Social insects – Department of Biology - University of Copenhagen

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What insect societies are

... and why they exist

Outside humans, the eusocial hymenopteran and isopteran societies are the most successful form of life in terrestrial ecosystems when measured in terms of collective biomass and ecological footprint. They have reproductive division of labour, with queens doing most or all of the reproduction, and largely or completely sterile workers doing most of the other tasks. Some species have several worker castes of different body size, including a caste of soldiers. Societies of ants, bees and wasps consist of females only, as the males are short-lived and survive only as stored sperm in the queens. Termite societies, however, have both a queen and a king and workers of both sexes.

Our current understanding of the evolution and elaboration of insect societies is primarily based on Hamilton's kin selection theory, which predicts that altruistic reproductive sterility evolves most readily in families of close relatives. The most advanced eusocial insects with physically differentiated castes have evolved from ancestors whose colonies had strict life-time monogamous parents, so that altruistic nursing workers and soldiers were always full siblings. After castes had become morphologically differentiated, secondary evolutionary elaborations have often lead to multiply inseminated queens (several ant genera, honeybees, some yellow jacket wasps) that produce half-sibling colonies, and to colonies with multiple co-breeding queens that maintain colonies with even less related workers (mostly in ants).

The social Hymenoptera are haplodiploid, which means that the haploid males arise from unfertilized eggs, whereas the diploid females (queens and workers) have both a father and a mother. Because of this, full sisters are related by 75%, whereas half-sisters and sisters and brothers are related by 25%. Queens do all the mate choice of their life on a single day early in adult life and never re-mate although they may survive for several decades in some ants. The males of ants, bees and wasps normally die directly after mating, but the queens store their sperm in a special organ where it remains viable for many years. The social Hymenoptera belong to the insects that have complete metamorphosis. This means that there are larvae of several instars, a pupal stage, and adults that look completely different from the larvae. Specialized soldier castes have evolved in a number of ants and a single species of stingless bee, but only as later, evolutionarily derived elaborations of social colony life.

The termites are diploid, just like most organisms, so there are no important extra differences between the sexes. Workers can be either male or female and are normally full siblings related by 50%, independent of their sex. The termites belong to the insects that do not have a pupal stage with complete metamorphosis, so that each individual develops gradually. This means that termite workers are all immatures (nymphs) that may later develop into replacement reproductives in the same colony or to dispersing reproductives. After finding a mate during massive nuptial flights, such dispersers found new colonies where they will continue to live side by side as queen and king in a so called royal chamber. Termite queens therefore do not need to store sperm for many years as they continue to mate monogamously with their male throughout their lives. The termites evolved soldier castes before they evolved an obligate worker caste.