Insect Sociality

Degrees of sociality:

• eusocial:
• individuals of the same species cooperate in caring for young
• a division of labor exists in which more or less sterile individuals work on behalf of relatively few reproductive nest mates
• at least 2 generations overlap enough for offspring to assist parents in performing colony labor during some part of their lives
• all ants, most advanced wasps & bees, and all termites exhibit these traits
• pre-social
• species characterized by 2 or fewer of the above traits
• represented by a series of social stages:
• parasocial sequence
• adults belonging to the same generation assist one another to varying degrees (see table below)
• may be the evolutionary pathway taken by bees

• subsocial sequence
• increasingly close association between mother and offspring (see table below)
• may be the evolutionary pathway taken by ants, termites, social wasps, & some social bees

Caste differentiation in some eusocial insects:

• Ants
• 3 basic castes - queen, major worker (or soldier), & minor worker

From left to right: Queen, Winged male, Major worker, & Minor worker.

• Queen - fully reproductive female whose main function is to lay eggs and, in some species, to start new colonies after mating
• Major workers:
• unfertilized females
• principal functions are colony defense and, in some species, food storage
• specialized forms in some species:
• many genera - mandibles designed for cutting integument & clipping off appendages of invading arthropods
• army ants - possess pointed or hooked mandibles designed for piercing an opponent's integument

• many species - possess shield- or plug-shaped heads used to block nest entrances
• Minor workers:
• unfertilized females
• functions include construction, brood care, care of the queen, and foraging
• in most ants, the division of labor is based at least in part on age:
• about first 50 days - in the nest caring for the brood, the queen, & other workers and cleaning the nest
• after about 50 days - shift to outside activities, particularly foraging and nest construction
• Males - not differentiated into castes & serve only as reproductives

• Termites
• different order (Isoptera) but caste system is similar to that of the social Hymenoptera
• castes comprised of both males and females (not just females as in social Hymenopterans)
• castes include:
• Primary reproductives
• consist of a monogamously mated female (queen, below left) and male (king, below right)

• mate on a nuptial flight following departure from their respective natal colonies
• construct a nest after mating & rear their first brood. After workers acquire their own digestive capability, they perform all construction, nursing, & foraging and begin feeding the primary reproductives.
• Minor workers
• division of labor based on size occurs in a few species, with larger workers foraging and smaller workers working in the nest
• temporal division of labor in some speices, with younger workers in the nest & older workers foraging & working on nest construction
• Major workers (soldiers)
• specialized for colony defense
• commonly, soldiers are mandibulate (with large armored heads, powerful muscles, & sharp, elongated mandibles; see photo to the right)
• in some species, soldiers employ chemical defenses (corrosive or stick secretions & sprays)

Origins of eusocial nesting - two potentially important factors:

• 1 - haplodiploidy and kin selection
• with haplodiploid sex determination (all Hymenoptera), assuming the same father, sisters are related by 3/4. As a result, females can propogate genes faster by helping rear sisters rather than daughters (an important factor in the evolution of sterile castes)
• haplodiploidy alters calculations of relatedness relative to diploid species:
• daughters - related to parents by 1/2
• sons - related to mothers by 1 & to 'fathers' by 0
• brothers - related to each other by 1/2
• sisters
• related to each other by 3/4 (assuming the same father)
• half of their genetic material from haploid father is always the same, while the other half (from diploid mother) is 'shared' by 1/2
• brothers & sisters
• related by 1/4
• half their genetic material is never shared (males receive no genetic material from 'fathers', while females do) & the other half (from mother) is 'shared' by 1/2
• full sisters are more closely related to each other than to their own offspring but this doesn't mean that a daughter should inevitably give up her own reproduction in favor of helping her mother:
• she must be able to contribute significant benefit to her mother's fitness (i.e., if she can rear 1.5 times more surviving offspring on her own than she is able to add to her mother's brood, then selection will favor nesting alone)
• genetic benefits from helping drop rapidly if a daughter is not helping to rear full sisters
• if large numbers of males are produced, this greatly reduces the genetic benefits of helping
• if daughters can bias sex ratio of mother's offspring, produce males by laying eggs of their own, or preferentially assist mother when she is producting daughters, then helping is more likely to be profitable
• the association between haplodiploidy and eusociality (i.e., eusociality has evolved several times in the order Hymenoptera and just once in other insects) provides support for the importance of haplodiploidy and kin selection in the evolution of eusociality
• 2 - parental manipulation
• sterile worker castes result from parental manipulation of offspring
• in all insects, mothers are in a position to control the development & behavior of offspring by:
• varying food supply
• by way of chemicals
• by fertilizing or not fertilizing eggs
• the control by queens over caste determination in many species of bees, wasps, ants, & termites provides support for the importance of parental manipulation in eusociality

Useful links:

Ant colony life cycle

Ant information

Living with Ants and The Science of E.O. Wilson

Myrmecology: The Scientific Study of Ants

Social insects

The Evolutionary Dynamics of Social Organization in Insect Societies

Back to Behavioral Ecology syllabus