Soldier-producing aphids have evolved at least nine separate times. The larvae of soldier-producing species can be organized into three general categories: monomorphic larvae, dimorphic larvae with a reproductive soldier caste, and dimorphic larvae with a sterile soldier caste. Here we report the discovery of a novel soldier type in an undescribed species of Pseudoregma that is morphologically similar to P. bambucicola. A colony of this species produced morphologically monomorphic first-instar larvae with a defensive behavioral dimorphism. These larvae attacked natural predators, and larval response to a simple assay, placing the tips of forceps in front of larvae, was correlated with this attacking behavior. Approximately one third of the first-instar larvae in the colony attacked and this proportion was uncorrelated with the time 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 they were older than non-attackers. This is the first example of a possible within-instar age polyethism in soldier-producing aphids. Canonical variates analysis of seven morphological measurements failed to discriminate between attacking and non-attacking larvae. The monomorphic larvae share some morphometric characteristics in common with the soldiers of P. bambucicola and other characteristics in common with normal larvae. We discuss these results with respect to the evolution and loss of soldier castes in the tribe Cerataphidini.

A number of aphid species produce individuals, termed soldiers, that defend the colony by attacking predators. Soldiers have either reduced or zero direct reproductive fitness. Their behavior is therefore altruistic in the classical sense: an individual is behaving in a way that incurs reproductive costs on itself and confers reproductive benefits on another. However, comparison with the better–known eusocial insects (Hymenoptera, Isoptera) indicates that there are important differences between clonal and sexual social animals.

Here we take a clone's–eye view and conclude that many facets of aphid sociality are best thought of in terms of resource allocation: for example, the choice between investment in defense and reproduction. This view considerably simplifies some aspects of the problem and highlights the qualitatively different nature of genetic heterogeneity in colonies of aphids and of other social insects. In sexually reproducing social insects, each individual usually has a different genome, which leads to genetic conflicts of interest between individuals. In social aphids, all members of a clone have identical genomes, barring new mutations, and there should be no disagreement among clonemates about investment decisions. Genetic heterogeneity within colonies can arise, but principally through clonal mixing, and this means that investment decisions will vary between different clones rather than among all individuals.

Aphid taxonomy is often frustrated by the host alternation and extensive polyphenism displayed by many species. Here we examine the utility of using molecular data to assist in life cycle and taxonomic determination. We found that a relatively small amount of DNA sequence data can greatly assist in these tasks. Molecular data have identified the synonymy of five species: Tuberaphis plicator (Noordam) is a junior synonym of T.takenouchii (Takahashi), T.taiwana (Takahashi) is a junior synonym of T.coreana Takahashi, Hamiltonaphis styraci (Matsumura) is transferred to Tuberaphis Takahashi, Astegopteryx roepkei Hille Ris Lambers is transferred to Ceratoglyphina van der Goot, and A.vandermeermohri Hille Ris Lambers is transferred to Cerataphis Lichtenstein. We have elucidated the complete life cycles of five species: A.basalis (van der Goot) alternates between Styrax benzoin and bamboos, Ceratoglyphina bambusae van der Goot alternates between S.benzoin and bamboos, Pseudoregma sundanica (van der Goot) alternates between S.paralleloneura and Zingiberaceae, T.coreana alternates between S.formosana and Loranthaceae, and T.takenouchii alternates between S.japonica and Loranthaceae. In all cases the molecular data agreed with available morphological data. This analysis demonstrates the utility of DNA sequence comparisons for elucidating complex life cycles and the taxonomy of difficult insect groups.