Crematogaster

Longino, J. T., 2003, The Crematogaster (Hymenoptera, Formicidae, Myrmicinae) of Costa Rica., Zootaxa 151, pp. 1-150 : 18-20

Crematogaster

WORKER CHARACTERS

The terminal segments of the antenna are enlarged to form the antennal club. The club may be composed of the terminal two segments and sharply differentiated from segments eight and nine, or it may be composed of the terminal three or four segments, which gradually decrease in size. Although the number of segments in the club has been used as a subgeneric character (Santschi 1918), I find continuous interspecific variation (Figure 3) that does not correlate with other characters, and I do not use the character in the key.

The petiole varies greatly in shape and seems to be one of the most useful characters for differentiating clusters of species (Plates 1-3). In dorsal view the petiole has a large, flat anterodorsal face. The posterior border is usually well defined by two posterolateral angles or tubercles, these usually surmounted by one or two setae. A transverse carina or ridge unites the posterolateral tubercles. A very much shorter posterior face drops to the cylindrical posterior aperture of the tergite, which tightly invests the anterior rim of the helcium (the specialized anterior portion of the postpetiole, which articulates with the petiole; see Bolton 1994). When the anterodorsal face of the petiole is viewed from above, perpendicular to the line of sight of the viewer (this can be achieved by pushing the gaster down or removing the metasoma entirely and remounting it) it may be (1) elongate and tapering anteriorly to a narrow neck just posterior to the condyles that articulate with the propodeum; (2) rectangular to quadrate, with subparallel sides and an abrupt constriction anteriorly; and (3) short with broadly convex sides, distinctly widest at the middle. Although these three character states are not always sharply differentiated and intermediate forms occur, clusters of species can be clearly placed into one of the three categories. A fourth condition is characteristic of the major radiation (or radiations?) of temperate zone species. The anterodorsal face is short, with strongly convex sides that are strongly expanded anteriorly, such that the petiole is widest anteriorly. Species with this character state (subgenus Crematogaster s.s., see Buren 1959) are restricted to the temperate zone and subtropics, and do not occur in Costa Rica.

The ventral margins of the petiole and postpetiole exhibit varying degrees of development of an anterior tooth. These anteroventral teeth are independent of each other, appearing on the petiole alone, on the postpetiole alone, on both, or on neither. They appear to be evolutionarily labile because they are not highly correlated with other characters and do not unite clusters of related species the way petiole shape seems to. Nevertheless, they are often reliable species-level characters: they correlate well with particular suites of characters that are hypothesized to be diagnostic for particular species. A clear view of the anteroventral margin of the petiole and postpetiole is often essential for species identification.

The postpetiole is comprised of the thickened rim of the anterior helcium, the narrow "neck" of the helcium, and the broadened and elevated node. The neck of the helcium may be short and cylindrical, forming a relatively sharp juncture with the node, like a stem on an apple. Alternatively, the neck may be relatively elongate and gradually tapering such that the juncture with the node is not sharply defined. In this case the dorsal view of the petiole is somewhat hour-glass or flask shaped. In the latter case, the node is globular, about as wide as long, and with no trace of a median longitudinal sulcus. In the former case the node is less globular, relatively more dorsally compressed, subquadrate in dorsal view, and usually distinctly wider than long. When the node is broad and subquadrate, it may exhibit some degree of development of a median longitudinal impression or sulcus. At extremes, the sulcus is very well defined, dividing the postpetiolar node into two distinct lobes, with a strong posterior emargination and a fully impressed sulcus.

Pilosity characters, like petiolar characters, are very useful and seem to unite clusters of related species. Pilosity on the face usually falls into one of three categories: (1) abundant long erect flexuous setae; (2) abundant short stiff setae, forming a stubble of straight or curved setae; or (3) largely or entirely bare, with a vestiture of sparse, very short, completely appressed hairs (a very dilute and appressed pubescence) and a small number of longer erect setae (Fig. 4). Pilosity on the meso and metasomal dorsum varies in density and may be composed of long flexuous or short stiff setae. Tibial and scape pilosity may be appressed or suberect and varies in length. When tibial pilosity is erect, it is usually subequal in length to the maximum width of the tibia, but in a few species there are 1-3 specialized longer setae that are about twice as long at the others.

Subgeneric classifications were proposed for Crematogaster by Santschi (1918), Emery (1922), and Wheeler (1922b). Wheeler's key was largely a translation of Santschi with few modifications. The main characters used to distinguish subgenera in the Neotropics were (1) the number of segments in the antennal club, (2) whether the anterodorsal face of the petiole was widest posteriorly or anteriorly, and (3) whether the postpetiolar dorsum had a median sulcus. In practice it is difficult to use any of these characters to sharply divide groups of species. Although I suspect these characters will prove phylogenetically informative, this work does not purport to establish phylogenetic hypotheses. Its purpose is to improve the species-level alpha taxonomy of the Neotropical species and provide a key for the Costa Rican fauna. Evaluating the status of subgenera must await further research, and they are ignored in this work.

QUEEN CHARACTERS

Queens are known for 27 of the 33 species that occur in or near Costa Rica. Normal queens are similar to workers in general shape, sculpture, and pilosity characters, differing in typical caste-specific traits: larger size, ocelli, enlarged mesosoma with additional sclerites, wings or wing scars, and enlarged gaster. As discussed in the Natural History Overview, acuta-group queens are more strongly differentiated from workers, often with highly polished integument and/or distinctive pilosity traits. There is variation in queen size (Fig. 5) and in the relationship of queen size to worker size (Fig. 6). Queens are given very cursory treatment in this work. Size relationships are depicted in Figures 5 and 6, and brief descriptions are provided in the species accounts.