The horned soldier aphids of the Cerataphidini, unlike most social insects that reside in nests, live on the open surface of plants. The lack of a nest and other obvious ecological correlates makes it unclear why secondary-host soldiers might have evolved. Here I present a molecular phylogenetic analysis of 32 species of the Cerataphidini, including 10 species from the genera Ceratovacuna and Pseudoregma that produce horned soldiers. The phylogeny suggests that horned soldiers evolved once and were lost once or twice. Most horned soldiers are a morphologically specialized caste and two species that have unspecialized soldiers are independently derived from species with specialized castes. The genus Ceratovacuna appears to have undergone a relatively rapid radiation. Mapping secondary-host plants and geographic ranges onto the phylogeny suggests that bamboos were the ancestral secondary-host plants and that the Asian tropics and subtropics were the ancestral geographic regions for the genera Astegopteryx, Ceratoglyphina, Ceratovacuna Chaitoregma, and Pseudoregma and possibly for the entire tribe. There is evidence for vicariant events that separate the tropical and subtropical lineages in all of the major lineages of the tribe and for dispersal of some lineages. Based on these results, I present hypotheses for the causes and consequences of horned-soldier evolution.

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.

Colonies of the aphidPseudoregma alexanderi produce morphologically-specialized first-instar larvae, termed soldiers, that defend the colony from predators. The environmental cues and physiological mechanisms governing soldier production are currently unknown. Here we present a morphometric study of soldiers and normal first-instar larvae ofP. alexanderi. Several morphological features (fore-leg length and width, hind-leg length, and horn length) plotted against body length display relationship that are similar to a sigmoidal curve. We found further support for an earlier finding that soldiers fall into two size categories, majors and minors, although both types of soldiers appear to follow the same allometry. The patterns of allometry in the soldier-producing aphids are very different from those found in other social insects and do not easily fit into the traditional categorization of allometries. We present two simple alternative models of soldier development as a framework for guiding future studies of the mechanisms of soldier production.

1. Defensive individuals, termed soldiers, have recently been discovered in aphids, Soldiers are typically early instar larvae, and in many species the soldiers are reproductively sterile and morphologically and behaviourally specialized. 2. Since aphids reproduce parthenogenetically, we might expect soldier production to be more widespread in aphids than it is. We suggest that a more useful way to think about these problems is to attempt to understand how a clone (rather than an individual) should invest in defence and reproduction. 3. Known soldiers are currently restricted to two families of aphids, the Pemphigidae and Hormaphididae, although they are distributed widely among genera within these families. We discuss the use of a phylogenetic perspective to aid comparative studies of soldier production and we demonstrate this approach using current estimates of phylogenetic affinities among aphids. We show that the distribution of soldier production requires a minimum of six to nine evolutionary origins plus at least one loss. 4. At least four main types of soldiers exist and we present and discuss this diversity of soldiers. 5. Most soldier-producing species produce soldiers within plant galls and we discuss the importance of galls for the evolution of soldiers. 6. We summarize the evidence on the interactions between soldiers and predators and between soldier-producing aphids and ants. 7. We present an optimality model for soldier investment strategies to help guide investigations of the ecological factors selecting for soldiers. 8. The proximate mechanisms of soldier production are currently very poorly understood and we suggest several avenues for further research.

We reanalysed Yang & Pattern's allozyme data, published in Auk in 1981, of Darwin's finches with a variety of distance and cladistic methods to estimate the phylogeny of the group. Different methods yielded different results, nevertheless there was widespread agreement among the distance methods on several groupings. First, the two species of Camarhynchus grouped near one another, but not always as a monophyletic group. Second, Cactospiza pallida and Platyspiza crassirostris formed a monophyletic group. Finally, all the methods (including parsimony) supported the monophyly of the ground finches. The three distance methods also found close relationships generally between each of two populations of Geospiza scandens, G. difficilis and G. conirostris. There is evidence for inconstancy of evolutionary rates among species. Results from distance methods allowing for rate variation among lineages suggest three conclusions which differ from Yang and Patton's findings. First, the monophyletic ground finches arose from the paraphyletic tree finches. Yang and Patton found that the ground finches and tree finches were sister monophyletic taxa. Second, Geospiza scandens appears to be a recently derived species, and not the most basal ground finch. Third, G. fuliginosa is not a recently derived species of ground finch, but was derived from an older split from the remaining ground finches. Most of these conclusions should be considered tentative both because the parsimony trees disagreed sharply with the distance trees and because no clades were strongly supported by the results of bootstrapping and statistical tests of alternative hypotheses. Absence of strong support for clades was probably due to insufficient data. Future phylogenetic studies, preferably using DNA sequence data from several unlinked loci, should sample several populations of each species, and should attempt to assess the importance of hybridization in species phylogeny.

Many diverse taxa have evolved independently the habit of living in plant galls. For all but some viral galls, it is unknown whether plants produce galls as a specialized plant reaction to certain types of herbivory, or whether herbivores direct gall development. Here I present a phylogenetic analysis of gallforming cerataphidine aphids which demonstrates that gall morphology is extremely conservative with respect to aphid phylogeny, but variable with respect to plant taxonomy. In addition, the phylogeny reveals at least three host plant switches where the aphids produce galls most similar to the galls of their closest relatives, rather than galls similar to the galls of aphids already present on the host plant. These results suggest that aphids determine the details of gall morphology essentially extending their phenotype to include plant material. Based on this and other evidence, I suggest that the aphids and other galling insects manipulate latent plant developmental programmes to produce modified atavistic plant morphologies rather than create new forms de novo.

We describe the life cycle and general biology of the tropical cerataphidine aphid Cerataphis fransseni. We demonstrate that this aphid migrates between trees of Styrax benzoin and various species of palms; palm-feeding populations have previously been known as C. variabilis and C. palmae, which now become synonyms of C. fransseni. On S. benzoin the fundatrix induces a relatively simple gall which can contain >6000 aphids at maturity with a large number of reproductively sterile soldiers that protect the gall from predators. These galls are apparently produced throughout the year. Colonies on the secondary host plants, palms, are apparently obligately tended by ants whereas colonies within galls on Styrax are never tended by ants. We discuss the life cycle of this tropical aphid with respect to hypotheses for the evolution and maintenance of host alternation.

The gall-forming aphidCerataphis fransseni produces soldiers that defend against predators. Soldiers are produced soon after colony foundation and the number of soldiers increases nonlinearly during colony growth. The number of soldiers scales to the square-root of the number of non-soldiers and linearly to the surface area of the gall. This suggests that soldiers are produced to defend an area, for example the perimeter of the colony or the surface of the gall, rather than individual aphids.