In social insects the division of tabour will reflect the trade-off between
behavioural flexibility and physical specialization of workers. We investi
gated this trade-off in two social aphids: Pseudoregma sundanica (Van der G
oot) 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 i
s also defended by tending ants. We determined the extent to which physical
castes are developed in both species by measuring the lengths of various b
ody parts of soldier and nonsoldier first instars. We also measured the def
ensive behaviour of soldiers and nonsoldiers by confronting them with aggre
ssive stimuli. The soldiers and nonsoldiers of P. nicolaiae were morphologi
cally and behaviourally more similar than those of P. sundanica, and the no
nsoldiers showed greater behavioural plasticity. This suggests a more flexi
ble division of tabour in P. nicolaiae, with nonsoldiers being recruited to
defensive tasks. Across both castes first-instar larvae were also more agg
ressive in P. nicolaiae than in P. sundanica. We relate these findings to t
he 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. (C) 2001 The Assoc
iation for the Study of Animal Behaviour.