Crematogaster

Blaimer (2010) - Crematogaster ants are widespread, but reach their highest diversity and abundance in tropical and subtropical regions. These ants are generally found in forest, woodland and shrubby habitats, where they form a conspicuous and often dominant element of the fauna. Most tropical Crematogaster species nest arboreally, but some tropical and many temperate zone species nest in the ground (e.g. Hosoishi et al., 2010). Worker ants of this genus are easily recognized by a few unique morphological features, including the dorsal attachment of the postpetiole to the 4th abdominal segment and the absence of a dorsal petiolar node, which give the ants the ability of flexing the gaster forwards over the mesosoma while the petiole is pressed tightly against the propodeum (Buren, 1959). This is an aggressive response to every invader, enabling the ants to use their remarkable spatulate sting for the topical application of their venom – which apparently is efficient in repelling, if not killing other ant opponents (Marlier et al., 2004).

Hita Garcia, Wiesel and Fischer (2013) - One of the most species-rich and widely distributed genera, with approximately 470 valid species and 300 subspecies (Bolton, 2012). Despite its cosmopolitan distribution, most species are found in the tropics. The extreme species richness together with the high intraspecific and geographical variability provides serious obstacles for the taxonomic understanding of this genus. Thus, it is not surprising that revisionary treatments are very scarce and have been thoroughly avoided in the past. Some regional faunas have been revised on genus or subgenus level (Buren, 1959; Longino, 2003; Hosoishi & Ogata, 2008, 2009; Blaimer, 2010, 2011). In tropical regions, most members of this genus are arboreal (Longino, 2003), although a minority of species nest and forage on the ground (Quinet et al., 2009; Hosoishi et al., 2010). Crematogaster can be found in a diversity of habitats, such as forests, woodlands, savannahs or shrublands (Blaimer, 2010) and they often play a dominant, aggressive and territorial role within the local ant fauna (Longino, 2003). Most Crematogaster seem to be highly generalistic and omnivorous (Longino, 2003), although the most important resource for many species is homopteran honeydew.

Identification
Crematogaster species can be difficult to identify. There are many parts of world where Crematogaster are quite abundant but the diversity and taxonomy of the genus is poorly known. A number of recent revisions have greatly improved our understanding of a few species groups and regions. Unfortunately there is no comprehensive revision that can provide any coherent accounting of the genus as a whole.

Eguchi, Bui and Yamane (2011) - Worker monomorphic, but sometimes varying widely in size; head round, subrectangular or subtrapezoidal; frontal carina and antennal scrobe absent; median portion of clypeus roundly expanded anteriad, partly overhanging basal part of mandibles when fully closed; posteromedian portion very broadly inserted between frontal lobes; no isolated, median seta on anterior clypeal margin; mandible narrow; masticatory margin oblique, with 4 teeth; antennae 11-segmented, with a 2-, 3-, or 4-segmented club, or gradually incrassate; eye medium sized or rarely consisting of a few ommatidia; promesonotum more or less raised; promesonotal suture absent or weakly present dorsally; metanotal groove usually distinctly impressed, sometimes margined laterally by a longitudinal carina or lamella; propodeal spine usually (but not always) present, varying in size and shape; propodeal spiracle located well posteriorly on posterolateral margin of propodeum, just below base of propodeal spine; petiole depressed dorsoventrally, without node; postpetiole with rounded node which often bears median longitudinal impression, attached to dorsal surface of gaster; gaster in dorsal view triangular or cordate; sting spatulate.

Crematogaster is easily distinguished from all other myrmicine genera known from Vietnam by the morphology of the waist and gaster. The worker of species belonging to the subgenus Orthocrema of Crematogaster is a little similar to that of Recurvidris, but in the latter the propodeal spines are weakly to strongly recurved, the propodeal spiracle is located far in front of the base of the propodeal spine, the postpetiole, in dorsal view, is broadly attached to the first gastral segment, and the first gastral segment behind the postpetiole is extremely dorsoventrally compressed in lateral view. Antennal club is 2-segmented in Orthocrema, but 3-segmented in Recurvidris.

Species Groups

 * Asian Crematogaster ranavalonae group
 * Crematogaster borneensis group
 * Malagasy Crematogaster Species by Subgenus and Species Group

Biology
Quinet et al. (2009) - The genus Crematogaster occurs throughout the tropical and subtropical regions of the world, and it is particularly diverse and abundant in the Neotropics. Crematogaster species that extend into the temperate zone are typically ground-nesting, while tropical species are typically arboreal or, in a few cases, nesting in dead wood in forest floor leaf litter.

Eguchi, Bui and Yamane (2011) - Many species are arboreal foragers, and nest in decayed parts of standing trees and hollows of tree trunks and branches or build carton nests. Some species nest in soil or rotting logs on the ground. Species of the subgenus Orthocrema forage both on and under the ground.

Longino (2003), provides the following perspective regarding the biology of Costa Rican Crematogaster, with much of the generalities made here extending to the neotropics as a whole: Crematogaster are often common ants, and they play a major ecological role in Neotropical forests. Colonies may be large, blanketing forest canopies, or small, contained within a single dead twig. Large colonies are usually polydomous, with multiple nests. Most species nest in dead wood, from narrow gauge hollow stems to large dead branches or trunks. One species, Crematogaster stollii, nests in live stems. Although major Crematogaster lineages in the Asian and African tropics are specialized plant ants, and at least one or two species are plant ants in the Amazonian region, none are known to be specialized plant ants in Costa Rica. Crematogaster bryophilia often nests under epiphyte mats. Although many species can make carton from masticated plant fibers, most use relatively small amounts to form partitions inside the nest or to restrict the opening of a nest in dead wood. Several Costa Rican species use carton more extensively. Crematogaster stollii makes carton galleries on tree trunks and branches, connecting their nests in the live branch tips. These carton galleries are indistinguishable from those of Azteca forelii, an ant species with similar nesting behavior, and both are very similar to the galleries of the arboreal termites that are so common in lowland forests. Crematogaster montezumia and Crematogaster arcuata make external carton nests that encircle small stems. These nests are plain carton, and lack epiphytes. In contrast, two Costa Rican species make carton nests that sprout epiphytes, forming ant gardens. Crematogaster longispina makes loose ant gardens on tree trunks (Kleinfeldt 1978), and Crematogaster jardinero lives in the high canopy, forming “archipelago” clusters of discrete ant gardens.

Although most species are arboreal, a few nest in the leaf litter. Species that nest in the leaf litter are usually yellow, nocturnal, and rarely encountered. One leaf litter species, Crematogaster sotobosque, is brown, forages diurnally on low vegetation, and is moderately abundant in lowland wet forest.

Most species are monogynous; a few are polygynous.

Ergatogynes or intercastes have been reported for Crematogaster minutissima (Holliday 1903), C. minutissima smithi, and a species tentatively identified as Crematogaster curvispinosa (Heinze et al. 1998, Heinze et al. 1999). The Heinze et al. studies of smithi revealed that these intercastes, morphologically intermediate between workers and queens, function mainly to provide trophic eggs for the colony. They never perform foraging, maintenance, or defensive duties. They mainly lay eggs, most of which are eaten by larvae. They lack a spermatheca and cannot be inseminated, but their eggs are viable and produce males if left to develop. In Crematogaster pygmaea and Crematogaster biroi, Peeters et al. (2013) showed that this is a specialized caste with a trophic function. Among the Costa Rican fauna, intermediates are known to occur in C. bryophilia, C. curvispinosa, and Crematogaster nigropilosa. Nests are often found with only intermediates, workers, and brood. It is unknown whether these are colony fragments with queenright nests elsewhere.

Two categories of queens occur among the Costa Rica fauna. In one group the propodeum is tall and narrow and drops very steeply from the scutellum (Fig. 1A), and sculpture and pilosity characters are similar to workers. I refer to these as “normal” queens. These species appear to have typical colony founding behavior, with standard nuptial flights and claustral colony founding by individual queens, and they are the most abundant species in communities. In another group the propodeum has a shallower slope and extends well beyond the scutellum (Fig. 1B), and sculpture and pilosity characters often differ greatly from workers. In particular, queens are often highly polished and shiny. Queens in this group also show varying degrees of development of falcate mandibles. Costa Rican species having these distinctive queens are Crematogaster acuta, Crematogaster arcuata, Crematogaster distans, Crematogaster evallans, jardinero, montezumia, and Crematogaster raptor. I refer to these as the “acuta-group.” These are all very low density species, and very little is known of their colony founding behavior. The morphology is similar to other ant lineages that are known to be temporary social parasites (Forel 1928, Hölldobler and Wilson 1990). Queens of temporary social parasites insinuate themselves into nests of other species, killing or incapacitating the host queen, and use the heterospecific worker force to establish their own colony. Two anecdotal observations are consistent with temporary social parasitism as a colony founding mechanism in the acuta-group. I observed a mixed nest in which a queen of C. montezumia occurred in a small nest with workers of C. curvispinosa, and Adrienne Nicotra, a student working at La Selva Biological Station, observed a queen of C. raptor in a small queenright nest of Crematogaster carinata. These are the only such observations so far, and the colony founding behavior of acuta-group species is in need of investigation.

Most species of Crematogaster, especially those with large polydomous colonies, are aggressive and territorial. Crematogaster carinata is exceptional in having large polydomous and polygynous colonies that overlap with many other ant species. Workers are not aggressive and may even share the same nest structures with other species. Forel (1898) observed C. carinata (as C. limata parabiotica) and Dolichoderus debilis inhabiting the same nest in Colombia, and coined the term parabiosis to describe the phenomenon of mutual nest sharing. In Costa Rica, C. carinata can be found coinhabiting ant gardens with Odontomachus panamensis and sharing nest space with D. debilis or Dolichoderus inermis. The nesting behavior and taxonomic uncertainties in the complex are further discussed under the C. carinata species account. Crematogaster limata may also exhibit an ability to overlap non-aggressively with other species because it has been observed in close association with the large tropical ponerine ant Ectatomma tuberculatum (Wheeler 1986).

Crematogaster appear to be very generalized and omnivorous foragers. Individual scouts search for resources and recruit nestmates when resources are encountered. They rapidly recruit to baits of sugar or protein (e.g., tuna, dead insects). Although rarely predators of active prey, I have often seen them attacking pupae or otherwise immobilized live prey. They readily tend Homoptera, and species vary in the degree of reliance on Homoptera. Crematogaster stollii appears to rely entirely on Homoptera and perhaps cryptic plant resources. Workers are found only inside of live stems or under their carton galleries and they never forage on the surface. Their chambers in live stems are packed with Coccoidea that are feeding from the inside of the stems. This phenomenon, in which ants live and feed entirely within live plant stems, with no external patrolling by the ants and no obvious myrmecophytic adaptations on the plant's part, has evolved convergently in several ant lineages, including species of Azteca and Myrmelachista.

In Costa Rica, Crematogaster are abundant in all lowland habitats. In mangroves, Crematogaster crinosa is often a dominant species. In lowland dry or wet forest sites, a community of over 15 species may occur. These are concentrated in second growth vegetation, forest edges, and forest canopy. Relatively few species are found in wet forest understory and forest floor litter. At higher elevations the dominant Crematogaster drop out by about 500 to 1000m, depending on the openness of the habitat. A few species, such as C. bryophilia, Crematogaster moelleri, and Crematogaster sumichrasti, are montane forest specialists that are more common at mid elevations than at sea level.

Nomenclature

 *  CREMATOGASTER [Myrmicinae: Crematogastrini]
 * Crematogaster Lund, 1831a: 132. Type-species: Formica scutellaris, by subsequent designation of Bingham, 1903: 124.
 * [Type-species not Formica acuta, unjustified subsequent designation by Emery, 1912d: 272; this error repeated in, for example, Emery, 1914c: 39, Arnold, 1920a: 482, Emery, 1922e: 128, Gallardo, 1934: 4; Soulié, 1965: 78.]
 * Crematogaster senior synonym of Acrocoelia: Roger, 1863b: 36; Mayr, 1863: 404; Dalla Torre, 1893: 79 (see also Buren, 1959: 125; Kempf, 1972a: 81).
 * Crematogaster (as subgenus) senior synonym of Atopogyne, Colobocrema, Decacrema, Oxygyne, Nematocrema, Paracrema, Physocrema, Sphaerocrema, Xiphocrema: Blaimer, 2012: 52.
 * Subgenera of Crematogaster: nominal plus  Orthocrema. .
 *  CREMASTOGASTER Mayr, 1861: 74 (and many later authors), incorrect subsequent spelling. Discussion of spelling: Emery, 1912d: 272 (footnote); Wheeler, W.M. 1913a: 78; Donisthorpe, 1941f: 36 and Buren, 1959: 125.
 * ACROCOELIA [junior synonym of Crematogaster]
 * Acrocoelia Mayr, 1853a: 147. Type-species: Acrocoelia ruficeps (junior synonym of Formica scutellaris), by subsequent designation of Wheeler, W.M. 1911f: 158.
 * Acrocoelia junior synonym of Crematogaster: Mayr, 1863: 404; Wheeler, W.M. 1911f: 158; Wheeler, W.M. 1922a: 828.
 * Acrocoelia revived from synonymy as subgenus of Crematogaster: Emery, 1922e: 140.
 * Acrocoelia junior synonym of Crematogaster: Buren, 1959: 126.
 * Acrocoelia revived from synonymy as genus: Soulié, 1964: 398.
 * Acrocoelia junior synonym of Crematogaster: Brown, 1973b: 178. [The type-species of Acrocoelia and Crematogaster are synonymous, the generic synonymy is therefore absolute.]
 * TRANOPELTOIDES [junior synonym of Crematogaster]
 * Tranopeltoides Wheeler, W.M. 1922e: 10. Type-species: Tranopelta huberi, by original designation.
 * Tranopeltoides junior synonym of Crematogaster: Kempf, 1960c: 173.
 * NEMATOCREMA [junior synonym of Crematogaster]
 * Nematocrema Santschi, 1918d: 182 [as subgenus of Crematogaster]. Type-species: Crematogaster stadelmanni, by original designation.
 * Nematocrema raised to genus: Soulié, 1964: 398.
 * Nematocrema junior synonym of Crematogaster: Hölldobler & Wilson, 1990: 13.
 * Nematocrema subgenus of Crematogaster: Bolton, 1995b: 38.
 * Nematocrema junior synonym of Crematogaster: Blaimer, 2012: 249.

North America
Deyrup and Cover (2007) - The first useful account of the North American Crematogaster was Creighton’s (1950) treatment of the genus in the “Ants of North America.” From the past Creighton inherited a confusing list of ill-defined taxa, the customary legacy to modern taxonomists from the “Bronze Age” of Ant Taxonomy (1750 - 1950), an era characterized by much honest descriptive effort that was critically undermined by the lack of a biologically realistic conceptual framework. Creighton began the process of bringing order to the North American ant fauna, and to the Crematogaster in particular, by employing the newly popularized biological species concept to elicit meaning from what seemed to be an intractable morass of names and morphological variation (see Buhs, 2000 for a discussion of this development). Expanding on this pioneering work, Buren (1958, 1968) revised the North American Crematogaster again, incorporating much newly available information. Like Creighton, Buren made extensive use of distributional data and natural history observations in making taxonomic decisions and, as a result, his work still forms the basis for our modern understanding of the genus in North America. Not much has been added since Buren’s studies. Johnson (1988) reviews the eastern species and presents a key to them. In an excellent new revision of the Crematogaster of Costa Rica, Longino (2003) makes several taxonomic changes that affect the North American fauna.