Odontomachus species groups

In 1976 Bill Brown published what remains as the definitive taxonomic treatment of the genus Odontomachus. His study assigned the known species to one of twelve species groups. Some of this arrangement has since been altered, i.e., Sorger & Zettel (2011), but much remains in place. Brown's ideas about the relationships of species within the genus is useful in understanding aspects of their biology, morphology, and the potential relatedness of various congeners.

Introduction to the groups and the rationale for their groupings

Brown (1976), beginning page 107:

The species of Odontomachus can be assigned among 12 more or less arbitrary groups. These groups can be arranged in a series in which the putatively more primitive ones come early, and the more advanced ones fall toward the end. Of course, any given species may exhibit a mixture of primitive and advanced character states, and the judgement about relative primitiveness of a taxon really boils down to a character-by-character assessment and correlations found among the states of the different characters. The main characters considered in a study of phylogeny within Odontomachus are the following:

1. Mandibular form and armament (female). It is difficult to separate mandibular characters into separate components. In general, I see the evolutionary progression as moving from long, slender mandibles with 3 slender, acute teeth in the apical trio, and a preapical series of large and sharp teeth diminishing in size basad along the mandibular shaft; toward a shorter, broad kind of mandible with the apical trio of teeth much shorter and thicker, quickly wearing to truncate or rounded, hammer- or bludgeon-like opposable surfaces, while the preapical masticatory border suffers the evolutionary reduction and eventual loss of at least the more distal tooth units as the border is transformed into a crenulate or cult rate shearing edge.

It is assumed that prey capture in the more primitive groups is mainly a matter of striking and puncturing the prey with the long, sharp teeth at the ends of the jaws (including the largest, most distal teeth of the preapical series). Thus, the primitive groups may have to depend more in prey with softer integument. We have no useful information on the prey or details of prey capture in the long-toothed groups, but by analogy with the sharp-toothed Strumigenys of subfamily Myrmicinae (Brown and Wilson, 1960) one might predict that they will be found to use the sting liberally against prey that have been struck and punctured, and sooner or later held impaled between the tips of the closed jaws (Ledoux, 1952). The longer jaws in the primitive groups may well be adaptive in holding protective all om ones released by the prey away from the captor.

In the higher groups of Odontomachus, the bludgeon-and-shears jaws can be used against small, soft prey simply to crush them with one or two strikes by the blunt tips, while the shearing action of the sharp preapical masticatory border is effectively used against the appendages of larger or tougher prey by repeated strikes, as long ago witnessed by Wheeler (1900), and many times by myself. This attack by dismemberment would hypothetically require less use of the sting. Unfortunately the differences in mandibular morphology did not impress me with sufficient force until a late stage of this study, and despite numerous opportunities in the field, I am afraid that I have no good behavioral data that bear upon the topic.

Morphological primitiveness of the long-toothed mandible is assumed from the widespread presence of the long-toothed state in the presumed ancestral taxon, Anochetus, from the direct application of the reduction rule ( << Lamarck's Arrow» or «Williston's Rule») to the number and size of teeth on the mandibles, from the correlation of the bludgeon-and-shears type of mandibles at the opposite end of the series with another reduction character, the 3-merous labial palpi, and from the greater morphological diversity and geographical discontinuity of the long-toothed groups.

2. Palpal segmentation (female). The 4, 4 formula is imdoubtedly primitive in Odontomachus, by both the ancestor-group and reduction rules. The haematodus group has 3-merous labial palpi, representing a segment lost by fusion.

3. Sculpture and pilosity - pubescence (female). It is difficult to decide whether in the genus as a whole striation of the entire dorsal surface of the head is a primitive or an advanced character state; this character apparently has suffered some reversals within the genus, as it may have done also independently in Anochetus. Extensive striation, or other fine sculpture departing from a smooth surface of the gaster, however, seems to be restricted in Odontomachus to the ruficeps and haematodus groups, which on grounds of the structure of mandibles, head and petiole appear to be closely related. As a correlated state, then, the smooth gaster appears to be primitive. The striate gastric surface (also coarse gastric punctation) appears in Anochetus, apparently as a convergent, or at most a parallel, development.

High-density pubescence (particularly of the gastric dorsum), and to some extent abundant longer pilosity, are derivative in Odontomachus, and are of course partly correlated with sculptural traits, but intra-group reversals have apparently occurred in both sculptural and pilosity-pubescence states.

4. Development of extraocular furrow and temporal ridge (female). These are of course just different aspects of the same character; without the ridge, there is no furrow defined. The ancestral genus Anochetus essentially is in state zero for this character, as are the tyrannicus group and coquereli in Odontomachus, but other Odontomachus groups all have the temporal ridge (weak in Odontomachus hastatus). The development of the character is partly tied to increased widening of the vertex, also an advanced state in Odontomachus, but head width varies by its own rules among Anochetus species.

5. Polyphenism of workers. There seems to exist a tendency, albeit irregularly expressed, for allometric and absolute size differences to be most marked in some middle-course groups. Thus the ruficeps group seems to show greater than usual allometric variation in shape of mandibular teeth, width of vertex and relative length of mandibles and antennae. Odontomachus ruficeps tends to have a low, broad, truncate or rounded subapical tooth in large workers, while the same tooth is slender and acute in the smallest workers, with intergrades in workers of intermediate sizes. In the rixosus group, the subapical tooth is truncate with a concave end in callow workers, but is worn to rounded-truncate in older individuals; smaller-sized workers have this tooth longer and more slender than do large workers.

Odontomachus assiniensis group

Odontomachus assiniensis

A variable species of wet tropical Africa. O. assiniensis has a broad head and relatively thick mandibles, a petiolar node more or less like that of the ruficeps or rixosus groups, and a smooth gastric dorsum. It may be the vicariant of either (or both) of these groups in Africa. Palpi segmented 4,4.

Odontomachus bradleyi group

Odontomachus bradleyi

Range limited to the western side of the Andes in Ecuador, palpi segmented 4, 4. Most similar to the Old World rixosus group.

Odontomachus coquereli group

Odontomachus coquereli

A single Malagasy species. It is a slender form related to the tyrannicus group by head form but with its own unique sculpture.

Odontomachus cornutus group

Odontomachus cornutus

Western Ecuador.

Odontomachus infandus group

Related to the tyrannicus and saevissimus groups, and like them, it has palpi segmented 4,4; long, acute, apical, intercalary and subapical mandibular teeth; and a strong preapical series of teeth. But it also shows variable broadening of the vertex and at times some tendency toward the formation of a dome-shaped petiolar node. The species belonging here are still not all satisfactorily defined; they are inhabitants of New Guinea, eastern Indonesia and the Philippines, with one known species in Vietnam.

Sorger & Zettel (2011) - Group Diagnosis of worker: Large and slender species with long antennae and legs. Palp formula 4, 4. Head long, with distinct temporal prominences. Mandibles long, with long and sharp apical and subapical teeth. Mesosoma depressed. Pronotum with species-specific striation. Petiole high, without false peduncle anteriorly, with long apical spine.

Philippine species of this group differ from described taxa from other areas by an impression of the first gaster tergite in the worker caste at the point where the petiolar spine meets the gaster (Brown 1976: 125 suggests impression during pupation). This character is not developed in gynes. Moreover, at NHMW we have seen three specimens of an undescribed taxon from Sulawesi (Indonesia) with this same impression. Such an impression is also developed in O. saevissimus Smith, 1858 from the Moluccas to the Solomon Islands (in the O. saevissimus species group). In almost all Philippine worker specimens, the anterior slope of gaster tergite 1 is flattened (while it is rounded in species from other regions); an exception is the single worker of Odontomachus sp. 2 (unnamed) from Mindanao.

Description of workers from the Philippines: (Note: This description is provided to present characteristics that all Philippine species of this group have in common. These characters are not repeated in following species descriptions.)

Head in dorsal view broadly rectangular, longer than wide, broadest at level of eyes which do not surpass outline of head in frontal aspect. Temporal ridge shallow but present. Eyes situated dorsolaterally in anterior third of head. Head partly striate (except between eyes and antennal insertions, and posterior extension species-specific); in areas without striation, microsculpture smooth, with scattered, very fine punctures. Head dorsally with pair of long standing setae approximately at centre of head (occasionally broken off). Head venter with some long hairs (broken off in a few specimens). Clypeus with some fine short white hairs. Mandibles long, slightly shorter than head; ca. 12 teeth with peculiar dentition: teeth slightly increasing in size towards apex of mandible, but three apical teeth enlarged with intercalary tooth significantly (less than half) shorter than apical tooth and subapical tooth creating the "trap-jaw". Mandibles very finely striate laterally and dorsally, but smooth and shiny mesally and apically; with long trigger hairs located ventrally and directed forward.

Mesosoma slender, in dorsal view broadest at level of pronotum, all edges rounded. Pronotum roughly oval in dorsal aspect, with fine, species-specific striation. Mesonotum with transverse striation coarser than on pronotum. Propodeum with coarse transverse striation. Mesopleuron usually with fine transverse striation (reduced in some species). Mesosoma void of standing setae, except pronotum sometimes with two setae located approximately at centre (broken off in some specimens).

Metanotal spiracle large and situated near dorsal outline; propodeal spiracle situated laterally, approximately midway between dorsal and ventral outline of propodeum; metapleural gland orifice situated ventrolaterally.

Petiole long and acute, formed into an acute spine apically; in lateral view anterior face with an upward slope, petiolar spine bent backwards, posterior face of petiole usually S-shaped. Petiole usually smooth and shiny (except in O. banksi with some striation basolaterally).

Gaster rounded to oval in dorsal aspect, anterior face of tergite 1 usually flattened in lateral aspect (except in Odontomachus sp. 2 from Mindanao), and with a small impression; this impression usually pit-shaped, sometimes linear, sometimes weak (as individual variations without diagnostic importance). Gaster smooth and shiny, with dispersed fine hair pits. Gaster tergites 1 and 2 usually with out standing setae, or with few setae near posterior margin; following tergites with increasing numbers of setae.

Description of gynes: Although gynes are known only of four out of twelve species in this group, we assume that the following differences within worker morphs are applicable to all species of the O. infandus group.

Head structures similar to worker, but ocelli present, located medially in front of ocular ridge. Eyes slightly larger than in workers. Mesosoma strongly developed, high, bearing wings (or wing insertions). Pronotum transversely striate (striae curved). Mesonotum with coarse, parallel, longitudinal ridges, scutellum and metanotum smooth and shiny. Petiole slightly stouter than in workers, but of similar shape. Gaster large, tergite 1 without impression.

[Sorger & Zettel (2011) state O. silvestrii should be excluded from this group ("included by Brown (1976) based on mandible structures, but it differs greatly in many other characteristics, e.g., by short head and a densely punctured gaster") but do not suggest a group where this species should be placed].

Odontomachus hastatus group

Odontomachus hastatus

Widespread in tropical America, palpi segmented 4, 4. Most similar to the Old World tyrannicus group.

Odontomachus haematodus group

Includes a large majority (19/23) of New World Odontomachus species. In the Old World, Odontomachus simillimus (Indo-Pacific) and Odontomachus troglodytes (Africa-Madagascar) are its only members.

These species are united by the 4,3 palpal formula, checked during this study for all except Odontomachus panamensis, Odontomachus biolleyi, and Odontomachus spissus, for which suitable material was unavailable. These are so obviously linked to the haematodus group by other characters that I have little doubt that the labial palpomere counts conform also.

Within the group we can recognize some indistinct subgroups: O. affinis, O. mayi and O. panamensis are obviously closely related by their smooth vertex and other characters. The tiny O. spissus of northern Mato Grosso also has the vertex smooth, but its short scapes and head shape suggest that it may be a specialized offshoot of something like O. meinerti or O. brunneus. O. chelifer, the largest species, is also the rnost slender (CI 52-61); it may be a relic of the primitive stock of the group in tropical America, but its relatively short, heavy mandibles are like those of its relatives.

If we except chelifer and such aberrant narrow-headed members as allolabis, we can characterize the haematodus group as with broad head (vertex) and short, stout mandibles with relatively short, blunt teeth (at least as worn in larger specimens) in the group of 3 (or 2) at each apex.

The subapical tooth is especially short; in large specimens that are at all worn, this tooth is scarcely or not at all projecting beyond the inner mandibular border. In callow workers, the subapical tooth consists of 2 low, obtuse points with a concavity between them — the whole thing apparently being a reduced homolog of the truncate or hollow-ended subapical tooth as seen in the rixosus group. The 2 points are made of thin material, easily worn away, so that the subapical tooth soon becomes, first an angular, then later a rounded, end to the inner mandibular border. The denticles along the inner mandibular (masticatory) borders are at an extreme of reduction for the genus; the apical third or more of this margin varies from edentate (cultrate) to crenulate, with worn, blunt denticles in the middle part, giving way to sharper but finer serration near the base of the mandible. Thus the mandibles, after considerable evolution, have arrived at being a short, heavy pair of shears with blunt, bludgeon-like tips.

The antennae tend to be shorter than in other groups, probably in coadaptation to the shorter mandibles, and this shows up especially in the scapes, which often in larger workers of haematodus-group species surpass the posterior border of the head only slightly when they are held straight back; sometimes they even fall short, and in O. spissus they always fail to reach the border by a wide margin.

Sorger & Zettel (2011) - Group Diagnosis: Palp formula 4, 3 (one labial palp segment lost by fusion; unique in Odontomachus, see Brown 1976). Head broad. Mandibles stout and relatively short, with short, blunt teeth; apical and especially subapical tooth short, tooth is scarcely or not at all projecting beyond the inner mandibular border (in callow workers, consists of two nae tending to be shorter than in other groups (especially scapes of larger species only slightly surpassing posterior margin of head). In most species dorsal surface strongly sculptured, mostly by striation, in many species also petiole and gaster tergites with some striation or reticulation. Mesosoma stout, not depressed. Petiole stout, without or with short spine.

Odontomachus malignus group

Western Pacific.

Sorger & Zettel (2011) - Group Diagnosis: Palp formula 4, 4. Head short, posteriorly with pair of tubercles. Mandibles long with long and sharp apical and subapical teeth. Antennae relatively short. Mesosoma not depressed. Pronotum with delicate sculpture. Petiole comparatively small and low.

Brown (1976) included O. malignus in the O. infandus species group, mainly because of its mandibular structures. However, the short head, short scape, and comparatively high and stout mesosoma differ considerably from both the O. infandus and the O. rixosus species group. The combination of morphological characters and the unique biology justify the erection of a separate species group.

Wang et al. (2020) - The following account compounds and adds on to the characters as already mentioned in Sorger and Zettel (2011). Considering the limited geographic range of specimen sampling and lack of stronger genetic evidence, this diagnosis should be deemed preliminary. The morphological and genetic bases for the treatment of this species group may be confirmed in the future when more samples and comprehensive genetic information are made available for assessment. Most of the known species of the Odontomachus infandus group, excluding those from the Lesser Sundas, New Guinea and Fiji, were examined to clarify morphological characters that distinguish Odontomachus malignus and the new species from other members of the group. The O. malignus group is recognised by the following set of characteristics in the worker caste; those peculiar to the group are shown in italics.

Worker. Head rather elongate, with CI 79–82. Temporal prominences and ocular furrow distinct. Vertex posteriorly with a protuberance on each side of median furrow. Mandible with a series of teeth throughout the length of its masticatory margin; apical, intercalary and subapical teeth elongate, none apically truncate, all somewhat acute; palp formula 4,4. Pronotal dorsum anteriorly with two or more long erect setae that are clearly discernible from short appressed or suberect hairs around them (this condition is occasionally seen in some members of the O. infandus group). Mesopleuron with anteroventral margin strongly developed, looking like a tubercle when seen from above. Petiolar spine rather short, not very sharply pointed apically. First gastral tergite not flattened dorsally, without anteromedian pit. Valviceps of male genitalia lacking dorsolateral carina.

Odontomachus mormo group

Odontomachus mormo

Range limited to the eastern slopes of the Andes in Peru, palpi segmented 4, 4. Most similar to the Old World infandus group.

Odontomachus rixosus group

Mainland eastern Asia and the Greater Sunda Islands, a distribution largely complementary to that of the infandus group. The rixosus group has a truncate or (as worn) low and rounded subapical mandibular tooth, at least in medium-sized and large workers, the head tends to be fairly broad behind, and there is a further tendency towards height reduction and doming of the petiolar node. The palpal formula is 4,4.

Sorger & Zettel (2011) - Diagnosis: Palp formula: 4, 4. Subapical teeth of mandibles short and truncate (O. latidens, O. monticola) or only slightly truncate and long (O. rixosus). Posterior portion of head tends to be fairly broad. Petiolar node tending to height reduction.

In its present concept, the O. rixosus group is almost complementary biogeographically to the O. infandus group, with one exception: an overlap on the island of Mindanao.

Odontomachus ruficeps group

Central and western Melanesia and Australia. The first 2 of these, and possibly even all 3, are doubtfully separable as species. The workers are broad-headed, with allometric mandibles; small workers tend to have more slender mandibles with the apical trio of teeth all slender and acute, while large workers have thick mandibles with the apical trio, especially the subapical tooth, short and blunt, or truncate. The head is broad and the petiolar node, while very variable, tends to be thick and often more or less dome-shaped. The gastric dorsum is more or less distinctly striate, coriaceous or otherwise superficially sculptured, at least in part. Palpi segmented 4,4.

Odontomachus saevissimus group

Large, slender, Melanesian species. Similar to the tyrannicus group, but the workers and queens have well defined temporal ridges and extraocular furrows; the petiolar node has a peculiar, extremely attenuated form.

Odontomachus tyrannicus group

Three closely related, large Melanesian species. Their lack of a temporal ridge, and therefore of an extraocular furrow, may well be primitive; the larvae (of tyrannicus, at least) have standard piligerous tubercles on the dorsal surfaces of abdominal segments IV and V in place of holdfasts.

References