Caste Terminology

Ants possess two female castes (workers, queens) in addition to males. While females are diploid (hatched from fertilized eggs, hence two sets of chromosomes), males are haploid (hatched from unfertilized eggs, hence just one set of chromosomes from the mother). This haplodiploidy is characteristic of all Hymenoptera, males do not have a father!


 * Queens: reproductive females that are fully winged in most species but can also be short-winged or wingless (in a minority of species). Generally one to a few queens are present in a nest, but many queens occur in some species. They have a lifespan of up to 25 years (and even more in the laboratory).


 * Workers: non-reproductive (sterile) females that are always wingless. They form the bulk of the individuals within a nest, and typically live a few years. In a proportion of species, virgin workers can lay haploid eggs (destined to be males) in orphaned colonies.


 * Males: fully winged (mostly), short-winged or wingless (in a few species). They are generally present within a nest for only a short time each year and typically live a few weeks, dying very soon after mating.

Both female castes show considerable variation in morphology and behaviour/physiology. Some authors (e.g. Buschinger & Winter 1976, 1978) define castes according to function while others (e.g. Peeters & Crozier 1988, Peeters 2019) advocate a strictly morphological definition. This dichotomy of form versus function can cause ambiguity, especially for reproductives, where authors can differ in their usage of the terms "caste" or "queen", for example referring to mated egg-laying workers as "queens". The existence of permanently wingless reproductives ("ergatoid queens") also causes confusion in terminology because of the uncoupling of 'flight' and 'reproduction' (Peeters 2019). Polymorphism of female reproductives in ants has been reviewed by Heinze (1998) and Peeters (2012).

Winged/alate
According to a morphological definition of castes, any alate or dealate (after mating) female with fully segmented flight thorax, regardless of its reproductive status. According to a functional definition of castes, any mated and egg-laying female of an ant colony. The evolution of wingless workers in ants allowed a huge divergence in body size relative to winged queens, and this is unknown in social wasps and bees (Peeters & Ito 2015).

Ergatoid (flightless)
Queens that emerge as adults without wings (see list of species). As a result of the 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 strategies of colony foundation. See current list of species

Dichthadiiform (flightless)
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 that emerge as adults with short non-functional wings. The thorax of brachypterous queens retains unfused flight sclerites (unlike ergatoid queens), and it is thus very distinct from that of conspecific workers. The short wings are often broken within hours of emergence, with wing stubs remaining, and the impression is left of a queen capable of flying. Known from at least 15 genera ([[Media:Peeters MN 2012.pdf|Peeters 2012]]). See current list of species



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



A Myrmecina graminicola winged gyne deposits tiny traces of a sexual pheromone (secreted from the poison gland) on the substratum. Males are attracted and search around until they find the gyne. After mating the wings are broken off and the queen founds a colony.



Mating sequence in Myrmecina graminicola. Both winged and ergatoid queens attract males by emitting a sexual pheromone onto the substratum or into the air.

Both winged queens and gamergates occur in various species such as Euponera sikorae, Harpegnathos saltator, Platythyrea quadridenta and Pseudoneoponera tridentata.

Male (fully winged)
The vast majority of ant species have fully-winged males.

Brachypterous (short-winged, flightless)
In a few ant species males have reduced wings and are unable to fly.

Ergatoid (wingless)
Ergatoid, wingless males are present in a few species.

Workers
In many species the worker caste can be divided into a number of subcastes. These subcastes are commonly based on distinct size-classes, and include:
 * Minor
 * Median
 * Major or soldier

A given species can be:
 * Monomorphic, with all workers approximately the same size and form
 * Polymorphic, with workers showing more or less continuous variation in size and shape from small to large, although distinctly different proportions can occur at the extreme ends of the variation range (see Crematogaster clariventris for an example)
 * Dimorphic, with two distinct size classes (often called "minor" and "major" or "soldeir")

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

Major or Soldier
This is the largest-sized subcaste of workers. The terms "major" and "soldier" apply to the same subcaste. The differing terms are based on either a morphological or functional perspective. In general, "major" is used when referring to morphology and "soldier" when placing the emphasis on function.

[[Media:Molet et al 2012.pdf|Molet et al. (2012)]] pointed out that majors/soldiers are a mosaic of queen-like and worker-like traits. Majors/soldiers are sometimes considered to be a third caste, not a sub-caste of workers (Peeters 2019). See also Baroni-Urbani (1998).

Morphological sense
Characterised by physical traits absent in minor workers. Heads are usually enlarged because of powerful mandible muscles, and mandibles can have specialised 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. Two sizes of majors/soldiers are known in Gesomyrmex and Pheidole obtusospinosa.

In some genera (e.g. Acanthomyrmex ferox, Crematogaster pygmaea), majors/soldiers have queen-like ovaries that allow a higher production of trophic eggs. The gaster is larger than workers' and this allows majors/soldiers to be efficient repletes (e.g. Carebara, Colobopsis nipponica, Pheidole desertorum).

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

A challenge in the use of the functional definition for this subcaste is that workers with the same morphology can show defensive behaviours in some species (and therefore should be called "soldier") while in other species similar workers specialise in crushing seeds or the store food in the gaster.





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.

Gamergate
A mated, egg-laying worker that reproduces like the queen caste ([[Media:Peeters 91 LinnSoc.pdf|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. See current list of species



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 (Peeters 2019). 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.

The Formica specimen here depicted and termed „intercaste“ is probably a pseudogyne (= secretergate). Such workers with vaulted mesosoma and partially gyne morphology are caused by a virus infection of the labial glands, the „labial gland disease“. Usually they have the size of workers and thus are smaller than true gynes. They are quite frequent in Formica, and particularly in Formica sanguinea. https://www.ameisenwiki.de/index.php/Pseudogynen_/_Secretergate_/_Viren (with pictures).

See also: https://www.tapatalk.com/groups/antfarm/pseudogyne-secretergate-due-to-labial-gland-diseas-t9507.html