Structural complexity of chemical recognition cues affects the perception of group membership in the ants Linephithema humile and Aphaenogaster cockerelli.

Hydrocarbon profiles on the cuticle of social insects act as multi-component recognition cues used to identify membership in a species, a colony or, within colonies, cues about its reproductive status or task group. To examine the role of structural complexity in ant hydrocarbon recognition cues, we studied the species recognition response of two ant species, Linepithema humile and Aphaenogaster cockerelli, and the recognition of conspecifics by L. humile. The cuticular hydrocarbons of ants are composed of molecules of varying chain lengths from three structural classes, n-alkanes, methyl-branched alkanes and n-alkenes. We employed species recognition bioassays that measured the aggressive response of both species of ants to mixtures of hydrocarbon classes, single structural classes of hydrocarbons (n-alkanes, methyl-branched alkanes and n-alkenes), and controls. The results showed that a combination of at least two hydrocarbon structural classes was necessary to elicit an aggressive species recognition response. Moreover, no single class of hydrocarbons was more important than the others in eliciting a response. Similarly, in the recognition of conspecifics, Linepithema humile did not respond to a mixture of n-alkane cuticular hydrocarbons presented alone, but supplementation of nestmate hydrocarbon profiles with the n-alkanes did elicit high levels of aggression. Thus both L. humile and A. cockerelli required mixtures of hydrocarbons of different structural classes to recognize species and colony membership. It appears that information on species and colony membership is not in isolated components of the profile, but instead in the mixture of structural classes found in cuticular hydrocarbon profiles.