A natural history of conspecific aggregations in terrestrial arthropods, with emphasis on cycloalexy in leaf beetles (Coleoptera: Chrysomelidae)

Aggregations of conspecifics are ubiquitous in the biological world. In arthropods, such aggregations are generated and regulated through complex interactions of chemical and mechanical as well as abiotic and biotic factors. Aggregations are often functionally associated with facilitation of defense, thermomodulation, feeding, and reproduction, amongst others. Although the iconic aggregations of locusts, fireflies, and monarch butterflies come to mind, many other groups of arthropods also aggregate. Cycloalexy is a form of circular or quasicircular aggregation found in many animals. In terrestrial arthropods, cycloalexy appears to be a form of defensive aggregation although we cannot rule out other functions, particularly thermomodulation. In insects, cycloalexic-associated behaviors may include coordinated movements, such as the adoption of seemingly threatening postures, regurgitation of presumably toxic compounds, as well as biting movements. These behaviors appear to be associated with attempts to repel objects perceived to be threatening, such as potential predators or parasitoids. Cycloalexy has been reported in some adult Hymenoptera as well as immature insects. Nymphs of the orders Hemiptera (including Homoptera) as well as larvae of the orders Neuroptera, Coleoptera, Diptera, Hymenoptera, and, in a less circular fashion, the Lepidoptera, cycloalex. There are remarkable convergences in body form, life habit, and tendencies to defend themselves in the social larval Coleoptera, particularly chrysomelids, social larval Lepidoptera, and social larval Hymenoptera. In immature insects, the cycloalexing organisms can be arranged with either heads or abdominal apices juxtaposed peripherally and other conspecifics may fill in the center of the array. In the Chrysomelidae, the systematic focus of this review, species in the genera Lema, Lilioceris (Criocerinae), Agrosteomela, Chrysophtharta, Eugonycha, Gonioctena, Labidomera, Paropsis, Paropsisterna, Phratora, Phyllocharis, Plagiodera, Platyphora, Proseicela, Pterodunga (Chrysomelinae), Coelomera (Galerucinae), and Acromis, Aspidomorpha, Chelymorpha, Conchyloctenia, Ogdoecosta, Omaspides and Stolas (Cassidinae) are reported to cycloalex although cycloalexy in other taxa remains to be discovered. Other types of aggregations in insects include stigmergy, or the induction of additional labor, and epialexy, or the positioning of conspecifics organisms over the midvein or an elongated aspect of a leaf.