The evolution of novel mouthpart interlocking mechanisms in ants (Hymenoptera: Formicidae)

Abstract

Ant lifestyles centre around the unique combination of eusociality, flightless workers, and often predatory habits. No morphological innovations in ants are known to relate to predation. Here, we explore an understudied ant novelty: the ability to close off their mouth. Ants lock their labrum in front of the maxillolabial complex through part of the maxilla pressing against labral hooks. Grooves on the maxillolabial complex additionally allow tighter labral fit in some groups. Variations in both mechanisms lead to different levels of mouthpart closure, with an evolutionary trend to switch from looser to tighter conditions across ants. We find predation to predict tight closure, but models of character evolution indicate that evolutionary dependence between these traits is not the only possible explanation for this pattern. Although tight interlocking is lost in some ants, we find no clear evidence for trade-offs with other functions, such as sensing with the palps. We discuss the origin of interlocking in ants and suggest that it might relate to a combination of predation, prognathous mouthparts, and multipurpose mandibles. We discuss the role of mouthpart closure in ant evolution but also highlight limitations of our comparative approach and emphasize the need for focused biomechanical investigations and enhanced natural history data to refine future inferences of adaptation.