Caste Terminology

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 & Crewe 1984, 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 a eusocial insect colony. According to a morphological definition of castes, any alate or dealate (after mating) female with fully developed flight thorax, notwithstanding its reproductive status.

Ergatoid (flightless) queens
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.

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

Brachypterous (=short-winged) (flightless) queens
Queens that emerge as adults with abnormally short wings. In some species these wings are retained during the beginning of adult life, but flight is impossible. In other species they break off soon after emergence. Unlike ergatoid queens, the thorax of short-winged queens is more segmented than conspecific workers.

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

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''.

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.

Queen polymorphism
Both winged and ergatoid queens co-occur within some species, though not necessarily within one nest. In a couple of species queen polymorphism is claimed to have a genetic basis (e.g., Harpagoxenus sublaevis, Myrmecina graminicola)

Intercastes
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.

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.

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 nipponicus, 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.