Myrmecophily Under X-Rays: The Exceptional Brain of an Exceptional Beetle, Paussus favieri (Coleoptera, Carabidae, Paussinae)

Among myrmecophilous insects, beetles represent the most specialised and diverse group. Myrmecophily is a complex evolutionary strategy encompassing a wide spectrum of inter- actions with ants, ranging from occasional to obligate relationships, and from mutualistic associations (e.g., trophobionts) to fully parasitic symbioses (social parasites). One of the most remarkable examples of an obligate ant parasite is Paussus favieri Fairmaire,1851 (Cara- bidae, Paussinae, Paussini), a West-Mediterranean ant-nest beetle. This species spends most of its life inside the nests of Pheidole pallidula (Nylander, 1849) (Hymenoptera, Formicidae), where it exploits the colony’s most valuable resources (ant larvae, pupae, and tenerals) through a suite of sophisticated chemical and structural adaptations that allow it to evade detection and integrate seamlessly into the host colony. For these reasons, P. favieri has re- cently emerged as a key model organism for studying host–parasite interactions in eusocial systems. In this study, we investigated possible correlations between the nervous system of P. favieri and its remarkable morphological and behavioural adaptations, shedding light on how an extreme environment such as the ant nest may have shaped the beetle’s brain. Our results, although requiring more in-depth analysis, reveal an exceptional development of the central body and the antennal lobes, which rank among the largest recorded across all insect species studied to date. We also report two previously undescribed morphological asymmetries affecting the optic lobes and mushroom bodies. Together, these findings provide new insights into the neuroanatomy of carabid beetles and, more broadly, into the biology of a unique model of ant parasitism, advancing our understanding of the evolu- tionary adaptations that characterise the highly specialised Paussinae subfamily, laying down the basis for further analysis.