1. Introduction

2. Social Insects

3. Worker Polymorphism

4. Temporal Polyethism

5. Push-Pull Model

6. References

Polyethism refers to the concept of the functional specialisation of members of a colony of social insects. Social insects include ants, vespidae, and termites. These different functions that are performed are either divided by morphology- caste polyethism, or age-temporal polyethism.(6) ^[1][https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/polyethism]  

Introduction

Polyethism is a mechanism by which social insects divide work to improve colonial living efficiency. This is achieved through the Task allocation and partioning of tasks. This refers to how tasks are assigned, chosen and divided within colonial social insects. Task allocation refers to the process of specific workers being assigned to specific tasks, in numbers proportionate to the current population of the colony that are needed to perform the task. Task partioning refers to the division of a task into sections to be performed by different units of workers.(9)^[2] Division of Labour refers to individuals propensities to complete a portion of the total net tasks that must be performed within a group. The difference between division of labour and task partioning is that the division of labour is the total tasks that must be performed by the insect colony shared between the total workforce of that colony, whereas task partioning is a discrete task shared between a group of workers.

A simple example of task partioning is the reception of nectar by honey bees. Nectar is collected by one group of workers known as ‘foragers’ and then once the nectar has been collected foragers give the nectar to ‘recievers’ or ‘storers’, who pack the nectar into cells within the hive. The example of task partioning is clear as we have the task of nectar collection being partioned into two tasks performed by different subgroups. This example also includes division of labour as the storers are younger bees, while foragers are more mature. This goes to show that task partioning and the division of labour are not mutually exclusive, and infact that task partioning leads to more intricate levels of division of labour. Finally, a novel feature of this process is the junction of forager and receiver. While division of labour can be a series of unrelated tasks performed by the collective, task partioning requires the sequential completion of a series of tasks that are related.^[3]

Social living provides a host of advantages to social insects including; procurement of resources, gathering of food, shelter construction and maintenance, environmental buffering and closer proximity to potential mates.

The highest level of organisation is eusociality. Eusociality is characterised by cooperative brood care, overlapping generations of adults within the colony, and a division of labour into reproductive and non-reproductive groups. With few exceptions, all species that practice eusociality are:

o   Hymenoptera (ants, bees and wasps)

o   Isoptera (termites)

o   Thysanoptera (thrips)

o   Hemiptera (aphids) (10) (11)^[4][5]

All eusocial insects practice polyethism. Polyethism is most successful when the colony has persistence that outlasts multiple lifespans of individuals, and the colony population is thousands or millions of individuals.(4)^[6] Thus, eusocial insects are the only species that practice polyethism as colonial living generally generates thousands of individuals and requires the delegation of labour. This delegation is divided by castes decided by polyethism forms; physical or temporal. The process of members developing alternative body types through different gene expression, is known as polymorphism.

Social Insects

Social insects refer to a range of insect species which live in colonies and display the following three characteristics: overlapping generations of individuals within the colony, task partioning and allocation and group integration.(8)^[7] As aforementioned, social insects are eusocial so the main groups that are known as social insects are:

o   Hymenoptera (ants, bees and wasps)

o   Isoptera (termites)

o   Thysanoptera (thrips)

o   Hemiptera (aphids)

Social insects are divided into castes contingent upon structure, function, behaviour or age. The main castes are reproductive members and sterile members.(8)^[7] Examples of each are The queen bee in terms of a reproductive member, and soldiers or workers for steriles. Sterile members engage in activities that support the incoming generations through nest defence, brooding care in nurturing the larvae and young, and collecting food for other members of the colony.

What is important in drawing parallels between social insects and polymorphism is that all colonies have implemented behavioural rules and subsequently behavioural adaptations which increase the efficiency and benefits of group living and seek to negate the liabilities. One of the main behavioural adaptations is task partioning and the allocation of various tasks to members within the colony, as without centralised control these tasks must be assigned within the colony This task allocation without central command is achieved through each individual having a genetically engrained sense to react to environmental cues and recognise the task that they must perform due to their caste. This distribution of knowledge within the colony is seen most specifically in next construction. Social insect nests boast architectural organisation far beyond the sensory or cognitive function of any one individual, displaying the ability for the insects to complete units of the task to culminate in the production of a stereotyped and organised structure.

Worker Polymorphism

The division of tasks requires member specification, and as a result, alternative morphological development can occur within members of the colony, referred to as polymorphism. Polymorphism occurs within colonies of ants, thrips, termites and aphids in the form of soldier castes for example. Within Polyethism, caste polyethism is achieved through worker polymoyphism. There are two types of worker polymorphism- monomorphism, by which division of workers occurs through size difference of workers, and allometry by which workers are divided by the bodily proportions.

An example of monomorphism is honey bees. Within monomorphic population size of individuals is distributed normally. As a result, there are a large number of moderately sized individuals around the average size for the colony and smaller number of either smaller, or larger individuals. In honey bee populations, smaller individuals within the monomorphic spectrum are tasked with brooding tasks, and thermoregulation of the hive. Larger honey bees, which can be up to ten times the size of the smallest members of the colony, are tasked with foraging. However, each body type has its advantage, larger bees tend to be better at learning, carry more weight and are able to fly at a greater range of temperatures, while smaller bees tend to be more resilient to starvation(12)^[8]

Within allometry, population proportional changes distribution is bimodal. An example of allometry is within colonies of ants and termites.(5)^[9] Approximately one in six ant genera have allometry present. Within allometry in ants there are two castes- the smaller caste is referred to as minors, and the larger caste referred to as workers or soldiers.(13)^[10] As a result, the workers or soldiers forage and food handle, and the smaller cast perform tasks such as brooding and environmental buffering through nest maintenance. In a small proportion of ant species there is a third, larger caste that is known as super soldiers. This is seen in select Pheidole species.

Temporal Polyethism

Temporal Polyethism or age polyethism, is a mechanism of task allocation which is uniform throughout all eusocial insects. Within temporal polyethism, tasks are allocated dependant on members age. As temporal polyethism is contingent upon differences that are internal and physiological, there is some cross over between temporal polyethism and physical polyethism.^[3]

Honeybees are an excellent example of a temporal polyethistic species as they age and their DNA methylation continues their roles change; the youngest members only clean cells, then between 2-11 days individuals participate is nest repair and brooding care. At 11-20 days of age members will pack food retrieved by the foragers, and once workers are over 20 days, they will retrieve food for the colony.(3)(7)^[11][12] This pattern of temporal polyethism of brood care, nest maintenance and foraging is seen in a host of ant species and primitive wasp species such as the Ropalidia Marginita up to the eusocial wasp Vespula Germanica. In regards to how temporal polyethism promotes task delegation genetically is seen within honeybees, where gene expression is affected by methylation of individual's DNA. This may result in a revert to nursing and brood care for individuals who have begun foraging when the foraging needs are met by new young foragers. Temporal polyethism is adaptive as it operates on the principle of putting individuals with the lowest life expectancy in the highest risk tasks, while the young members perform the less risky tasks.(3)^[11]TThis is seen in older individuals participating in foraging, which has a high chance of predation and parasitism. Furthermore, in ant colonies, if a young worker gets injured, and thus lowers their life expectancy, they will progress into foraging before those healthy individuals of the same age. However, temporal polyethism is not a system that operates at maximum efficiency, as it is true in most ant species that older members of the colony are more efficient at brood care.

Push-Pull Model

A model put forward by Seeley (1982) endeavours to explain a difference between honeybees in the high season of spring and summer in contrast to winter and autumn.(7)^[12] It is known as the push-pull model and describes how physical castes, as a result of temporal polyethism, may actually be temporary physical castes as members of the species are able to move between task specified castes in the summer and spring to generalists in winter. This temporary change in physical caste is due to the fact that the goals of each season vary; in Summer honeybees endeavour to build food stores, as opposed to Winter where a survival focus is adopted and bees become generalists, being able to perform a range of tasks that need to be completed. The push-pull model assumes that all physical castes of honeybees are able to jump to the winter generalist state dependant on the season. There are information rich experiments to support the push-pull model and it is generally accepted within polyethism.(7)^[12] The means by which the physical castes change in the honey bees is through an a colony wide release of information to stay as generalists, thus disallowing them to develop into the next caste due to a colony wide release of information that disrupts the priming mechanisms that trigger endroncinological cascades to develop into the next temporal caste. This model is built on mutually positive feedback mechanisms being displayed within the colony to reinforce the prevention of development into the next caste.

References

2.   Robinson, G. (1987). Regulation of honey bee age polyethism by juvenile hormone. Behavioral Ecology And Sociobiology, 20(5), 329-338.

3.   Rangel, J., Strauss, K., Seedorf, K., Hjelmen, C., & Johnston, J. (2015). Endopolyploidy Changes with Age-Related Polyethism in the Honey Bee, Apis mellifera. Retrieved from https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0122208

4.   Gautraist, J. (2001). Emergent Polyethism as a Consequence of Increased Colony Size in Insect Societies. Retrieved from http://cognition.ups-tlse.fr/IMG/pdf/41.pdf

5.   Badertscher, S., Gerber, C., & Leuthold, R. (1983). Polyethism in food supply and processing in termite colonies of Macrotermes subhyalinus (Isoptera). Behavioral Ecology And Sociobiology, 12(2), 115-119.

6.    Polyethism | Encyclopedia.com. (2018). Retrieved from https://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/polyethism (Links to an external site.)Links to an external site.

7.   Thomas D. Steeley (1982). “Adaptive Signifigance of the age polyethism schedule in honeybee colonies”. Behavioural Ecology and Sociobiology. Retrieved from: https://link.springer.com/article/10.1023%2FA%3A1016651613285

8.   The Editors of Britannica Encyclopedia. (2017). social insect | Definition & Facts. Retrieved from [https://www.britannica.com/animal/social-insect https://www.britannica.com/animal/social-insect]

9.    Francis L. W. Ratnieks & Carl Anderson (1999). "Task partitioning in insect societies". Insectes Sociaux. 47 (2): 95–108. Retrieved from: 10.1007/s000400050119.

10. John R. Krebs & Nicholas B. Davies (1987). An Introduction to Behavioural Ecology (2nd ed.). Blackwell Scientific Publications. p. 291.

11. Ross H. Crozier & Pekka Pamilo (1996). "Introduction". Evolution of Social Insect Colonies. Sex Allocation and Kin Selection. Oxford Series in Ecology and Evolution. Oxford University Press.

12. Couvillon, MJ; Jandt JM; Duong NHI; A Dornhaus (2010). "Ontogeny of worker body size distribution in bumble bee (Bombus impatiens) colonies". Ecological Entomology. 35: 424–435. Retrieved from:10.1111/j.1365-2311.2010.01198.x.

13.   Rissing, Steven W (1984). "Replete Caste Production and Allometryof Workers in the Honey Ant, Myrmecocystus mexicanus Wesmael (Hymenoptera: Formicidae)". Journal of the Kansas Entomological Society. 57 (2): 347-350.

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