Proceratium

The genus Proceratium comprises rare ants distributed irregularly in the tropical and temperate areas of the world. Together with two even rarer genera, the pantropical and subtropical Discothyrea and the extinct Baltic amber Bradoponera make up the tribe Proceratiini. These three genera and a fourth, Probolomyrmex, constitute the subfamily Proceratiinae. Despite its rarity in collections, it is a diverse genus. The taxonomy of the genus was revised recently (Baroni Urbani & De Andrade 2003) on a global basis and includes species keys. Most Proceratium nest in rotten wood, the ground, under stones, or in trees and colonies appear to be comparatively small with usually less than 200 individuals (Brown, 1974; Baroni Urbani & de Andrade, 2003). Their diet seems similar to Discothyrea. Both genera are specialized predators of arthropod’s eggs, mostly spider eggs (Brown, 1958b, 1980). Colonies of Proceratium are small, usually with 10-50 workers (Brown 1958b). In the north temperate zone nuptial flights are usually in late summer (Brown 1958b).

Identification
Baroni Urbani and de Andrade (2003) - According to our observations the genus Proceratium appears to be characterized by at least the following three synapomorphies:

1. Worker eyes either absent or composed by minute, reduced, flat ommatidia or by a single convex facet (ommatidia salient in Discothyrea and in Bradoponera).

2. Worker and gyne with a short last funicular joint (much longer in both Discothyrea and in Bradoponera).

3. Worker and gyne with dentate mandibles (edentate in both Discothyrea and in Bradoponera).

The hammer-shaped second joint of the maxillary palps of both female castes and males Proceratium appears to be unique among ants. This character was first described by Kennedy and Talbot (1939), pointed out as a typical for Proceratium by Brown (1958a) and stressed again by Onoyama and Yoshimura (2002). Its condition in Discothyrea, however, is not linear, as it can be surmised from a figure of the palps of Discothyrea sexarticulata by Borgmeier (1954). We dissected specimens of Discothyrea stumperi and of an unidentified Discothyrea species from Manazuru, Kanagawa Pref., Japan. The second joint appears to be variably curved in these species, though never hammer-shaped as in Proceratium. Unfortunately, the synapomorphic state for Proceratium could be confirmed only by presence of the plesiomorphic state in both its most closely related genera: Discothyrea and Bradoponera. All Bradoponera specimens we studied appear to share a slender second joint of the maxillary palps. In all these specimens the palps are well visible in profile only and recognition of the hammer-shaped structure in ventral view appears unlikely but can not be categorically excluded. Mayr (1868) also observed the maxillary palps of Bradoponera meieri without mentioning the presence of hammer-shaped structure. We suppose that absence of this structure in Bradoponera is highly probable, although no completely demonstrated. The morphology of the second maxillary palp joint should also be regared as a very conceivable synapomorphy of Proceratium.

Hita Garcia, Hawkes and Alpert (2014) - One commonly used character for species level diagnostics is in fact relatively variable; we observed a lot of intraspecific variation in the surface sculpture of several species throughout most regions. This was already pointed out by Fisher (2005b) who found Proceratium avium from Mauritius to display noticeable differences in density and depth of surface sculpture. Our study supports this and extends it to the majority of species examined. The differences are not extreme, however, which means that there are never very different types of sculpture in the same species, but the type encountered can vary from very weakly developed and almost absent to very strongly developed, dense and conspicuous. Consequently, surface sculpture is not recommendable as a primary diagnostic character for the separation Proceratium species.

Characters that have proven to be comparatively stable at species level are general dimensions of the head and petiole, the shape of the ventral process of the petiole, the relationship between abdominal segments III and IV, eye size and also pilosity and pubescence. Like in most ants, the shape of the head and petiole turned out to be very stable within species-specific ranges and are useful for diagnostics. The relationship between the lengths of abdominal segments III and IV proved to be valuable, too. The development of the eyes is normally not very important in Proceratium since most species have very reduced eyes consisting of one ommatidium, a tiny cluster of indistinct flat ommatidia only distinguishable at high magnifications, or no eyes at all, but a few species, such as Proceratium burundense, have slightly larger compound eyes. At first glance the use of pilosity/pubescence might seem challenging since most species of Proceratium are very hairy and possess different types of hairs throughout their bodies. However, despite some small variation in density, which can also be attributed to specimen processing, we could not observe any significant variability in pilosity/pubescence within species. Especially the lack or presence of abundant, longer, standing pilosity on top of the very dense mat of subdecumbent to decumbent hairs seems to be species-specific.

Species Groups
Proceratium species groups
 * Key to Afrotropical Proceratium species groups

Fossils
Fossils are known from:, , , , , (an unidentified species, Wang et al., 2021).

Biology


Almost all species of Proceratium spend their lives in concealment, usually in rotten wood or deep humus, but sometimes under stones in colder climates. Captive colonies will accept, usually reluctantly, various types of insect prey, but the principle diet appears to be the eggs of arthropods, particularly spiders (Brown 1958a and 1980). The peculiarly modified gaster seems to be primarily defensive in function. The hypertrophied second gastral tergite conceals the terminal gastral segments, it presents a hardened surface to enemies approaching from the rear, and it is equipped with a gland that probably produces defensive chemicals (Baroni Urbani and de Andrade 2003). Although unusual in ants, insects in other groups have convergently evolved similar adaptations to provide a rigid armor for the gaster, while maintaining flexibility for the terminal abdominal segments. This can be seen in numerous Coleoptera, parasitic Hymenoptera, and even some Diptera. The anteriorly-directed terminal segments of the gaster have also been seen helping in the manipulation and transportation of spider eggs (Brown 1980), but nobody claims that the abdominal modifications are a necessary adaptation for this purpose, as other groups of ants have no difficulty dealing with round objects.

Aftrotropical region
Hita Garcia, Hawkes and Alpert (2014) - Considering the biogeography of the genus in sub-Saharan Africa and the rarity of collections, the available data about the distribution patterns of most species is very limited. This is especially true for more than half of the species that are only known from the type locality (P. burundense, P. nilo, P. sali, P. sokoke, P. terroni and P. toschii). The data for the other four species (P. arnoldi, P. boltoni, P. carri, and P. lunatum) is a bit better, even though they are also only known from a few localities each. The widest known distribution is seen in Proceratium lunatum, which occurs in Cameroon and Gabon, but is also found in Uganda. We expect that this species will also be found in the rainforests of the Democratic Republic of Congo, Congo, and Central African Republic with further sampling in these countries. Leston (1971) noted that for the three then known species (P. arnoldi, P. boltoni and P. toschii) it seemed as if most of the Afrotropical Proceratium prefer drier savannah habitats. Nonetheless, P. lunatum and P. terroni were later described from rainforests in Cameroon (Terron 1981), and the four new species presented in this study inhabit coastal, montane or sandy forest types. So, it seems as if Proceratium can be found in most sub-Saharan habitats with the exception of semi-deserts and deserts.

Due to the cryptobiotic lifestyle and small colony size, it is not easy to collect Proceratium, but their rarity might also be due to sampling artefacts. For example, intensive sampling in two localities in Kenya (Kakamega Forest and Mpala Research Centre, listed in Hita Garcia et al. 2013) yielded not a single Proceratium worker. However, each study found an unidentified male showing clearly that the genus is present in both localities. Consequently, we expect that intensive sampling in the soil stratum in a wide range of habitats throughout the Afrotropical region will very likely yield additional material of the currently known species and from new, yet unknown forms, as can be seen for the Malagasy region, where more than ten undescribed species have been collected within the last decade due to very intensive sampling efforts to collect subterranean ants in general (Fisher 2005a; Yoshimura and Fisher 2014; FHG, unpublished data).

The knowledge on natural history or behaviour for the ten Afrotropical species is extremely limited. Leston (1971) provided the only available data on any of them. He collected a relatively small colony (one queen with 42 workers and a few larvae and pupae) of Proceratium boltoni in a piece of rotten wood in the ground and he was also able to collect the same species around 600 m away in the topsoil at the base of a tree. He did not mention more on its natural history except that one greenish dipterous egg and one live nematode were with the colony.

Castes
At least one species from Madagascar ("fhg-beta") has ergatoid queens, with a worker-like thorax but a larger gaster (C. Peeters & B.L. Fisher unpublished).

Nomenclature

 *  PROCERATIUM [Proceratiinae: Proceratiini]
 * Proceratium Roger, 1863a: 171. Type-species: Proceratium silaceum, by monotypy.
 * Proceratium senior synonym of Sysphingta: Dalla Torre, 1893: 18; Brown, in Borgmeier, 1957: 118.
 * SYSPHINGTA [junior synonym of Proceratium]
 * Sysphingta Roger, 1863a: 175. Type-species: Sysphingta micrommata, by monotypy.
 * Sysphingta junior synonym of Proceratium: Mayr, 1886d: 437; Dalla Torre, 1893: 18.
 * Sysphingta revived from synonymy: Emery, 1895c: 262.
 * Sysphingta junior synonym of Proceratium: Brown, in Borgmeier, 1957: 118; Brown, 1958g: 241.
 * [ Sysphincta Mayr, 1865: 12 (and most subsequent authors), incorrect subsequent spelling.]

Worker
Baroni Urbani and de Andrade (2003) - Monomorphic but variable in size. Head subglobose, generally longer than broad, rarely as long as broad or broader than long, variable in shape, i.e. with sides convex, subparallel, diverging posteriorly or converging anteriorly. Median part of the clypeus either unarmed (truncate), or armed with an anterior projection varying from small to developed, convex, rectangular or triangular in shape. Frontal carinae generally apart from each other, rarely attached and never or only partially covering the antennal insertions. Lateral expansions of the frontal carinae broad or narrow, diverging backwards, rarely touching each other posteriorly, gently convex or straight. Genal carina absent or present but variably marked aid generally delimiting a variably impressed sulcus. Gular area impressed or not. Eyes absent or present, but, if present, always reduced to small pigmented spots or a clear convex facet, and placed approximately around the mid line of the head. Ocelli absent, rarely the workers of a few species may have only the anterior ocellus vestigial or as developed as in the gyne. Antennae 12-jointed. Scapes surpassing, reaching or much shorter than the vertexal margin and always thickening apically. First funicular joint about as long as broad or slightly broader than long or longer than broad. Funicular joints 2-10 broader than long, or about as broad as long, or slightly longer than broad. Last funicular joint as long as the sum of joints 6-10, or 7-10 or 8-10. Mandibles subtriangular. Basal margin of the mandibles straight or convex distally. Masticatory margin of the mandibles with 2-13 denticles of variable size followed by a pointed apical tooth. Palp formula (counting the protuberance fused with the stipes as the basalmost segment as already done by Brown, 1980 and Onoyama & Yoshimura, 2002) 2,2, 3,2, 33, or 4,3. Second maxillary palp joint hammer-shaped.

Mesosoma short to elongate. Dorsum of the mesosoma sloping posteriorly or variably convex dorsally. Promesonotal and metanotal sutures absent, obsolete or rarely impressed. Promesopleural and meso-metapleural sutures more impressed ventrally. Propodeum unarmed, simply angulate or denticulate or toothed, rarely with a pair of spines. Area between basal and declivous faces of the propodeum variably concave medially and with or without carina. Each side of the declivous face of the propodeum with a lamella or a variably marked carina. Propodeal lobes simply convex or truncate, with or without a pointed or rounded dorsal tooth. Propodeal spiracle rou nd or tubuliform and placed at mid height in lateral view. Petiole variable in size, width and height. Petiolar node in shape of an erect scale of variable width and thickness, or loaf-shaped. Ventral petiolar process small or large, truncate, triangular or spiniform. Postpetiole (abdominal segment III) anteriorly as broad as or broader than petiole. Postpetiole in dorsal view anterolaterally with diverging, convex or angulate sides. Postpetiolar sternite anteromedially with a variably marked subtriangular projection bearing or not a longitudinal carina prolonged posteriorly. Constriction between postpetiole and gastral segment I variably impressed. Gastral tergum I of variable length, variably convex, continuously curved dorsoventrally or with the posterior half clearly separated by a curve from the dorsal part. Sides of the gastral sternite I strongly or slightly projecting anteriorly. Remaining gastral tergites and sternites curved ventrally. Sting developed, curved downwards. Legs short or variably elongate. Tibiae of fore and hind legs with a large, pectinate spur. Mid tibiae with or without a pectinate spur variably developed. Spurs of fore legs with or without a basal spine. Mid basitarsi shorter or at most as long as the fore ones. Second tarsomere of the three pairs of legs slightly or much longer than each third and fourth tarsomeres. Pretarsal claws simple. Arolia of variable size. Head, mesosoma, petiole, postpetiole and gastral tergum I variably reticulate and/or granulopunctate; this sculpture superimposed or not by irregular rugosities or by foveae. Remaining gastral tergites variably smooth, reticulate-punctate, punctate, or punctate-foveolate. Legs variably smooth, with superficial or deep punctation-granulation. Body with at least three main types of hairs: (1) short and generally dense, suberect or subdecumbent on the whole body, (2) long erect or suberect on the whole body, sometimes subdecumbent, dense or sparse, rarely absent and (3) very short, decumbent or appressed on the funicular joints only. Colour yellowish light brown, reddish-brown, dark brown or black. Legs concolourous with or lighter than the body.

Queen
Baroni Urbani and de Andrade (2003) - Similar to the worker but slightly differing from it, even in the usual caste-dependant characters. The most salient of these characters are the following: size generally larger; compound eyes larger and with ocular pilosity; ocelli always present; mesosoma always robust; promesonotal and propodeal sutures impressed; metanotum sometimes spiniform. Basal face of the propodeum very short or slightly prolonged backwards.

Wings. Fore wings variably pigmented, in some species infuscate, brown or whitish-yellow hyaline. The wing venation appears to follow five distinct patterns as follows:

1. m-cu marked and Rsf3 not reaching Rsf1. 2r Rsf4, Rsf4, r-m, Mf4 marked. Rsf5 reaching the Costa.

2. m-cu marked, Rsf3 and Rsf4 absent. 2r, Rsf5, r-m, marked. Rsf5 not reaching the Costa.

3. m-cu marked, Rsf3, Rsf4, r-m and Mf4 absent. 2r, Rsf5 marked. Rsf5 not reaching the Costa.

4. m-cu, Rsf3 and Rsf4 absent. 2r very long, Rsf5, r-m, and Mf4 of variable length. Rsf5 not reaching the Costa.

5. m-cu Rsf3, Rsf4, r-m and Mf4 absent. 2r very long, Rsf5 of variable length. Rsf5 not reaching the Costa.

Hind wings pigmented as the fore wings and showing three venation patterns:

1. r-m distinct and R reaching the anterior margin. M and CuA variably marked.

2. r-m distinct and R not reaching the anterior margin. M and CuA variably marked.

3. r-m absent. M and CuA variably marked.

Male
Baroni Urbani and de Andrade (2003) - Size variable, generally smaller or nearly as large as the gyne. Head about as broad as long, broader than long or longer than broad. Vertex variably convex. Clypeus anteromedially variably projecting; it can be simply straight, slightly convex, subrectangular or subtriangular. Frontal carinae reduced, never hiding the antennal socket and generally separated each other, rarely close each other. Sides of the frontal carinae subparallel, or slightly diverging posteriorly. Antennae 13-segmented. Ocelli large. Compound eyes very large and mostly on the anterior half of the head sides. Scapes variable in size, not reaching or surpassing the anterior ocellus or slightly surpassing the vertexal margin. First funicular joint shorter than or as long as the second; distal joint as long as the sum of joints 10-11 or 9-11. Mandibles slender, edentate or minutely and irregularly denticulate, and with a visible apical, pointed tooth. Palp formula (stipes protuberance included) 3,2, 4,3, 5,2 or 5,3. Second maxillary palp joint hammer-shaped. Mesosoma robust. Mesonoturn and scutellum convex. Propodeum with or without differentiate basal and declivous faces. Sides between basal and declivous faces of the propodeum separated by similar projections as in the worker and gyne but much more reduced. Metanotum spiniform or not. Propodeal lobes similar to those of the worker and gyne. Petiole variably convex, rarely scale-like, lower and narrower than in the worker and gyne. Subpetiolar process and postpetiolar sternite with structures resembling those of the worker and gyne but usually much more reduced. Postpetiole broader than the petiole. Gastral segments generally less curved than in the worker and gyne. Legs long and slender. Sculpture similar to the one of the worker and gyne but generally more superficial; few species with the sculpture on the first gastral tergite more impressed than in the worker and gyne. Pilosity resembling the one of the worker and gyne but slightly less dense.

Wings. Gynes and males have similar fore and hind wings. Male wings are usually smaller than those of the gynes but their venation follows the same patterns.

Colour generally darker than the worker and the gyne. Some species black with lighter legs.

Larva
Baroni Urbani and de Andrade (2003) - The larvae of Proceratium crassicorne and Proceratium croceum were described by Wheeler & Wheeler (1952). According to the same authors (Wheeler & Wheeler, 1976) the Proceratium larvae differ from those of Discothyrea by having the integument beset with large hemispherical bosses instead of with only one pair of bosses or without bosses. Notice that these differences are drawn from the study of three Proceratium and one Discothyrea species.