Profound head modifications in Claviger testaceus (Pselaphinae, Staphylinidae, Coleoptera) facilitate integration...

Clavigeritae is a group of obligate myrmecophiles of the rove beetle subfamily Pselaphinae (Staphylinidae). Some are blind and wingless, and all are believed to depend on ant hosts through feeding by trophallaxis. Phylogenetic hypotheses suggest that their ancestors, as are most pselaphines today, were free-living predators. Morphological alterations required to transform such beetles into extreme myrmecophiles were poorly understood. By studying the cephalic morphology of Claviger testaceus, we demonstrate that profound changes in all mouthpart components took place during this process, with a highly unusual connection of the maxillae to the hypopharynx, and formation of a uniquely transformed labium with a vestigial prementum. The primary sensory function of the modified maxillary and labial palps is reduced, and the ventral mouthparts transformed into a licking/'sponging' device. Many muscles have been reduced, in relation to the coleopteran groundplan or other staphylinoids. The head capsule contains voluminous glands whose appeasement secretions are crucial for the beetle survival in ant colonies. The brain, in turn, has been shifted into the neck region. The prepharyngeal dilator is composed of an entire series of bundles. However, the pharynx does not show any peculiar adaptations to taking up liquid food. We demonstrate that far-reaching cephalic modifications characterize C. testaceus, and that the development of appeasement glands and adaptation of the mouthparts to trophallaxis determine the head architecture of this extreme myrmecophile.