Eusociality: Social Behavior of Insects

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           Cooperative social behavior, or eusociality, is a term marked for organisms that work together in a hives, colonies, or other groups that divide labor and resources for the good of the whole and is the highest level in animal social organization. The precise characteristic of eusociality is the existence of castes in a population of organisms. Castes determine a specific role for an individual that is set for its entire mature life (Crespi and Yanega 1995). In a eusociety, two functions are essential: an infertile group aiding the other insects that are reproducing, as well as totipotency in either the reproductive group (called facultative eusociality) or neither group (called obligate eusociality) (Crespi and Yanega 1995). Of the insects, termites and various species within the Order Hymenoptera, such as bees, wasps or ants, are known for their eusocial societies.
            There are four distinctives of eusocial organisms: group living in adult life, cooperative care of young, division of labor (reproductively and resource-gathering), and generational overlap (Plowes 2010). The Order Hymenoptera have the most eusocial insects existing: there are a few bees species (Family Apidae), wasps (with different origins for Family Sphecidae and Vespidae), and all ants species (Plowes 2010). By far, the ants have the most eusocial species and are the most successful eusocieties. There are a few other insects that exhibit eusociality including certain aphid and thrip species.
            Many diverse variations exist within eusociality. For example, within the Order Hymenoptera, species can have a different number of queens with reproductive capacities, such as in the fire ants Solenopsis invicta or the honeybee (Ross and Keller 1995). The differences in this number can be explained by a variety of factors such as queen phenotype, pattern of sex allocation, or breeding strategy (Ross and Keller 1995). There are also habitat limitations that restrict the number of reproductive queens that the colony can support which is established when the colony is first formed. The number of queens actually changes the genes that exist in the reproductive population of the colony (Ross and Keller 1995).
http://hymenopteragenome.org/solenopsis/
            Another diversity that exists in eusociality is the difference between eusocial societies of the Order Hymenoptera versus the Order Isoptera. Order Hymenoptera are haplodiploid, whereas, the sexes of Isoptera are both diploid (Thorne 1997).  Haplodiploid sex determination is a strategy where females are derived from diploid eggs and male offspring come from unfertilized haploid eggs (Plowes 2010). Order Isoptera are the termites, which, because of their diploid existence, do not have asymmetric degress of genetic relatedness. Their eusociality probably arose from an earlier lineage of termites where small populations had helpers that were dynamic developmentally and reproductively whereupon they evolved into a group where certain individuals were asexual and remained as helpers of the reproductive castes their entire adult life (Thorne 1997).
http://exxeltermite.com/the-termite/
            One study attempted to derive the genetic source of eusocial behaviors of wasps (Order Hymenoptera). To have a caste of workers in a population of insects that cannot reproduce and who care for their own siblings is an interesting evolutionary conundrum. It would not seem evolutionarily advantageous to relinquish reproductive abilities. This study researched whether or not worker behavior was genetically derived from maternal care behavior genes (Toth et al. 2007). The gene expression of the eusocial wasp, Polistes metricus, were studied and it was found that the workers brain gene expression was closer genetically to foundresses, which exhibit maternal behavior, rather than queens or gynes which do not display maternal behavior (Toth et al. 2007). Instead of individually trying to pass on one’s own genes, a eusocial individual is encouraged by group selection or kin selection.  Reproductively, they are driven by kin selection, but they other advantages exist in eusocieties, such as the fact that tasks in a colony can be done by multiple individuals and enabled by cooperation. (Nowan et al. 2012). Also, defense of the individual is easier with colony support, as well as higher competitive advantages against competitors due to their large size as well as cooperation (Plowes 2010).
            It has been found that once eusociality is exhibited in a species, there is little to no possibility that this species will regress evolutionarily to a rudimentary social society or exhibiting solely solitary behavior (Wilson and Hölldobler 2005). When the first caste of workers have differentiated into individual roles within a species, the bridge of returning to presocial society has been burned. Of the 2,600+ families of insects, only 15 families have eusocial families (Wilson and Hölldobler 2005). Even though the species that exhibit eusociality are extremely successful and prolific, this is still a rare phenomenon. The biomass of all eusocial insects is half of all insects and more than all terrestrial nonhuman vertebrates (Nowak et al. 2012). The species of termites that display eusociality were evolved from one lineages; however, it is bee-lieved that eusociality evolved from three separate lineages in the Order Hymenoptera (Wilson and Hölldobler 2005). One of the proposed reason for the uniqueness and infrequency of eusociality is the strength of the individual direct selection force. Below is an image describing the evolution to eusociality, illustrating the factors contributing to eusociality as well as the point of no return. 
http://www.nature.com/scitable/knowledge/library/an-introduction-to-eusociality-15788128

Sources:
1) Crespi, Bernard J., and Douglas Yanega. "The Definition of Eusociality."Behavioral                                  Ecology 6.1 (1995): 109-15.
2) Nowak, Martin A., Corina E. Tarnita, and Edward O. Wilson. "The Evolution of                                        Eusociality." Nature 466.7310 (2010): 1057-062.
 3)Plowes, Nicola. "An Introduction to Eusociality." Nature.com. Nature Publishing Group,                          2010. Arizona State University.
4)Ross, K. G., and L. Keller. "Ecology and Evolution of Social Organization: Insights from Fire                  Ants and Other Highly Eusocial Insects." Annual Review of Ecology and Systematics 26.1                    (1995): 631-56.
5) Thorne, Barbara L. "Evolution Of Eusociality In Termites." Annual Review of Ecology and                      Systematics 28.1 (1997): 27-54.
6) Toth, A. L., K. Varala, T. C. Newman, F. E. Miguez, S. K. Hutchison, D. A. Willoughby, J. F.                  Simons, M. Egholm, J. H. Hunt, M. E. Hudson, and G. E. Robinson. "Wasp Gene Expression              Supports an Evolutionary Link Between Maternal Behavior and                                                            Eusociality." Science 318.5849 (2007): 441- 444.
7) Wilson, E. O., and B. Holldobler. "Eusociality: Origin and Consequences."Proceedings of the                  National Academy of Sciences 102.38 (2005): 13367-3371. 

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