Changes of division of labour along the eusociality spectrum in termites, with comparisons to multicellularity

Abstract

Eusocial insects are characterized by reproductive division of labour, with one (or a few) individuals specialized in reproduction (queen and in termites, also a king) and the other individuals performing all other tasks (workers). Among workers, further division of labour can occur. Termites have three main castes: reproductives, comprising a queen and king; morphologically differentiated sterile soldiers; and workers. Task division among workers varies greatly depending on lifestyle and degree of workers’ reproductive potential, which varies from totipotency to reproduce up to sterility. In wood-dwelling species, which do not forage outside the nest, all tasks are performed by totipotent workers, comprising multiple-instars with less further division of labour. Foraging species with pluripotent workers also have a multi-instar worker caste, but some division of labour between brood care versus foraging and defence exists. The first task seems mainly to be done by smaller—and potentially younger—instars, while the latter two tasks are performed by larger—and potentially older—workers. The highest degree of division of labour occurs in foraging species with sterile workers. Here, morphological worker castes with defined tasks and age polyethism occur. Comparisons with Metazoa reveal striking similarities with termites concerning gradients in germline/soma differentiation and cell totipotency.

This article is part of the theme issue ‘Division of labour as key driver of social evolution’.

1. Introduction

Reproductive division of labour is a hallmark of social insect societies. Within a social insect colony only one (or a few) individuals reproduce (queen, and in termites also a king), while all other tasks are performed by workers and sometimes soldiers. Among workers, further division of labour can occur, commonly associated with morphological size polyphenism (e.g. minor and major workers) and/or age polyethism (see e.g. [1–4]). In age polyethism, young workers generally perform less risky tasks within the nest (like nursing brood and reproductives), while older workers switch to more risky tasks outside the nest (like foraging and defence) (e.g. [5]). Such age-based division of labour is considered adaptive as older individuals generally have a reduced life expectancy, and thus a lower reproductive value (or its inclusive fitness equivalent) than young ones [2,6–8].

As typical for social insects, termites are also characterized by division of labour. However, they differ in several aspects from the better-studied social Hymenoptera (ants and some bees and wasps). Termites (Isoptera) are ‘social cockroaches’ which evolved eusociality independently from social Hymenoptera within the Blattodea. This different ancestry is reflected in their social organization. Termites are hemimetabolous insects, and workers are, in fact, immatures, not adults as in social Hymenoptera. Termites have an unparalleled developmental plasticity [9,10]. Besides progressive development when individuals develop gradually via larval (without wing buds) and nymphal instars (with wing buds) into winged adults, many species can do stationary (i.e. development without any change of size or morphology) and regressive moults (i.e. ‘backwards’ development into an apparently earlier instar of smaller size and with reduced wing buds) [9–11]. As workers of many species are composed of several instars, size polyphenism can occur that can reflect age (but see below for exceptions), a phenomenon that has been called ‘temporal polymorphism’ [12] (hereafter called ‘temporal polyphenism’ as the term polymorphism should be restricted to heritable phenotypic variation). In addition, termite colonies are composed of males and females, generally with king and queen and male and female workers and soldiers, while males in social Hymenoptera are often transient and play a limited social role. All termites are considered eusocial, but the degree of workers’ reproductive potential varies widely across taxa. Workers can be (i) totipotent immatures from which all reproductives develop, (ii) individuals that have lost some but not all reproductive options (pluripotent), or (iii) sterile individuals without reproductive potential ([13] and references therein) (for details, see below). This variation reflects a ‘eusociality spectrum’ with the position of a species being determined by the workers’ probability of foregoing reproduction (sensu [14]), which directly relates to the workers’ reproductive potential. It aligns with the degree of workers’ evolutionary altruism as the importance of indirect fitness gains increases from totipotent to sterile, while direct fitness gains decrease—the latter being zero in sterile workers. Thus, the eusociality spectrum defined by workers’ reproductive potential is a categorization based on first evolutionary principles (i.e. in inclusive fitness terms) that should be used in social evolution (e.g. see [15]). The observed variation in termites makes them good models to test how division of labour changes across the eusociality spectrum with workers’ reproductive potential. Most evolutionary scenarios imply that social evolution in termites evolved from totipotency via pluripotency to sterility of workers; however, some uncertainty and debates exist (reviewed in [16]).

At one extreme end, the eusociality spectrum of termites comprises species that can be considered true superorganisms (sensu [17]) (box 1). These species have sterile workers and soldiers (true neuters) and all reproduction—and thus evolutionary fitness—is realized via the queen and king. Thus, selection and adaptation mainly work at the colony level and not at the individual level, as the true neuters cannot gain direct fitness. This differs from systems in which workers can reproduce, creating (potential) conflict over reproduction [27] and leading to selection at the individual and colony level. Systems with sterile workers reflect a major evolutionary transition (MET) (e.g. [15,17,18,28]), similar to the evolution of multicellularity from unicellular eukaryotes (box 1). The transition to multicellularity is similarly characterized by division of labour between reproduction and viability functions (defence, food acquisition, etc) (e.g. [26,28–32]). As in workers of social insects, viability functions in multicellular organisms are further subdivided when cells differentiate into diverse somatic tissues during ontogenetic development specializing on specific task. The parallels between termites (and other social insects) and multicellular organisms, which also vary considerably in cell differentiation, are striking (e.g. [26,28,29]), and I will explore them using Metazoa as examples of multicellularity.

Box 1. Major evolutionary transitions and reproductive division of labour.

The evolution of social insects has been proposed to represent a major evolutionary transition (MET) [18]. Some discussion has emerged on whether this truly qualifies as a MET and whether colonies are correspondingly true superorganisms, in the sense that they function as higher-level units on which selection acts (e.g. [19–21], reviewed by Boomsma & Gawne [22]). Boomsma & Gawne [22] proposed that not all social insects should be considered superorganisms that passed a MET, only those with differentiated morphological queen and worker castes, in which workers lost totipotency. This definition is based on a phenotypical trait (i.e. morphological caste differentiation), which might be an epiphenomenon of a more general evolutionary principle. In order to apply a common evolutionary concept guiding METs in general (i.e. a first-principle explanation after [15]), Bernadou et al. [17] proposed to use worker/soldier sterility (i.e. true neuters) as criterion for a MET in social insects (see also [23]). Only if workers/soldiers are sterile is the lower-level fitness of colony members (lower-level units; i.e. workers/soldiers and queen as well as kings in termites) completely transferred to the colony level (higher-level unit). This approach uses the general theoretical multi-level selection framework, in which a MET is only reached when the complete fitness of its lower-level units is transferred to the higher level ([24]; multi-level selection 2 sensu [25]). Both criteria, morphological caste differentiation and neuter sterility, often coincide and they are commonly used equivalently (e.g. [22,26]). Yet, they differ and the variation that exists in social insects and, as recently stressed also in Metazoa (see main text of [26]), can make it difficult to apply either as criterion for a MET. They coincide for Metazoa and social insects with an early germline/soma, respectively reproductive/neuter differentiation, resulting in ‘sterile’ soma/neuters. Yet, for instance, social insect species like foraging termites with differentiated, though pluripotent workers have passed a MET according to the morphological caste differentiation criterion but not according to the sterility criterion. The degree of conflict among lower levels—another common criterion used to characterize a MET—largely differs depending on whether they can reproduce or not (see also [27]). Sterile workers or a separated disposable soma is the major factor reducing conflicts and aligning evolutionary interests among related units [26]. The recent recognition of the diversity in the degree of germline/soma separation in Metazoa [26] parallels that found in social insects, and especially termites. This offers new insights about driving evolutionary factors but also blurs the signal of what makes a MET.

In the following, I will outline termites’ eusociality gradient associated with workers’ reproductive potential, summarize what is known about associated division of labour and analyse whether it changes along termites’ eusociality spectrum. Finally, I will draw comparisons with the evolution of multicellularity, and especially Metazoa, and reveal some striking parallels. In this review, division of labour is defined not as an ‘all or nothing’ specialization but a measurable continuum of how often tasks are performed across castes/stages, considering several tasks within a colony.

2. Sociality in termites: the eusociality spectrum and workers’ reproductive potential

Termites, like all social insects, are characterized by reproductive division of labour. Within a colony, most individuals refrain from reproduction and only a few individuals reproduce (kings and queens). These reproductives can be of two types: (i) primary reproductives that originate from winged adults that disperse and found a new nest, or (ii) neotenic replacement or supplementary reproductives (i.e. reproductives that develop from worker instars generally via a single moult within the natal nest) [11,16]. In addition to the reproductive castes, termites have two non-reproducing castes that further characterize division of labour. These are morphologically differentiated sterile soldiers and workers that vary in degree of evolutionary altruism. Workers and soldiers can be of both sexes or one sex only, depending on species (e.g. [12,13,33]).

Soldiers are morphologically differentiated castes. They are a developmental endpoint (i.e. terminal caste) and are always sterile. Note, so-called ‘reproductive soldiers’, which reproduce, are not soldiers but replacement reproductives with defensive traits [34]. Soldiers represent the first altruistic caste, which evolved once at the origin of termite eusociality and which has been lost only secondarily in some taxa, like the soldier-less termites (e.g. the Anoplotermes group within the Apicotermitinae) [35,36]. Soldiers mainly defend the colony and their nestmates but also can take over tasks like scouting for food resources and communicating new food resources to nestmates (e.g. [37–39]). Age polyethism can occur in soldiers, for example, when old soldiers do more risky tasks like front-line defence, while young soldiers preferentially defend the central nest [40]. Species that have (secondarily) lost soldiers are generally nesting in confined nests, often as inquilines in other termites’ nests, where they are less exposed to predation [35,36].

Overall, termite workers are more diverse than soldiers, both concerning the diversity of tasks performed and their development, and this affects division of labour. Termite workers are always developmentally immature stages. Thus, they can, for instance, never reproduce without, at least one, further moult. Their developmental plasticity and reproductive potential vary highly among different taxa, resulting in the eusociality spectrum observed in termites [14] (figure 1). They can comprise a range of totipotent immature instars, from which all terminal castes (i.e. neotenic and primary reproductives, as well as soldiers) develop (totipotent workers). At the other extreme, workers are instars that are arrested in their development and which are therefore sterile. Accordingly, workers’ functions vary from potential future reproductives to specialized altruistic worker tasks (see §3). In species in which workers are comprised of a range of instars, age and instar size can co-vary during development (temporal age polyphenism). This makes it difficult to strictly distinguish between age- and morphological size-polyethism [37,41]. The distinction between age- and size-polyethism is further complicated in species in which workers can do stationary and regressive moults (figure 1) (see below for details). Then, individuals of the same instar might not be of similar age and smaller instar individuals might even be older than those of larger instars. This developmental complication has been forgotten when interpreting termite data (see below). To summarize, in species where workers are composed of several instars, (i) instar generally aligns with size and age, when individuals cannot moult regressively or stationarily, while (ii) it aligns with size, but not necessarily with age, when they have lost these developmental options. (iii) Only when workers are composed of fixed instars, can age polyethism be unambiguously determined as size is fixed and individuals can be followed over time relatively easily.

Figure 1.

Eusociality spectrum of termites characterized by workers’ development and workers’ reproductive potential. (a) Wood-dwelling termites are characterized by totipotent workers with a linear development. Early larval instars (= Larva) develop into workers composed of later larval (= Worker (larval), orange in online version) and nymphal instar workers (= Worker (nymphal), orange in online version), which differentiate into winged sexuals that disperse and found a new nest (= Primary reproductive). Nymphal and larval workers can also do stationary and regressive moults (=${\displaystyle \leftarrow }$ ). Both larval and nymphal workers can also moult into neotenic reproductives within the natal nest. (b) In contrast, foraging species with pluripotent or sterile workers are characterized by a bifurcated development. Here, during early ontogeny, larvae develop along either an apterous or a nymphal line. In species with pluripotent true workers (blue in online version), these still can commonly develop into neotenic reproductives that reproduce within the natal nest, but they cannot develop into winged sexuals and primary reproductives. Sometimes nymphal instars can also develop into neotenic reproductives (not indicated). By contrast, in species with sterile workers (red in onlne version), these cannot become functional reproductives. Simplified cartoon, development of soldiers not shown. For further information, see text.

Termite workers’ developmental plasticity and reproductive potential are largely associates with ecology and life type, and three broad categories can be distinguished [14] (figure 1):

• —Wood-dwelling species, which nest in a single piece of dead wood that serves at the same time as shelter and food ([42]; one-piece nester, sensu [43]). These species do not forage for food outside the nest. They have totipotent workers with a linear development (also called ‘false workers’/pseudergates sensu lato) [11,16], comprising a range of larval and nymphal instars that can perform stationary and regressive moults, besides progressive moults that lead to an increase of size and instar with age. As stationary and regressive moults are very common (e.g. [44,45]), it is difficult to study strict age polyethism as a cause of division of labour in these species, and conclusions based on the assumption that instar is equivalent to age should be interpreted with caution.
• —Foraging species, which have a nest that is separate from their foraging ground ([42]; multiple-pieces nester, sensu [43]). Although nests in some species might be initiated in a piece of wood, workers leave the nest to forage for food outside (central place foraging), at least during later stages of colony development. These species typically have ‘true’ workers that have lost developmental totipotency owing to a bifurcated development of early instars (figure 1). The bifurcated development leads to (i) a nymphal line resulting in winged dispersing sexuals and sometimes nymphoid neotenic reproductives, and (ii) an apterous line leading to wingless individuals comprising the true workers as well as soldiers, and sometimes apterous neotenic reproductives [16,34]. The true workers engage in altruistic brood and royal care and foraging [46]. The foraging species can be further subdivided into two sub-categories (figure 1):

  1. Foraging species with pluripotent workers. Here, (true) workers can still become fully functional replacement/supplementary neotenic reproductives within the natal nest (then called ergatoids) but they cannot develop into winged sexuals that disperse and found a new nest. Workers generally comprise several apterous instars, and temporal polyphenism exists. Regressive moults can occur, but apparently only in the nymphal line. Species with asexual queen succession (AQS) can be grouped here, where clonal-produced workers commonly develop into replacement queens [47]. AQS evolved repeatedly in termites.

  2. Foraging species with sterile workers. Here (true) workers are sterile. They are generally arrested in their development and comprise defined instars; even if they can develop into replacement reproductives, it is unlikely that they gain direct fitness as they rarely do in nature and are never fully functional reproductives. Sterile termite workers are functionally most similar to typical ant workers. Here, age polyethism can clearly be distinguished from polyphenism-based polyethism. In several species, morphologically distinct worker castes (such as majors or minors) can exist that develop from either different instars or different sexes.

There are few exceptions to this association of life type with development. Psammotermes and Prorhinotermes have a flexible linear development, but foraging can occur in these species, which are generally considered to be wood-dwelling [48,49].

Along these three categories that reflect termites’ eusociality spectrum, the workers' reproductive potential decreases, and hence the workers’ evolutionary altruism increases as the importance of direct to indirect fitness benefits decreases. A recent comprehensive study by Revely et al. [50] furthermore strengthens the importance of workers’ reproductive potential for termite social evolution. It showed that pluripotency (called ‘functional sterility’) explains about 40%, and sterility (called ‘obligate sterility’) 13%, of the variation in social complexity traits of termites (like colony size, caste polyphenism and nest complexity).

3. Division of labour among termite workers across life types and the eusociality spectrum

Studies on division of labour among termite workers addressing a broader behavioural repertoire are confined to a few species. In addition, a number of studies exist that concentrate specifically on tunnelling and repair behaviour (e.g. [51–54]). These studies are addressed less in this review as it is difficult to infer colony-level division of labour from studies that consider behaviour only in one specific context.

(a). Wood-dwelling species

A laboratory study of incipient colonies of the dampwood termite Zootermopsis angusticollis (Archotermopsidae) revealed less division of labour among worker instars, using focal sampling of single individuals within their colony to reveal their behavioural repertoire [55]. Except for the first two larval instars, which were rather inactive, all other instars performed all tasks. Thus, the authors concluded that only a single multi-age worker caste exists. A close look at the data reveals that the third instar larvae (i.e. the youngest worker instar) performed relatively more reproductive care than older instars (fig. 3 in [55]). This might hint at some slight specialization of younger worker instars for reproductive care. Another laboratory study on the congeneric species Zootermopsis nevadensis similarly revealed that the first two larval instars are rather inactive, while the older instars function as workers performing all other tasks [56]. Also here the youngest worker instar spends more time licking and grooming others than do older instars. Note, Howse [56] called licking and grooming ‘trophallaxis’, yet true proctodeal (anal feeding) and stomodeal (mouth feeding) trophallaxis was not observed in the study [56]. Strikingly, although both studies had similar results, they were interpreted differently by the authors. While Rosengaus & Traniello [55] stressed that all tasks were performed by all worker instars, concluding an absence of task division, Howse [56] emphasized that the occurrence frequencies differed slightly for some tasks. This is not mutually exclusive as the former work stresses behavioural plasticity of Zootermopsis workers while the latter focuses on frequency differences and task division.

Cryptotermes workers of the wood-dwelling drywood termites (Kalotermitidae) seem to show less/no altruistic brood care behaviour but rather reciprocal interactions among workers, including feeding and being fed by the reproductives [46,57]. Laboratory studies similar to those for Z. angusticollis [55] using focal sampling of all instars in complete colonies have been done for Cryptotermes secundus, including all colony stages from incipient to mature, alate-producing colonies [44,57–59]. They revealed that all larval and nymphal instars, except for the first three larval and the ultimate nymphal instar, were active and performed all behaviours [44,57–59]. Similar results exist for Cryptotermes cavifrons, which concentrate on larval instars [60], and Kalotermes flavicollis (Kalotermitidae), which include early nymphal instars [61,62]. Yet, some slight task differentiation was revealed in more detailed C. secundus studies. Nymphal worker instars performed more butting (a dominance behaviour consisting of fast backward and forward movement of the whole body with or without bumping into other individuals) than larval worker instars. Furthermore, during intraspecific intercolonial encounters, earlier/smaller worker instars were less involved in aggressive interactions (e.g. biting) with non-nestmates than later/larger worker instars [63]. Assuming that instar size aligns with age, this is in line with theoretical expectations for the evolution of age polyethism, namely that older individuals perform more risky tasks than younger individuals [2,7,8]. Intraspecific, intercolonial encounters occur when neighbouring colonies that nest in the same tree meet each other during nest expansion. They commonly lead to fusion of colonies and the death of individuals, including reproductives [64].

(b). Foraging species with pluripotent workers

For foraging species in which workers still commonly become neotenic reproductives but cannot develop into winged sexuals (figure 1), the most intensive studies about division of labour among workers have been done with Reticulitermes fukienensis (Rhinotermitidae) [41,65] and Coptotermes formosanus [66–68]. In contrast to the studies of the wood-dwelling termites (Z. angusticollis, Z. nevadensis, C. secundus and K. flavicollis), which used focal sampling of individuals to reveal their behavioural repertoire, the authors of the R. fukienensis studies applied behavioural assays in which multiple tasks were tested for different instars. In Crosland et al. [65], three worker size classes (small, medium, large) were studied, assuming that they reflect three different instars and thus temporal polyphenism. All tested tasks (e.g. foraging, care of brood and queen, alarm-giving) were done by at least two size classes of workers, with the larger workers always being most active. The second study [41] concentrated on division of labour of foraging-related tasks (e.g. tunnel construction, gallery repair, feeding). In addition to the three worker size classes, it included two larval (non-worker) instars. Like in the studies on wood-dwelling termites, the larval stages were rather inactive, not contributing to the tested foraging-related tasks. For the three worker size classes, some division of labour was observed. Galleries and tunnels were mainly built by medium and large workers, which were also most efficient at feeding (especially the large workers). Small workers seemed to be unable to burrow into the soil. Both R. fukienensis studies led to the conclusion that the majority of tasks are performed by older workers (i.e. age polyethism), assuming that larger workers are older.

A study on Reticulitermes speratus (Rhinotermitidae) that observed inter-individual behaviours (mainly allogrooming, procto- and stomodeal trophallaxis, carrying brood) of experimental colonies in Petri dishes found that besides the first two larval (non-worker) instars also the first worker instar was rather inactive [69]. All other worker instars (here four, W2–W5, rather than the two stages of [41,65]) as well as nymphal instars performed all other behavioural interactions, with the intermediate instars (W3 + W4) being more active than the other stages [69]. This led to the conclusion of slight polyethism. It is important to note that this study concentrated on ‘indoor’ tasks and did not investigate foraging-related tasks, which were specifically performed by medium and large workers in R. fukienensis. Thus, the differences in the results between both studies (intermediate versus large workers being the most active instars) may mean that there is age polyethism, with younger (intermediate worker instars) mainly performing indoor tasks like trophallaxis and allogrooming (R. speratus: [69]) and older individuals (medium and especially large workers) doing most foraging-related tasks (R. fukienensis: [41,65]). This again assumes that instar/size is associated with age.

For C. formosanus, colonies were videotyped, and the behavioural frequency of different tasks was determined for the first two larval (non-worker) instars, the first worker instar (W1) and all other workers (≥W2) combined, as well as the reproductives and the soldiers [66]. The reproductives, soldiers and the larval instars were less active but passive partners in interactions. Workers performed most of the tasks, with first-instar workers concentrating on allogrooming, while higher-instar workers provided more care for the reproductives, and did more nest maintenance and hygienic behaviour (like removal of faecal pellets). Foraging (wood consumption) was done by W1 and ≥W2 workers equally. In a second study, which concentrated on the distribution of termites around the reproductives/eggs cluster versus a feeding site, ≥W2 workers were more common at the feeding site, while W1 workers predominantly occurred around the reproductives/eggs cluster [67]. Finally, when higher-instar workers are removed from the feeding area they are replaced by earlier-instar workers, the latter having lower food consumption rates [68]. These studies show division of labour among instars. Assuming worker instar associates with age, these studies imply age polyethism, with older workers performing the riskier tasks away from the nest centre while younger individuals perform tasks within the nest.

(c). Foraging species with sterile workers

The best and strongest evidence for division of labour among workers comes from foraging termite species with sterile workers. First, age polyethism with self-sacrifice was clearly shown for Neocapritermes taracua (Termitidae) [70]. Old workers rather than young workers explode in defence of their nestmates. This supports predictions that older rather than younger individuals should perform the costliest tasks [2,7,8].

Second, the best-studied termites with sterile workers are Macrotermes spp. and they have extensive division of labour among workers, including age polyethism and task division among morphological castes. Macrotermes spp. belong to the fungus-growing termites (Termitidae: Macrotermitinae). They live in an obligate symbiosis with Termitomyces fungi, which they culture inside their nest [71]. The termites provide the fungi with (pre-digested) dead plant material, which the fungus turns into nitrogen-enriched food for the termites. Besides two soldier castes (both females), Macrotermes species have two worker castes, minor (females) and major workers (males), which are sterile and develop from defined instars. Their extensive division of labour has been revealed in diligent studies by Reinhold Leuthold and his group for Macrotermes bellicosus from Ivory Coast and Macrotermes subhyalinus from Kenya [37,72–75]. These studies were able to distinguish true age polyethism within a caste/instar from caste polyethism, for instance, by following marked individuals over time. Thus, they are not based on the assumption that instar reflects age as studies on wood-dwelling termites and foraging species with pluripotent workers did (see above).

Besides one king and one (or sometimes a few) queens, M. bellicosus colonies are composed of about 75% minor workers, 20% major workers, 3.5% minor soldiers and 1.5% major soldiers. There is relatively strict division of labour among worker castes, which is associated with sex. Minor workers take care of the brood, feed dependent castes (i.e. larvae, royal pair, soldiers) and perform all building activities. The latter include mound construction and repair, construction of underground gallery networks during the search for food, and enclosure of discovered food in soil sheetings. By contrast, major workers process the food for fungus cultivation and forage for food. Tasks shared by both castes are water collection and carrying food items back to the nest. In addition, in both castes age polyethism exists. Indoor tasks (minor workers: brood care and allo-feeding; major workers: food processing) are done when young, while older workers switch to outdoor tasks (minor workers: construction of galleries and soil sheetings; major workers: foraging). Mound construction is probably an activity of minor workers before they transition to outdoor tasks. This is supported by recent physiological and gene expression studies, which indicate that foraging workers are older than builders [76]. The transition from in- to outdoor occurs after about 20 days, which corresponds roughly to 30–40% of worker median lifespan in the laboratory. Task division between minor and major soldiers is less well studied, but our own observations indicate that minor soldiers build a first frontline and mainly alarm nestmates about potential attacks by drumming their heads on the substratum [77], while major soldiers are then recruited to defend the colony against attackers/predators.

Division of labour in M. subhyalinus is similar to that of M. bellicosus, but some differences exist. Age polyethism also occurs in M. subhyalinus. Young major workers do indoor tasks like food processing and fungus cultivation and start to leave the nest to forage at the age of about 20 days. However, in contrast to M. bellicosus, foraging is only done by these old major workers, which explore the area, collect food and bring it back to the nest. Minor workers are not involved in foraging. This can be explained by the fact that M. subhyalinus emerges from foraging holes to explore the soil surface for food. Thus, soil sheetings that are built by minor workers in M. bellicosus are less common in M. subhyalinus (own observation). In line with this, M. subhyalinus colonies have a lower proportion of minor compared with major workers than M. bellicosus [37]. In contrast to M. bellicosus, the (young) major workers, and not the minor workers, also nurse the colonies’ offspring. The role and tasks of minor workers have not been studied in detail in M. subhyalinus; these workers are involved, for instance, in mound building (own observation).

Although less intensively studied, results for the fungus-growing termite Odontotermes distans (Termitidae: Macrotermitinae) imply a similar pronounced division of labour between major (males) and minor workers (females) [78]. Minor workers are over-represented among workers attending the queen, while major workers make up nearly 99% of all foraging workers and 70% of all workers at swarming-site exit holes.

Data for other termite species with sterile workers also indicate that there is division of labour among workers, although theses studies did not address age polyethism [79–81]. Most concentrated on division of labour during foraging, with no data on brood care behaviour. For example, in Australian harvester termites Drepanotermes spp. (Termitidae: Termitinae), larger workers are more common at repair sites of galleries after a breach than are smaller workers [80]. Nasutitermes costalis (synonymous to Nasutitermes corniger [82], Termitidae: Nasutitermitinae) has two worker castes, majors and minors, the first comprising two (SW1, SW2) and the second three instars (LW1, LW2, LW3). During foraging, minor workers are involved in gallery construction; however after a breach the largest/oldest major worker instar is mainly present [79]. Jones [81] recorded for the same species that SW1 and LW3 are moderate in abundance but the most vigorous builders at a breach, while SW2 is the most abundant caste. Caste-specific division of labour among soldiers (males) and minor (males), medium (females) and major (females) workers was also observed during foraging in processional lichen-feeding Hospitalitermes species (Termitidae: Nasutitermitinae), which develop from different instars [83–85].

The studies on foraging species with sterile workers imply that sex can play an important role in division of labour among workers. For species with totipotent or pluripotent workers, less is known about the role of sex-specific division of labour. In wood-dwelling species, both sexes are generally equally represented among the totipotent workers Rosin 2001 [16], which might imply an absence of sex-specific division of labour. In foraging species, there can be strong effects of sex on caste differentiation, so that, for example, only one sex becomes workers [33], reflecting a strong sex effect on division of labour. However, this pattern is inconsistent even across closely related taxa [16,33], so that it is difficult to draw conclusions.

(d). Conclusion: division of labour and the eusociality spectrum

All termites are characterized by division of labour between reproduction (queen and king), defence (soldiers; exception: secondary loss of soldiers in some humivorous species) and other tasks (workers) (table 1a). Beyond this, the current data might imply that division of labour among workers for other tasks increases along the eusociality spectrum associated with loss of workers’ reproductive potential (table 1b). In wood-dwelling species, which have totipotent workers, all other tasks are performed by all worker instars beyond a certain stage (generally, the third larval instar), forming a multi-instar caste (table 1b). Some indication exists that older/higher instars do the more dangerous tasks like defending a colony (C. secundus: [63]), while yonger/smaller instars disproportionally take care of the reproductives (Z. angusticollis: [55]). In foraging species with pluripotent workers, there still is a multi-instar worker caste that performs all tasks. However, more specialization seems to occur than in wood-dwellers. Older/higher instars perform more defence and foraging, while younger/smaller instars do more brood care (table 1b). This is especially clear in C. formosanus [66–68]. For foraging species with sterile workers, good evidence exists for age polyethism with younger workers performing low-risk indoor activities like nursing while older workers leave the nest to forage outside or defend the colony (e.g. [37,70]). In addition, several morphologically differentiated worker castes can occur, such as major and minor workers. The most elaborate and fine-tuned division of labour seems to exist in fungus-growing termites, like Macrotermes spp., which have sterile workers and colony sizes of a few million individuals. Here, not only do different morphological worker and soldier castes (majors and minors of each) perform different tasks but age polyethism also occurs, at least, among workers [37].

Table 1.

Eusociality spectrum of termites and division of labour. (a) The major castes in termites with their major tasks (in italics): reproductives (queen/king), soldiers, workers. (b) The degree of division of labour among workers differs greatly among termites depending on life type and workers’ reproductive potential: wood-dwelling species with totipotent workers, foraging species with pluripotent workers, and foraging species with sterile workers. inst., instar; —, not applicable as this task/behaviour does not occur in these species; ?, unknown, has not been studied; ? with a term, maybe, evidence not strong yet. For more information, see text.

(a) all termites: castes
queen/king		soldiers			workers
reproduction		mainly defence (intra-/inter-specific)			all other tasks
(b) workers (according to life type and workers’ reproductive potential)
wood-dwelling, totipotent workers
	worker interactions		brood care	defence		foraging
Archotermopsidae	multi-instar caste		multi-instar caste	?		—
Kalotermitidae	multi-instar caste		—	older?		—
foraging, pluripotent workers
	worker interactions		brood care	defence		foraging
Rhinotermitidae	multi-instar worker caste		smaller (younger)	larger?		larger (older)
foraging, sterile workers
	others (building, fungus care)		brood care	defence		foraging
Macrotermitinae	majors/minors		young workers	?		old workers
others	morph. castes		?	old workers		larger workers

There are some caveats associated with this conclusion of increasing division of labour with increasing loss of workers’ reproductive potential (i.e. along the eusociality gradient). First, data are limited and often studies are difficult to compare as they apply different observation approaches (e.g. focal sampling of individuals within colonies, specific behavioural assays) without studying all castes and/or tasks. Second, for wood-dwelling species and foraging species with pluripotent workers it is difficult to distinguish division of labour based on size/instar from age polyethism. Stationary, and particularly regressive moults can result in smaller individuals/instars being older than larger instars. To rigorously test for age polyethism in these species, individuals need to be followed over long periods to analyse how their behaviour changes with age. This is challenging as especially workers of wood-dwelling species are long-lived (at least 4−5 years) (reviewed in [13]) and following individuals is difficult as marks get lost during moulting (e.g. paint dots) or hinder development (e.g. metallic marks or radio-frequency identification devices (RFIDs)). Finally, the number of tasks ‘available’ in a colony can affect the evolution and differentiation of division of labour as the following three examples will illustrate. (i) In wood-dwelling termite species, no foraging occurs because colonies nest inside their food. Hence, age polyethism associated with foraging as in foraging termite species or social Hymenoptera (see this issue) cannot exist [86]. (ii) Soldier-less termites that live in safe, relatively predator-free environments have lost soldiers, although they belong to the highly social Termitidae that have sterile workers. (iii) Compared with all other termites, fungus-growing termites have the additional task of fungus cultivation and they have the most elaborate division of labour so-far known among all termites, including, for instance, several nest-building Nasutitermitinae (e.g. N. corniger) with equally large colony sizes. To determine the importance of these factors for the evolution of division of labour, species of the same eusociality category should be studied that vary in task availability. For instance, it would be interesting to test whether mound-building (i.e. new task) that evolved repeatedly among the Nasutitermitinae [87] is associated with more elaborate division of labour.

4. Multicellularity and division of labour in Metazoa: parallels to social insects and termites

As shown above, termite species vary in relation to the degree of workers’ reproductive potential (i.e. across the eusociality spectrum) and this might align with division of labour (table 1). Termites with sterile workers as well as social Hymenoptera with sterile workers have been considered ‘superorganisms’, in analogy to multicellular organisms (for a thorough treatment of the topic, see e.g. [15,17,22,27]), and these systems can reflect MET (box 1). Here I will point out major similarities between Metazoa and termites/social insects with regard to division of labour (table 2). For further comparisons of the evolution of social insects and multicellularity (e.g. importance of relatedness and conflict), I refer to recent work by Boomsma [15], Howe et al. [26] and Bourke [27].

Table 2.

Parallels between termites and Metazoa with regard to division of labour (DOL). There are striking parallels between termites and Metazoa concerning reproductive division of labour and further division of labour among soma/neuters. Several apply similarly to social Hymenoptera, which are, however, not addressed in this paper. For further information, see text..

	termites	Metazoa
reproductive DOL	queen/king vs neuters	germ cells/germline vs soma
gradient in…	reproductive/neuter separation	germline/soma separation
timing and separation	complete reproductive/neuter separation aligns with early separation during ontogeny	complete germline/soma separation aligns with early separation during ontogeny
further DOL	workers, soldiers, morphol. castes	tissues, organs
variability in potency of lower units	totipotent, pluripotent, sterile individuals	totipotent, pluripotent, sterile cells
mechanism	caste differentiation	cell differentiation

Multicellularity evolved in at least 16 different eukaryotic lineages, including animals and plants (e.g. [88,89]). They are all characterized by division of labour between reproduction and viability functions, the basic components of fitness (e.g. [31,32,90]). During ontogenetic development, the controlled process of cell differentiation demarcates division of labour between final cell types [89]. Thus, for example, adult extant Metazoa typically have at least five distinguishable cell types [91]. This allows task specialization, within the constraints of a shared genome. Tasks that might have been performed sequentially by an unicellular eukaryote can be carried out in parallel. Two hypotheses have been proposed to explain the evolution of animal cell differentiation at the proximate, mechanistic level: the temporal-to-spatial transition hypothesis (e.g. [92]) and the division of labour hypothesis (e.g. [93]). From an ultimate, evolutionary perspective, both hypotheses are linked and stress the importance of task allocation and division of labour for the evolution of multicellularity. The temporal-to-spatial transition hypothesis, however, proposes that cell differentiation mechanisms evolved before multicellularity and then temporally alternating phenotypes were converted into spatially segregated cells in animal ancestors. By contrast, the division of labour hypothesis suggests that cell differentiation only evolved after the evolution of multicellularity, by loss of function from multifunctional cells. Evidence for both hypotheses exists, though it remains largely descriptive (e.g. [89,94]).

The process of cell differentiation in animals is reminscent of caste differentiation in social insects, when different castes develop from a shared ‘family genome’ (e.g. [95–98]). For social insects, it has been shown that molecular pathways present in solitary insects have been co-opted during social evolution and became heterochronically expressed during caste differentiation (e.g. [99,100]), providing some evidence for a social insect analogy of the temporal-to-spatial transition hypothesis. Strikingly, in Metazoa and social insects the first division of labour concerns reproduction. This led to the evolution of sequestration of a germline separated from the soma in animals, reminiscent of reproductives (queens, kings) versus neuters (workers, soldiers) in social insects (table 2). However, not all Metazoa have a strict germline/soma separation and, if they have, the timing of segregation varies [26]. Animals, like Porifera, Cnidaria and ctenophores have toti-/pluripotent cells which can still differentiate into many cell types. For instance, hydroids (Cnidaria) have interstitial cells that can differentiate into all kind of cells, including germ cells from which dispersing propagules derive (e.g. [101,102]). As outlined above, a similar variation in the degree and timing of reproductive/neuter separation exists in termites: from totipotent individuals that differentiate late into reproductives (wood-dwelling species), via pluripotent individuals with an early differentiation into dispersing reproductives versus non-dispersing individuals that nevertheless can still become neotenic reproductives (foraging species with pluripotent workers), to species with an early differentiation into reproductives versus 'somatic' workers (foraging species with sterile workers) (figure 1). Besides, reproductive division of labour, division of labour among soma cells occurs when cells differentiate into different tissues and organs. Thus, even arthropods and craniates, which have a strict and early germline/soma separation [26], have pluripotent stem cells, able to differentiate into different soma cell types that perform different task. This resembles morphological castes, like minor and major workers, in social insects. The analogies between the evolution of multicellularity and social insects are striking and have been pointed out for decades (e.g. [29,30]). Yet, with recent advances in technology, knowledge, and extension of model organisms, the diversity within Metazoa and social insects has become more obvious, allowing testing for common underlying principles and evolutionary forces (see also [26]).

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This work did not require ethical approval from a human subject or animal welfare committee.

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I have not used AI-assisted technologies in creating this article.

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I declare I have no competing interests.