Caste Terminology

Ants generally have 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 (thus males do not have a father!).


 * Queens: reproductive females that are fully winged in most species but can be short-winged or wingless in other species. According to species, one or several mated queens reproduce in a colony. 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 individuals within a colony, and typically live one or a few years. In a proportion of species, virgin workers can lay haploid eggs (destined to be males) in orphaned colonies. Several genera have workers that completely lack ovaries.


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

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. Ants can show huge divergence in body size between winged queens and wingless workers, and this is not the case in social wasps and bees (Peeters & Ito 2015).

Helper Queens
Nehring et al. (2012) report that virgin queens (gynes) of the leaf-cutting ants Acromyrmex echinatior and Acromyrmex octospinosus switch to carrying out worker tasks such as brood care and colony defense when they fail to mate and disperse. Thus, while morphologically queens, these individuals act as "workers" within the colony. Helper queens are also known in Probolomyrmex longinodus (where virgin dealate queens which fail to disperse may engage in non-reproductive activities) and Linepithema humile (where dealate queens are commonly found in foraging columns).

Johnson et al. (2022) note that queens of Veromessor andrei are unusual because dealate queens sometimes occur in foraging columns. Creighton (1953) observed this behavior and suggested that these dealate queens were taken into established nests after mating. These observations were studied by Brown (1999), who documented that dealate queens occur in foraging columns for about one month, and that all of these dealate queens were uninseminated. The mechanism that causes queens to forego mating and to perform worker-like tasks is unknown, especially in V. andrei, where this behavior cannot be attributed to lack of rains that trigger mating flights. The only other study to examine this behavior in detail found that dealate foraging queens of Pogonomyrmex pima also were uninseminated (Johnson, Holbrook, Strehl, & Gadau, 2007). Dealate queens from mature colonies in several other ant genera also leave the nest to forage (Johnson, 2015; Peeters, 1997).

Ergatoid (flightless)
Queens that emerge as adults without wings. As a result of the lack of wing muscles, the thorax is reduced (fusion of flight sclerites) and worker-like. However, reproductive organs are fully functional and allow sexual reproduction. Known from at least 49 genera (see current list of species). Can co-occur with winged queens in species where independent colony foundation alternates with colony fission (see Life History).

Ergatoid queens can be placed in two categories:
 * 1) 'Single-purpose': Few are produced in each colony, they are larger than workers and function exclusively for reproduction, e.g. Leptogenys, Monomorium algiricum.
 * 2) 'Multi-purpose': Many are produced in each colony, they are similar in size to workers, only one or few mate and reproduce while others remain virgin and function as labourers, e.g. Mystrium oberthueri, Ocymyrmex.

Satria & Yamane (2019) discuss ergatoid queens within the genus Myrmecina: Despite the difficulty in recognizing the ergatoid queen among dead specimens, some morphological characters are used for the recognition of ergatoid queen, such as the presence of ocelli and vestiges of wing articulation on mesosoma. The vestige of wing articulation was observed in the ergatoid queen of Myrmecina andalas as a small hole in place of the forewing near small round pale area on the mesosoma, and the same feature was also reported in ergatoid queens of Myrmecina nipponica which were determined by a direct observation of the ovaries and spermatheca (Ohkawara et al., 1993). The ergatoid queen has developed eyes as in the dealate queen, but smaller in body size with a smaller number of ommatidia than in the dealate queen. On the other hand, the worker has smallest eyes and number of ommatidia among the three castes, but ommatidia each being larger than those in both the dealate queen and ergatoid queen in M. nipponica (Miyazaki et al., 2015). Although in M. andalas the ergatoid queen has larger eyes and number of ommatidia than in the worker, the size of each ommatidium was similar in both castes.

Dichthadiiform (flightless)
The special form of ergatoid queen peculiar to army ants, with a very large gaster to accommodate massive egg production. Winged queens do not exist in army ants.

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, giving the impression 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 various species (e.g. Chelaner leae, Technomyrmex brunneus ), though not necessarily within one colony. Queen polymorphism is claimed to have a genetic basis in Harpagoxenus sublaevis and Myrmecina graminicola.



Mating sequence in Myrmecina graminicola. Both winged and ergatoid queens attract males by emitting a sexual pheromone (secreted from the poison gland) onto the substratum or into the air. Males are attracted and search around until they find the gyne. After mating the wings are broken off and the queen founds a colony.

Both winged queens and gamergates (see below) occur in various species such as Euponera sikorae, Harpegnathos saltator, Platythyrea quadridenta, Pseudoneoponera tridentata and Rhytidoponera confusa.

Form or Function in Queens
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.
 * Functional
 * Reproductive (sexual breeders)
 * Sterile (helpers)
 * Morphological
 * Wings present (alate)
 * Wing absent (apterous)

This split in usage (form or function) can lead to ambiguity in the literature, with different uses of the terms "caste" or "queen". The existence of permanently wingless reproductives ("ergatoid queens" and mated egg-laying workers) also causes confusion in terminology due to the uncoupling of 'flight' and 'reproduction' (Peeters 2019). Polymorphism of female reproductives in ants has been reviewed by Heinze (1998) and Peeters (2012).

Workers
Many species have a monomorphic worker caste, with all individuals approximately the same body size and shape. Other species have polymorphic workers that show more or less continuous variation in body size. These can be divided into a number of subcastes based on distinct size differences. Individuals at the extreme ends of the range of size variation can have differently shaped body parts (= allometry) (see Crematogaster clariventris for an example). Dimorphic species have two distinct size classes, either called "minor" and "major", or "worker" and "soldier".

Worker subcastes
Wilson (1953b) described five major steps in the evolution of the worker caste:

(1) Monomorphism. The workers of the normal mature colony display isometry (with a log-log curve slope of approximately 1.0) and very limited size variability. A plot of their size-frequency distribution is symmetrical and has only a single mode. In other words, the properties of variation are not basically different from those in a typical random collection for nonsocial insects. The worker castes of most ant genera and species are monomorphic. Also, within the majority of genera and higher taxonomic groups monomorphism is evidently the primitive state.

(2) Monophasic allometry. The relative growth is nonisometric, meaning that the allometric constant a is greater or less than unity to a subjectively noticeable degree. In the most elementary form of monophasic allometry, and hence of worker polymorphism generally, feeble nonisometric variation is displayed over a short span of size variation; this variation in turn is grouped around a single mode with possible skewing in the direction of the major caste. A more advanced stage, involving an increased variation in size together with an apparent bimodal size-frequency distribution, is exemplified by the fire ant Solenopsis invicta. The two modal groups are still overlapping, and hence the majors and minors are connected by intermediate-sized workers, or “medias.”

(3) Diphasic allometry. The allometric regression line, when plotted on a double logarithmic scale “breaks” and consists of two segments of different slopes that meet at an intermediate point. In the several species known to possess this condition, including the leafcutter ant Atta texana (Wilson, 1953), African driver ant Dorylus nigricans (Hollingsworth, 1960), and carpenter ant Camponotus sericeiventris (Busher et al., 1985), the size-frequency curve is bimodal. Also, the saddle between the two frequency modes falls just above the bend in the allometry curve. Diphasic allometry permits the stabilization of the body form in the small caste while providing for the production of a markedly divergent major caste by means of a relatively small increase in size. The lower segment of the relative growth curve is nearly isometric, so that individuals falling within a large segment of the size range are nearly uniform in structure; but the upper segment leading to the major caste is strongly nonisometric, with the result that a modest increase in size yields a new morphological type.

(4) Triphasic and tetraphasic allometry. The allometric line breaks at two points and consists of three straight segments. The two terminal segments, representing the minor and major castes respectively, deviate only slightly from isometry while the middle segment, encompassing the media caste, has a very steep slope. The effect of triphasic allometry is the stabilization of body proportions in the minor and major castes. An example is the weaver ant genus Oecophylla. Baroni Urbani (1976b) has reported a case of tetraphasic allometry in the antennal length of the West African ant Camponotus maculatus. The curve resembles that of triphasic species except that in the largest size classes a high degree of allometry is resumed. The biological significance of this extreme pattern is wholly unknown.

(5) Complete dimorphism. Two morphologically very distinct size groups exist, separated by a gap in which no intermediates occur. Each class is either isometric or non-isometric, but the allometric regression curves are not aligned, a condition suggesting that complete dimorphism can arise directly from triphasic allometry. Examples include most queen-worker differences in ants. They also include minor-major divisions in no fewer than nine phylogenetically scattered genera: the myrmicines Acanthomyrmex, some Carebara (the old Oligomyrmex species), Pheidole, and some Cephalotes (the old Zacryptocerus species); the aneuretine Aneuretus; a few species of the dolichoderine Tapinoma (Zatapinoma); and the formicines Camponotus (subgenus Tanaemyrmex), Colobopsis and Pseudolasius.

Majors and Soldiers
These are the largest-sized infertile individuals in a colony. The terms "major" and "soldier" are used interchangeably in the literature. However, polymorphic workers (same growth rules but small and large individuals differ in shape due to allometry) are distinct from soldiers having morphological traits absent in workers. In Cataglyphis bombycina and various Colobopsis, workers vary extensively in body size, but the majors are morphologically distinct from the soldier caste.

Morphological sense of Soldiers
Characterised by physical traits absent in (minor) workers. [[Media:Molet et al 2012.pdf|Molet et al. (2012)]] showed that soldiers are a mosaic of queen-like and worker-like traits, and can be considered as a third caste, not a sub-caste of workers (Peeters 2019). See also Baroni-Urbani (1998). 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 needed to carry heavy heads. Two sizes of soldiers are known in Gesomyrmex and Pheidole obtusospinosa.

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 jajoby, Colobopsis nipponica, Pheidole desertorum).

Functional sense of Soldiers
Workers that perform the task of 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 is that workers with the same morphology can show defensive behaviours in some species (logically should be called "soldier") while in other species similar workers specialise in crushing seeds or storing food in the gaster.

Repletes
Workers or soldiers whose crop is greatly distended with liquid food and functioning as living reservoirs; this food is shared with other colony members by regurgitation (trophallaxis).

Phragmotic
Phragmosis is a defensive technique that involves specially modified body structures used to block nest entrances. The most common form is the flattening of the upper surface of the head of a worker, which is then used to block entrances to prevent intruders from entering nests or to push intruders out of entrance galleries. In a few rare cases other parts of the body parts may be involved, such as the gaster of the queen in Proceratium melinum.

Gamergates
Mated, egg-laying workers that reproduce sexually, like the queen caste ([[Media:Peeters 91 LinnSoc.pdf|Peeters 1991]]). In about 1% of species, across several ant subfamilies, all workers in a colony can mate and store sperm in the spermatheca (see list of species). Dominance interactions regulate which individual(s) can mate and lay eggs.

Intercastes
Developmental anomalies that are 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.

Labial Gland Disease


The Formica intercaste here depicted 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.

Terms Used for Reproductive and Non-Reproductive Females
This table summarises terms used to describe female ants, both winged and wingless, reproductive and sterile. Note that terms referring to winged/dealate reproductive females vary across researchers, with the same term having different meanings in different studies.

Fully winged males
The vast majority of species have fully-winged males. Ant males are optimized for flight and are highly effective agents of dispersal (reviewed in Hakala et al. 2019).

Non-flying males
A very small number of ant species have wingless or short-winged males. Consequent to the lack of wing muscles, the thorax is reduced in volume and sclerites fuse in some species. Hence “ergatoid” (worker-like) is appropriate to describe such males. However, in other species, wingless males retain flight sclerites