Behaviour, morphology and the division of labour in two soldier-producing aphids

In social insects the division of labour will reflect the trade-off between behavioural flexibility and physical specialization of workers. We investigated this trade-off in two social aphids: Pseudoregma sundanica (Van der Goot) and P.nicolaiae (Takahashi) (Hormaphididae: Cerataphidini). They are sibling species that produce dimorphic first-instar larvae of soldiers and nonsoldiers. Defence is ostensibly the task of soldiers, but P.sundanica is also defended by tending ants. We determined the extent to which physical castes are developed in both species by measuring the lengths of various body parts of soldier and nonsoldier first instars. We also measured the defensive behaviour of soldiers and nonsoldiers by confronting them with aggressive stimuli. The soldiers and nonsoldiers of P.nicolaiae were morphologically and behaviourally more similar than those of P.sundanica, and the nonsoldiers showed greater behavioural plasticity. This suggests a more flexible division of labour in P.nicolaiae, with nonsoldiers being recruited to defensive tasks. Across both castes first-instar larvae were also more aggressive in P.nicolaiae than in P.sundanica. We relate these findings to the differences in ant tending in the two species. The data suggest that the caste structure of the social aphids is more complex than expected, and is perhaps more similar to that of the social Hymenoptera.