Crematogaster

The genus Crematogaster is a widespread and distinctive lineage of myrmicine ants (Wheeler 1910, Forel 1928, Buren 1959, 1968, Hölldobler and Wilson 1990).

Most species are tropical, where they form dominant elements of the arboreal fauna. Some species groups have crossed the frost line, radiating in temperate zones, where they more often nest in the ground and beneath stones (Wheeler 1906).

The genus as a whole is clearly monophyletic, with a unique and apomorphic arrangement of postpetiole and gaster. The gaster has a tear drop shape, pointed posteriorly, and the postpetiole attaches to the dorsal surface of the fourth abdominal tergite. Thus the gaster is suspended beneath the postpetiole rather than being clearly posterior to it. The petiole has no dorsal node, and when the gaster is elevated the petiole fits flush against the propodeum. This combination of characters is probably related to defensive or offensive behavior in which workers wave their gasters in the air, exuding a droplet of venom onto the spatulate sting (Buren 1959). Not all species exhibit this behavior (D. Davidson, pers. comm.) but most do. Workers are monomorphic or exhibit continuous size polymorphism, but there is no distinct major worker caste. The antennae are 11-segmented with the exception of one lineage of Asian and African species, in which there are 10 segments. The antennae have a terminal club of 2-4 segments. The propodeum usually has a pair of dorsal spines (lack of spines is rare among New World species), but otherwise the workers lack armor.

At the species level the genus can appear somewhat monotonous because there is little variation in major shape characters. Nevertheless species diversity is high, both within local communities and globally. Species differ in details of petiole and postpetiole shape, overall pilosity characteristics, and surface sculpture. Because the genus is a common and conspicuous element of most faunas, especially in the tropics, and varies both within and among regions, 889 available names have been generated (Bolton 1995). Nearly all of these have accumulated as unconnected species descriptions and there are very few synthetic works (Buren's 1968 review of North American Crematogaster s.s. a notable exception). Attempts have been made to recognize subgenera, but they are not well supported nor well defined. The high species diversity and lack of higher level taxonomic structure in the genus has made it one of the intractable messes in the world of ant taxonomy, on par with Solenopsis and Pheidole. It is a group generally avoided by students of systematics seeking manageable projects.