At present, P. arnoldi is only known from two localities in Zimbabwe and South Africa. These localities are relatively far apart, but we expect that the species is more widespread and will be collected in the area between. Despite that it was described more than 100 years ago, there is no information available about its natural history. (Hita Garcia, Hawkes and Alpert 2014)
Hita Garcia, Hawkes and Alpert 2014 - The following character combination distinguishes P. arnoldi from the other Afrotropical members of the P. arnoldi clade: eyes very small, consisting of a single dark ommatidium (OI 3–5); head clearly longer than broad (CI 85–87); maculae on vertexal angles of head well developed and conspicuous; mesopleurae weakly to moderately inflated posteriorly; petiolar node high nodiform, anteroposteriorly compressed, with anterior face relatively straight; petiole in dorsal view between 1.0 and 1.2 times wider than long (DPeI 106–114); ventral process of petiole lamelliform, subrectangular, anteroventral corner blunt and posteroventral corner conspicuously projecting posteroventrally; abdominal segment IV around 1.2 to 1.3 times longer than abdominal segment III (ASI 116–132); head, mesosoma and petiole with mat of short decumbent to subdecumbent pubescence only, without any longer, fine suberect to erect hairs.
Proceratium arnoldi is well recognisable within the clade. Its relatively thin head in full-face view (CI 85–87) groups it close to Proceratium carri (CI 85–86) and separates both from the other five species that have thicker heads (CI 91–95). However, P. carri is not likely to be confused with P. arnoldi. The latter possesses a mat of short decumbent to subdecumbent pubescence but without numerous much longer, fine standing hairs. These hairs are present in P. carri, which also has a much longer abdominal segment IV in relation to abdominal segment III (ASI 156–159) than P. arnoldi (ASI 116–132).
We only observed some minor, very normal variation in body size in the known material of P. arnoldi with the specimens from South Africa being somewhat larger (WL 0.98–1.03) than the ones from Zimbabwe (WL 0.91–0.92). Otherwise, there is no observable intraspecific variation.
Baroni Urbani and de Andrade (2003) - A member of the arnoldi clade. Outgroup of Proceratium lunatum and Proceratium burundense, and differing from both species, in the worker, by the sculpture more superficial and by the first gastral tergite more than 1/4 longer than the postpetiole instead of at least 1/7 of the length of the postpetiole. Proceratium arnoldi also differs from lunatum and burundense by the smaller value of CI (87.2-87.3) instead of 91.1-94.4. arnoldi, lunatum and burundeasse share the palp formula 3,2 and the lower mesopleurae shining and inflate. The shape of the head of arnoldi is more similar to the one of the outgroup species, Proceratium galilaeum, than to lunatum and burundense.
Keys including this Species
- Key to Afrotropical Proceratium Species
- Key to Afrotropical species of Proceratium arnoldi clade
- Key to Proceratium workers of the world
Distribution based on Regional Taxon Lists
Distribution based on AntMaps
Distribution based on AntWeb specimens
Check data from AntWeb
Very little is known about the biology of Proceratium ants. They nest in soil, rotten wood, under deep-set stones and, in a few cases, tree branches. For many species the nest consists of small rounded chambers hollowed out of soft rotten wood or in the soil. Toward the cooler limits of the range, particularly in North America, nests and foraging workers are found under deep set rocks instead of in rotten wood. The nest site is usually in forest shade, in old moist gardens, or similar habitats that are constantly moist. Some species of known to be egg predators of arthropods, especially of spiders.
Most Proceratium are relatively rare but this is not the full explanation for why they are not commonly collected. Colonies of most species are small. Based on anectdotal natural history information from a few species, it was once thought that most Proceratium would likely be found to have mature colonies that contain somewhere between 10 - 50 workers. Yet nests with more than 50, and in some cases up to 200, workers have been been reported. Besides small colonies, these ants also do not appear to forage in places where they are readily encountered.
Males and females are though to be produced in small numbers but we generally do not have enough data for colonies of any species to know what might be typical. Reproductive flights have been observered toward the end of the summer in some northern temperate areas. In these regions the nuptial flight occurs during the last half of August. Both sexes climb some distance from the nest entrance before taking flight. Workers too issue from the nest during the nuptial flight, as is often the case with otherwise cryptobiotic ants.
The following information is derived from Barry Bolton's New General Catalogue, a catalogue of the world's ants.
- arnoldi. Proceratium (Sysphincta) arnoldi Forel, 1913j: 210 (w.) ZIMBABWE. Combination in Sysphincta: Arnold, 1915: 35; in Proceratium: Brown, 1958g: 247. See also: Baroni Urbani & De Andrade, 2003b: 297.
Unless otherwise noted the text for the remainder of this section is reported from the publication that includes the original description.
Baroni Urbani and de Andrade (2003) - Head longer than broad, with the sides gently diverging posteriorly. Vertex in full face view weakly convex. Clypeus medially reduced, gently convex, between the and slightly longer than the antennal sockets. Anterior border of the clypeus gently convex. Antennal socket with broad torulus. Frontal carinae slightly far from each other, partially covering the antennal insertions. Frontal area concave on the posterior third. Lateral expansions of the frontal carinae narrow, raised, diverging on the two anterior thirds and subparallel on the posterior third. Genal carinae marked, each carina correspoilding to the external border of a deep sulcus. Eyes small, visible as a dark dot below the integument. Eyes placed on the mid line of the head. First funicular joint 1/4 longer than broad. Funicular joints 2-10 broader than long. Scapes short of the vertexal margin and gently thickening apically. External border of the mandibles concave. Masticatory margin of the mandibles with 3 denticles before the pointed apical tooth. Palp formula 3,2.
Mesosoma gently convex in profile and as long as the maximum head length (mandibles included). Promesopleural and meso-metapleural sutures impressed ventrally only. Basal face of the propodeum declivous posteriorly. Declivous face of the propodeum gently concave anteriorly. Basal and declivous faces of the propodeum separate laterally by a lamelliform tooth. Sides of the declivous face of the propodeum with a broader and posteriorly subangulate lamella. Lower mesopleurae with well-defined sutures, its posterior half inflate and smooth. Propodeal spiracle round amid above mid height in lateral view.
Petiolar node convex in profile, the dorsum of the node convex and bearing postero-medially a small, faint point. Petiole in dorsal view with the sides diverging on the anterior fifth and convex posteriorly. Anterior border of the petiole almost straight and carinate, the carina angulate on each side. Ventral process of the petiole lamelliform, subrectangular, pointed anteriorly and posteriorly. Postpetiole anteriorly broader than the petiole; its sides diverging posteriorly. Postpetiolar dorsurn with a posteromedial, very faint, semitransparent, flat bulla below the integument. Postpetiolar sternite anteromedially with a marked subtriangular projection and strongly convex posteriorly in side view. Constriction between postpetiole and gaster impressed. Gastral tergite I convex on the curvature and with a postero-medial, very faint, semitransparent, flat bulla below the integument. Remaining gastral tergites and sternites curved ventrally.
Legs slender and slightly elongate. All tibiae with a pectinate spur. Spurs of fore legs without basal spine. Fore basitarsi longer than the mid ones. Hind basitarsi 1/4 shorter than hind tibiae. Second tarsomere of hind legs subequal in size to the pretarsus. Pretarsal claws simple. Arolia large.
Body covered by hairs of three main types: (1) short, dense, subdecumbent on the whole body, sparse and suberect on the funicular joints; (2) longer than type (1), sparse and subdecumbent on the whole body, slightly shorter and rare on the scapes, absent on the funiculi; (3) shorter than hair type (1), dense and decumbent on the funicular joints only. In addition the funicular joints bear whitish, thick, appressed, short, sparse hairs.
Sculpture. Head, mesosoma, petiole, postpetiole and gaster punctate and superficially foveolate, the punctures very sparse and superficial on the gaster which is slightly shining, the foveae shallower, smaller, sparser and more irregular on the head.
Colour. Light brown.
Measurements in mm and Indices: TL 3.24-3.29; HL 0.78-0.79; HW 0.68-0.69; EL 0.02-0.03; SL 0.5 1; WL 0.90-0.92; PeL 0.33-0.34; PeW 0.35; HFeL 0.57; HTiL 0.48-0.49; HBaL 0.36-0.37; LS4 0.25-0.26; LT4 0.66-0.67; CI 87.2-87.3; SI 64.5-65.4; IGR 0.38-0.39.
Baroni Urbani and de Andrade (2003) - Type locality: Bulawayo, Zimbabwe. Type material: holotype worker labeled: "Bulawayo (Arnold). Sysphincta Roger, sp. Proceratium (Sysphincta) arnoldi For., type" in Musee d'Histoire Naturelle Genève, examined. A worker from Bulawayo, S. Rhodesia, 29.III.1913, G. Arnold, Arnold Coll. B. M. 1934-354, labelled as syntype in The Natural History Museum is likely to be the specimen on which Arnold (1915) based his description and was probably never examined by Forel.
- Arnold, G. 1915. A monograph of the Formicidae of South Africa. Part I. Ponerinae, Dorylinae. Ann. S. Afr. Mus. 14: 1-159 PDF (page 35, Combination in Sysphincta)
- Baroni Urbani, C., de Andrade, M.L. 2003. The ant genus Proceratium in the extant and fossil record (Hymenoptera: Formicidae). Museo Regionale di Scienze Naturali, Monografie, 36, 1–492. (page 297, fig. 123 worker described)
- Brown, W. L., Jr. 1958g. Contributions toward a reclassification of the Formicidae. II. Tribe Ectatommini (Hymenoptera). Bull. Mus. Comp. Zool. 118: 173-362 PDF (page 247, Combination in Proceratium)
- Forel, A. 1913k. Ameisen aus Rhodesia, Kapland usw. (Hym.) gesammelt von Herrn G. Arnold, Dr. H. Brauns und Anderen. Dtsch. Entomol. Z. 1913(Su Suppl: 203-225 PDF (page 210, worker described)