For insects that live in colonies, social structure appears to be elevated to an art form. Numerous studies have documented that in insect societies the division of labour is typically based on age, whereby developmental maturation determines a specific job delegation, thus dividing colony members into ‘temporal castes’. However, some species (ants and termites) have also been shown to use distinct morphological attributes for certain tasks, thus allocating their members into ‘physical castes’. In the highest level of social organization, for example in eusocial insects, the division of labour into specialized castes is considered paramount to the success of the species.

Until now, behavioural specializations utilized by social bees and wasps were thought to be established solely based on age. However, Christoph Grüter and Francis L. W. Ratnieks, from the University of Sussex, UK, and Cristiano Menezes and Vera L. Imperatriz-Fonseca, from Preto University of São Paulo, Brazil, report for the first time in PNAS that the division of labour amongst eusocial neotropical stingless Tetragonisca angustula bees appears to be based on morphological specialization for one particular task – defence.

Stingless bees are unique in that they have a sophisticated defence system involving two groups of worker guards: hovering guards (stationed in the air near the nest entrance) and a set of soldier guards that stand inside and around the wax entrance tube of the nest. While the research team was carrying out another study on the bees, they noticed a possible morphological distinction between the various worker bees. Some workers looked larger than the forager bees and this led the team to hypothesize that a subset of workers might include a distinct physical sub-caste made up of larger-bodied guards. When they compared the size and shape of guards with that of foragers and waste-removing workers from 12 different colonies at two locations in Brazil, they found that both types of guard are 30% heavier than the foragers. The guards also display different-sized structural features – while the foragers have larger heads, the guards have larger hindlegs, and the waste-removing workers are intermediate in size. The team wondered how prevalent these larger-bodied workers are within a given population. To determine the overall size distribution of the different categories of workers in a colony, the researchers measured 300 workers (from five colonies) and found that forager-sized workers predominated, followed by waste-removing bees; specialized guard-sized workers represented only 1% of the bees in their sample.

Given that guards represent only a small subset of emerging workers, the researchers hypothesized that the colony must benefit somehow from maintaining a tightly regulated caste of larger-sized workers within the population. In fact, when they tested the fighting performance of soldier guards, they found that the larger-bodied worker bees were able to fight for longer when presented with a potential enemy, the Lestrimelitta limao robber bee. However, further research is needed to confirm whether the soldiers’ distinct morphology, and size, correlates with a more successful defence (advantage) for the colony as a whole.

These observations also generate intriguing questions for future study, including how the colonies manage to produce morphologically distinct workers and to precisely maintain them in appropriate ratios. Towards addressing this question, the authors hope to determine whether other species of stingless bees employ similar defence tactics and to understand how a select physical sub-caste of able-bodied troops evolved in eusocial bees.

V. L.
F. L. W.
A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee
Proc. Natl. Acad. Sci. USA
doi: 10.1073/pnas.1113398109