Specialised placement of morphs within the gall of the social aphid Pemphigus spyrothecae

BACKGROUND: The strategic placement and aggregation of certain castes within the nests of eusocial insects such as bees and ants results in complex colonies that enjoy increased fitness through improved efficiency of social tasks. To examine if this advanced social phenomenon might apply to social aphids, the location of the numerous morphs within the nests (plant galls) of the aphid species Pemphigus spyrothecae was examined. RESULTS: A strong, almost exclusive tendency for soldiers to occupy the gall section nearest to the opening was detected. In addition, it was found that the most reproductively valuable morphs, the mature gall reproductives and the adult gall emigrants, tended to locate themselves in the gall section furthest from the opening. CONCLUSION: The defensive morphs are thus ideally placed at the point most vulnerable to predation while the morphs most directly responsible for the colony's fitness are located in the safest area of the nest. Furthermore, the propensity for soldiers alone to be located at the opening provides excellent supporting evidence that they are also the agents of gall cleaning and repair. These observations demonstrate that relatively high levels of spatial complexity can occur within the galling colonies of the social aphids, just as they occur within the advanced societies of other insect orders.