Caste Terminology

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The term “caste” is used to describe either the physical or the behavioural/physiological phenotype of a eusocial individual, or both (Hölldobler & Wilson 1990: Glossary). Some authors, e.g., Buschinger & Winter (1976, 1978), define castes according to function alone, whereas others (e.g., Peeters & Crozier 1988) advocate a strictly morphological definition. This dichotomy of form vs. function can cause ambiguity, e.g. when comparing the writings of different authors that use "caste" or "queen" differently. The existence of permanently wingless reproductives in ants also causes confusion in terminology. Polymorphism of female reproductives in ants has been reviewed by Heinze (1998) and Peeters (2012).

Queen

According to a functional definition of castes, any mated and egg-laying female of an ant colony. According to a morphological definition of castes, any alate or dealate (after mating) female with fully segmented flight thorax, regardless of its reproductive status.

Alate queen of Pheidole bicarinata.
Dealate queen of Camponotus novaeboracensis.

Ergatoid queen (flightless)

Queens that emerge as adults without wings (see list of species). As a result of a lack of wing muscles, the thorax is reduced and worker-like. However, reproductive organs are fully functional and allow sexual reproduction. Can co-occur with winged queens in species with alternative life histories.

Ergatoid queen of Parasyscia schoedli.

Dichthadiiform queen (flightless)

The special form of ergatoid queen peculiar to army ants, always wingless and with very large gaster to accommodate massive egg production.

Dichthadiiform queen of Aenictus laeviceps and workers.

Brachypterous (=short-winged) queen (flightless)

Queens that emerge as adults with short non-functional wings. In some species these wings are retained during the beginning of adult life, but there is no aerial dispersal. In other species they break off soon within hours of emergence. The thorax of brachypterous queens retains more flight sclerites than ergatoid queens, and are thus very distinct from conspecific workers, e.g. Aphaenogaster senilis. Known from at least 15 genera (Peeters 2012).


Note the short non-functional wings.

Gamergate

A mated, egg-laying worker that reproduces like the queen caste (Peeters 1991). In about 1% of ant species, belonging to several subfamilies, all workers in a colony have a functional spermatheca (see list of species). Dominance interactions regulate which individual(s) can mate and lay eggs.

Dominance interaction between high-ranking workers in Dinoponera quadriceps. Photo by Thibaud Monnin

Queen polymorphism

Both winged and ergatoid queens co-occur in some species, though not necessarily within one nest. Queen polymorphism is claimed to have a genetic basis in Harpagoxenus sublaevis and Myrmecina graminicola.

Mg Gyne Sterz.JPG

A Myrmecina graminicola gynomorph deposits tiny traces of a sexual pheromone (poison gland content) on the substratum. Males are attracted to this place and search around until they find the gyne. After mating this gyne sheds off the wings and founds a colony.

Mg Int sterz Kop.jpg

Mating sequence of Myrmecina graminicola intermorphic females. As in gynomorphs, the intermorphs attract males by emitting a sexual pheromone onto the substratum or into the air. These intermorphs have fully functionalovaries and receptacula seminis. When mated they become intermorphic queens. Intermorphs are not the same as gamergates.


Both winged queens and gamergates occur in some species.

Major worker

The largest subcaste of worker ants; sometimes also referred to as a "soldier" because often specialized for defense, but may also be specialized to crush seeds or store food in the gaster.

Major and minor workers of a Pheidole species from Queensland, Australia. Photo by Steve Shattuck.

Minor worker

In species with pronounced variation in body size among workers, this is the smallest size category.

Monomorphic

In many species, the worker caste does not vary in body size or form.

Polymorphic

Refers to variability in body size and/or shape within the same caste. Individuals of distinctly different proportions occur at the extreme ends of the variation range, and they may have different functions in the colony. Numerous examples belonging to many lineages include Crematogaster clariventris

Soldier

Several species (mostly in subfamilies Dorylinae, Formicinae and Myrmicinae) have infertile helpers that are bigger than ordinary workers.

morphological sense

Characterized by traits absent in workers. Heads are usually enlarged because of powerful mandible muscles, and mandibles can have specialized shapes (for seed-milling, wood-boring, or defence). In various unrelated genera, the head is modified to be door-shaped (phragmosis) (e.g. Carebara, Cephalotes, Colobopsis). The prothorax is enlarged in many species due to larger neck muscles. In some genera (e.g. Acanthomyrmex ferox, Crematogaster pygmaea), soldiers have queen-like ovaries that allow a higher production of trophic eggs. The gaster is larger than workers' and this allows soldiers to be efficient repletes (e.g. Carebara, Colobopsis nipponica, Pheidole desertorum).

Molet et al. (2012) pointed out that soldiers are a mosaic of queen-like and worker-like traits. Considered to be a third caste, not a worker sub-caste. See also Baroni-Urbani (1998)

functional sense

Workers that are specialized for colony defense. The entire emphasis is on adaptive function (i.e. ultimate causes of evolution), regardless of developmental mechanisms. Includes major workers that show distinct shapes resulting from allometry within worker growth rules.

Soldier of a Pheidole species from Borneo. Photo by Steve Shattuck.
Cataglyphis bombycina dealate queen, minor worker and soldier. Photo by Patrick Landmann.
Cephalotes varians soldier showing the specialized plug-like head (phragmosis). Photo from AntWeb.

Replete

A worker ant whose crop is greatly distended with liquid food and functioning as a living reservoir; this food is shared with other colony members by regurgitation. Another mechanism of food transfer within colonies is for some workers to produce trophic eggs.

Replete workers in Myrmecocystus. Photo by Greg Hume.

Intercaste

Developmental anomalies morphologically intermediate between winged queens and workers. Infrequently reared in colonies, and poorly studied. Intercastes are conspicuous in ants because of the divergence winged/wingless, and also because queen-worker dimorphism in body size is often pronounced. In species where large samples of intercastes were examined, e.g. Temnothorax nylanderi, there is considerable heterogeneity in morphology (ocelli, thorax segmentation, gaster size). Reproductive organs (ovaries and spermatheca) may or may not be functional.

References