The Ecological Significance of Aphid Cornicles and Their Secretions.

Aphid cornicles are abdominal appendages that secrete an array of volatile and nonvolatile compounds with diverse ecological functions. The emission of alarm pheromones yields altruistic benefits for clone-mates in the aphid colony, which is essentially a superorganism with a collective fate. Secreted droplets also contain unsaturated triglycerides, fast-drying adhesives that can be lethal when smeared on natural enemies but more often impede their foraging efficiency. The longest cornicles have evolved in aphids that feed in exposed locations and are likely used to scent-mark colony intruders. Reduced cornicles are associated with reliance on alternative defenses, such as the secretion of protective waxes or myrmecophily. Root-feeding and gall-forming lifestyles provide protected feeding sites and are associated with an absence of cornicles. In some eusocial gall-formers, soldier morphs become repositories of cornicle secretion used to defend the gall, either as menopausal apterae that defend dispersing alatae or as sterile first instars that dispatch predators with their stylets and use cornicle secretions as a construction material for gall repair. Collectively, the evidence is consistent with an adaptive radiation of derived cornicle functions molded by the ecological lifestyle of the aphid lineage.