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Note: The genus Cephalotes was just revised by Andrade and Baroni Urbani. This is a substantial body of work (an 889 page volume) that covers the taxonomy, phylogeny, and ecology/natural history of the genus. There are keys to species, with separate keys for each caste, and each species has a full species account. Most species are accompanied by SEM photos. The revision also includes many extinct species, based on fossil material in amber. Their revision relies heavily on the pioneering revisionary work of Kempf (1951, 1952, 1958, 1973), and in many cases Kempf's detailed species descriptions are used verbatim in the species accounts of Andrade and Baroni Urbani.
Synopsis of biology
The tribe Cephalotini is a strictly New World radiation of arboreal ants. They all nest inside of live or dead stems of plants. All the species are heavily armored, and the majority are dorsoventrally flattened. The species with flattened workers (the great majority) appear to rely on passive defense, stopping and pressing themselves to the surface when molested, and many may rely on chemical defense that makes them distasteful to predators (Coyle 1966). They are relatively unaggressive ants, often coexisting with and using the same runways as other more aggressive ant species (e.g. Adams 1990, see under maculatus). Species vary in average colony size, ranging from a few tens of workers (e.g. C. texanus; Creighton and Gregg 1954) to over 10,000 (Cephalotes atratus; Weber 1957).
The feeding habits of cephalotines are not well understood. They appear to be generalized omnivores and scavengers. They frequent extrafloral nectaries, and have been observed at carrion, bird droppings, and various kinds of proteinaceous and sugar baits. Oddly, laboratory colonies typically show no interest in freshly killed arthropods. Workers will harvest and feed upon pollen, and various lines of evidence suggest that pollen is a major component of cephalotine diet (Andrade and Baroni Urbani 1999).
Many species of Cephalotes are dorsoventrally flattened, and the integument is black with a dense covering of silvery scales, giving the body a silvery-gray appearance. Hespenheide (1986) found these to be models for an extensive Batesian mimicry complex involving at least 40 species of arthropods, including beetles (Buprestidae, Anthribidae, Bruchidae, Cerambycidae, Cleridae, Colydiidae, and Curculionidae), wasps (Orussidae), and true bugs (Lygaeidae).
Generic boundaries and generic nomenclature have been relatively unstable. Andrade and Baroni Urbani (1999) recognize two genera: Procryptocerus and Cephalotes . Prior to this revision, there were four recognized genera in Cephalotini: Cephalotes, Eucryptocerus, Procryptocerus, and Zacryptocerus (Bolton 1995), with the majority of species in Zacryptocerus. Andrade and Baroni Urbani synonymized Eucryptocerus and Zacryptocerus under Cephalotes, greatly expanding the concept of Cephalotes.
Caste polymorphism (soldiers)
Species of Cephalotes may be monomorphic, polymorphic, or dimorphic with a discrete major worker (soldier) caste (Figure). In the most dramatic examples, the major workers have a large, concave disc on the head (the cephalic disc), which is used to block the nest entrance (a phenomenon termed phragmosis; see Wheeler 1927, Delabie 1994 and included references). In some species the queens also have a cephalic disc. Wheeler (1942) noticed that in some species the cephalic disc of the major workers was covered with a dull encrusting layer (examples include porrasi and setulifer), and he hypothesized that this served a protective function, camouflaging the nest entrance. Wheeler and Hoelldobler (1985) found that these species have specialized setae that bind the encrusting layer to the surface, and they gave circumstantial evidence that the encrusting material was a glandular product of the ants themselves, and not gathered from the environment.
Structurally, the main character used to define Cephalotes major workers is a transverse ridge or crest on the pronotum (Figure). In Costa Rica, all species except C. atratus have major workers. Cephalotes atratus is a largely South American species whose occurrence in Costa Rica is uncertain (see atratus species account).
Andrade and Baroni Urbani (1999, pg. 793) state that workers can always be separated into minor and major workers, and that there may be large minor workers and small major workers that overlap in size. Their view is related to a debate about the origin of the major worker caste. Baroni Urbani and Passera (Baroni Urbani and Passera 1996, Baroni Urbani 1998) proposed that the major and minor worker castes are independently derived from queens. Ward (1997) opposed this view and favored the traditional interpretation, in which a monomorphic worker caste becomes differentiated from queens first, and subsequently there may be diversification within the worker caste. In the case of a worker origin, discrete minor and major worker castes evolve from intermediate stages that have more continuous size variation (polymorphic species).
The principal criterion used by Andrade and Baroni Urbani to differentiate major workers is a transverse carina on the pronotum. I examined this trait and I am not convinced of its discrete nature. It is informative to discuss two different transverse carinae on the pronotum. A presumably plesiomorphic character is the presence of an anterior carina that separates the dorsal surface of the pronotum from a narrow pronotal collar, the latter forming a ball joint that fits into the head. This anterior carina is present in minor workers of nearly all cephalotines (including Procryptocerus), and is particularly well developed in minor workers formerly placed in Zacryptocerus. In C. alfaroi, the anterior carina is weakened, and there is the development of a posterior carina between the humeral spines. The posterior carina is weakly developed in minor workers, becomes more prominent in major workers, and is present as a tuberculate crest in queens. In C. atratus, the anterior carina is well developed, and there does not appear to be any development of a posterior carina. In the species formerly placed in Zacryptocerus, the minor workers have a strong anterior carina, but the major workers and queens have completely lost it and instead have a strongly developed posterior carina. It is this posterior carina that Andrade and Baroni Urbani use to differentiate major workers.
I do not always see a discrete division into minor and major workers, and I see workers that I would call intermediate. In C. minutus, for example, there are workers with size and shape of the cephalic shield perfectly intermediate between minor and major workers. These workers have a very weak anterior pronotal carina, and a weak posterior carina. If major workers were derived from the queen, I might expect to find a weakened median transverse carina on small majors, but I would not expect any trace of an anterior carina.
Using the keys
First you must determine whether the specimen you want to identify is a minor or major worker, using the criteria discussed above. The identification tools provided in the keys are for "typical" minor workers and "typical" major workers. An isolated intermediate worker may be difficult to identify by someone not already familiar with the species. For identification purposes, it is always best to have a series of specimens, but this is not always possible. Specimens are often encountered as isolated foragers. However, isolated foragers are usually typical minor workers, and are usually readily identifiable, especially within a restricted geographic area such as Costa Rica.
Adams, E. S. 1990. Interaction between the ants Zacryptocerus maculatus and Azteca trigona: interspecific parasitization of information. Biotropica 22:200-206.
Andrade, M. L. de, and C. Baroni Urbani. 1999. Diversity and adaptation in the ant genus Cephalotes, past and present (Hymenoptera, Formicidae). Stuttgarter Beitrage zur Naturkunde Serie B (Geologie und Palaontologie) 271:1-889.
Baroni Urbani, C. 1998. The number of castes in ants, where major is smaller than minor and queens wear the shield of the soldiers. Insectes Sociaux 45:315-333.
Baroni Urbani, C., and L. Passera 1996. Origin of ant soldiers. Nature 383:223.
Bolton, B. 1995. A New General Catalogue of the Ants of the World. Harvard University Press, Cambridge, Mass.
Coyle, F. A. 1966. Defensive behavior and associated morphological features in three species of the ant genus Paracryptocerus. Insectes Sociaux 13:993-104.
Creighton, W. S., and R. E. Gregg. 1954. Studies on the habits and distribution of Cryptocerus texanus Santschi (Hymenoptera: Formicidae). Psyche 61:41-57.
Delabie, J. H. C. 1994. Cooperative shield phragmosis by minor workers of Zacryptocerus pusillus (Hymenoptera; Formicidae; Cephalotini). Etologia 4:99-102.
Hespenheide, H. A. 1986. Mimicry of ants of the genus Zacryptocerus (Hymenoptera: Formicidae). Journal of the New York Entomological Society 94:394-408.
Kempf, W. W. 1951. A taxonomic study on the ant tribe Cephalotini (Hymenoptera: Formicidae). Rev. Entomol. (Rio J.) 22:1-244.
Kempf, W. W. 1952. A synopsis of the pinelii-complex in the genus Paracryptocerus (Hym. Formicidae). Stud. Entomol. 1:1-30.
Kempf, W. W. 1958. New studies of the ant tribe Cephalotini (Hym. Formicidae). Stud. Entomol. (n.s.)1:1-168.
Kempf, W. W. 1973. A new Zacryptocerus from Brazil, with remarks on the generic classification of the tribe Cephalotini (Hymenoptera: Formicidae). Stud. Entomol. 16:449-462.
Ward, P. S. 1997. Ant soldiers are not modified queens. Nature 385:494-495.
Weber, N. A. 1957. The nest of an anomalous colony of the arboreal ant Cephalotes atratus. Psyche 64:60-69.
Wheeler, D. E., and B. Hoelldobler. 1985. Cryptic phragmosis: the structural modifications. Psyche 92:337-353.
Wheeler, W. M. 1927. Physiognomy of insects. Q. Rev. Biol. 2:1-36.
Wheeler, W. M. 1942. Studies of neotropical ant-plants and their ants. Bul. Mus. Comp. Zool., Harvard 90:1-262.
John T. Longino, The Evergreen State College, Olympia WA 98505 USA.email@example.com
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