The Evolution of the Alarm-Defense System in the Formicine Ants

Abstract

1. The higher formicine ants base their chemical alarm-defense systems primarily on a limited array of acyclic terpenes discharged from the mandibular glands, and alkanes and ketones discharged from Dufour's gland. All of these substances appear to be utilized in defense, and most, especially those at the lower end of the range of molecular weight (C9-C13), also function as alarm pheromones. The active space of the pheromones reaches over a distance of centimeters and is relatively short-lived. The alarm response of the various species can be classified roughly as either "panic" or "aggressive" in nature. 2. Two major adaptive alterations in the basic alarm-defense system have occurred within the higher Formicinae. In the genus Acanthomyops, the mandibular gland has been enlarged and made the site of storage of unusually large quantities of citronellal and two isomers of citral. We suggest that the changes are causally linked to the development of a strongly aggressive form of alarm communication. The response thresholds are also higher than in the one species (Lasius alienus) displaying a panic form of communication for which similar measurements have been taken. Lasius subgenus Chthonolasius, which is considered phylogenetically intermediate between Acanthomyops and the more primitive Lasius subgenus Lasius, also has an intermediate set of alarm-defense chemicals. Lasius subgenus Dendrolasius has evolved a system similar to that of Acanthomyops, except that cyclic terpenes (furans) are the substances produced in excess. 3. The second major adaptive change has occurred in certain species of the slave-making Formica sanguinea group. In these ants the Dufour's glands have been enlarged to become the site of large quantities of decyl, dodecyl, and tetradecyl acetates. The acetates are highly effective alarm substances, with extensive, long-lived active spaces. When discharged at target colonies during slave raids, they also alarm and help disperse the defending workers. Thus they serve both as a colonial signal and as an offensive form of "propaganda."