other ponerine genera ( Discothyrea and Proceratium ), which have similar oral and anterior head structure to that of Probolomyrmex , are evidently obligatory arthropod egg predators (Brown, 1957). All known sexual forms of Probolomyrmex are of the normal winged type, so that colony proliferation probably includes a mating flight, as is usual in ants.

(4) Systematic Position of the Genus

Until recently Probolomyrmex was affiliated with Proceratium , Discothyrea , and other genera synonymous with them, in the spurious tribe Proceratiini Emery. This group was disbanded by Brown (1952, 1958), who showed that the " proceratiine habitus " of its included genera has evidently been convergently derived in several unrelated stocks. Proceratium and Discothyrea should apparently be included in the tribe Ectatommini, and Probolomyrmex appears to be related to Platythyrea and Eubothroponera , constituting with them the tribe Platythyreini . The close similarity between Probolomyrmex and some Discothyrea species, in frontoclypeal structure and other characters, is explained in these arguments as being due to convergent resemblance.

Brown's platythyreine assignment was based on a comparison of Probolomyrmex with Platythyrea , in which characters of habitus and the details of pilosity and sculpturation were considered. He concluded that " the point-by-point agreement is so close that I must consider Probolomyrmex to represent a direct derivative of Platythyrea modified for a highly cryptobiotic existence ".

The present paper contains much new information, including details of palpal formulae, wing venation, and male and larval characters. Unfortunately these facts shed little further light on the possible affinities of Probolomyrmex ; they neither strengthen the argument for a platythyreine placement, nor do they imply a better alternative assignment.

Although the additional female characters of palpal formula and wing venation and structure assist in the taxonomic diagnosis of the genus, they have little value as phylogenetic indicators. The 4: 3 palpal formula probably also occurs in Platythyrea (counts of 6: 4, 3: 2 and possibly 2: 2 were given by Brown (1952 )), but this formula is also produced in other lines of ant evolution. The wing venation is exceptional in its extreme reduction, to a point where all trace of affinities is lost.

The Probolomyrmex male has a decidedly " proceratiine habitus ", with the frontoclypeal process at least as well developed as that of any known Discothyrea male. Other apparently correlated features include the mandibular structure, the relatively large ocelli and the elongated antennal scapes. Considerable variation is shown in the structural complexity of the frontoclypeal region among females of Proceratium , and this variation is closely paralleled in the available males, each being similar to conspecific females. Moreover, the more extreme " proceratiine " head structure of Discothyrea females is also reflected in their males. Thus, it is not too surprising to find that the frontoclypeal structure of the Probolomyrmex male is similar to that of the females, and the similarities between the Probolomyrmex and Discothyrea males need in no way weaken Brown's argument. The palpal formula and wing venation are no more valuable as phylogenetic markers than in the female castes, and the genitalia are quite unspecialised, conforming to a basic ponerine plan. Similar simple genitalia occur in at least some males of Proceratium , Discothyrea and Platythyrea , as well as in those of other genera.

The Probolomyrmex male differs from those of Platythyrea in the characters discussed above and in the following additional features: it has single pectinate spurs on the middle and hind tibiae, and it lacks cerci, a terminal pygidial spine and an anal lobe on the hind wing. These same characters occur in males of Proceratium and Discothyrea as well as in those of many other ponerine genera; all are probably correlated with the small size of these animals and do not provide good phylogenetic markers. The lack of a median tooth on the pretarsal claws of all castes of Probolomyrmex need not preclude a platythyreine ancestor, since these structures occur in many ants as secondary adaptations to epigaeic foraging behaviour.

Ant larvae are very plastic organisms and may exhibit extreme modifications in response to specialised needs. Because of this, it is often difficult to evaluate the phylogenetic significance of their characters. Probolomyrmex larvae are extremely specialised, and very perplexing in this regard. The body form is unique among ponerines, and is no doubt correlated with the peculiar method by which the larvae are suspended from the ceiling of the nest by their terminal abdominal tubercles. The mandibles are rather ordinary but at least do not resemble those of Proceratium (G. C. and J. Wheeler, 1963, fig. 18, Ilia). The absence of papillae on the maxillary and labial palps is known elsewhere in only one other ponerine genus, Onychomyrmex (tribe Amblyoponini ) (G. C. and J. Wheeler, 1959, p. 638); this is almost certainly a convergently developed specialisation. A posteroventral tail is known only in two other Ponerine genera, Platythyrea and Proceratium ! The low boss-like tubercles of Probolomyrmex larvae somewhat resemble those of Proceratium ; however, similarly distributed, probably homologous, tubercles of diverse shape frequently occur in ponerine larvae (G. C. and J. Wheeler, 1952,1964) so that the possibility of convergence in this character is very likely. Platythyrea larvae have a series of paired protuberances on the ventral side of the body. These appear to be homologous with the midventral series of tubercles in Probolomyrmex and other ponerines; they may possibly indicate that the ancestral platythyreine larva was more generally tuberculate. The finely spinulose and papilligerous cuticle of the Probolomyrmex larva resembles that of Platythyrea , but similar cuticular structure occurs elsewhere in the Ponerinae and this resemblance could be convergent.

Although considerable information on the characters of Probolomyrmex is now available, a decision on the taxonomic position and phylogenetic affinities of the genus must still be largely subjective, dependent on the bias involved in " weighting " the various characters that could possibly represent phylogenetic indicators. Like Brown, I favour a platythyreine relationship for the genus, thus giving less weight to the characters of its " proceratiine habitus " than to the similarities with Platythyrea .

III. Measurements and Indices

In a genus as structurally reduced as Probolomyrmex , the use of detailed measurements and indices calculated from them is essential in providing objective characterisation of the various species. All measurements cited in this paper were made with a stereomicroscope fitted with an ocular scale reading in units of 0.1 and 0.01 mm. directly, at a magnification of 100 x. The various measurements and indices are defined as follows: -

Head length (HL): maximum mid-line length of head in full-face view, from median occipital border to clypeal apex.

Head width (HW): maximum width of head in full-face view, excluding eyes in the female castes, but including them in the male.

Scape length (SL): maximum measurable length of scape, not including its articular boss and condyle.

Cephalic index (CI): HW x 100 / HL.

Scape index (SI): SL x 100 / HW.

Weber's length of mesosoma (WL): diagonal length of mesosoma in lateral view, from the anterodorsal pronotal margin (i. e., point where pronotum joins cervix) to the posteroventral apex of the inferior lobe or flange on either side of the propodeal declivity.

Pronotal width (PW) (workers only): maximum width of pronotum viewed from directly above.

Mesonotal width (queens only): maximum width of mesoscutum viewed from directly above.

Dorsal petiole width: maximum width of petiolar node viewed from directly above.

Petiolar node index (workers only): dorsal petiole width x 100 / PW.

Petiole height: maximum height of petiolar segment in side view, measured vertically from the posteroventral corner of the subpetiolar process to the level of the petiolar apex.

Petiolar node length: maximum length of the node, measured longitudinally from the level of the spiracular process to that of the posteriormost extension of the petiolar tergum, where it surrounds the gastric articulation.

Lateral petiolar index: petiolar node length x 100 / petiole height.