BEHAVIOR AND MORPHOLOGY OF MONOMORPHIC SOLDIERS FROM THE APHID GENUS PSEUDOREGMA (CERATAPHIDINI, HORMAPHIDIDAE) - IMPLICATIONS FOR THE EVOLUTION OF MORPHOLOGICAL CASTES IN SOCIAL APHIDS

Soldier-producing aphids have evolved at least nine separate times. Th e larvae of soldier-producing species can be organized into three gene ral categories: monomorphic larvae, dimorphic larvae with a reproducti ve soldier caste, and dimorphic lar;ae with a sterile soldier caste. H ere we report the discovery of a novel soldier type in an undescribed species of Pseudoregma that is morphologically similar to P. bambucico la. A colony of this species produced morphologically monomorphic firs t-instar larvae with a defensive behavioral dimorphism. These larvae a ttacked natural predators, and larval response to a simple assay, plac ing the tips of forceps in front of larvae, was correlated with this a ttacking behavior. Approximately one third of the first-instar larvae in the colony attacked and this proportion was uncorrelated with the t ime of day, the ambient temperature, or the diel migratory behavior of the aphids. Migrating larvae rarely attacked. Attacking behavior was correlated with another defensive behavior, hind-leg waving. Attackers were more likely to possess the next-instar skin, suggesting that the y were older than non-attackers. This is the first example of a possib le within-instar age polyethism in soldier-producing aphids. Canonical variates analysis of seven morphological measurements failed to discr iminate between attacking and non-attacking larvae. The monomorphic la rvae share some morphometric characteristics in common with the soldie rs of P. bambucicola and other characteristics in common with normal l arvae. We discuss these results with respect to the evolution and loss of soldier castes in the tribe Cerataphidini.