Comparative analysis of body and eye morphology in three congeneric Nebria species (Coleoptera: Carabidae)

Body size is one of the most representative traits of insects and a key factor underlying the ecological preferences of species. Carabid beetles occupy a wide range of ecosystems, from dense forests to extreme alpine and polar environments, and exhibit considerable intraspecific variation in morphology, behaviour, and physiology, reflecting their adaptation to diverse ecological niches. A close correlation exists between body size and eye morphology across insects, highlighting the strong association between visual performance and ecological specialisation. Morphological features of compound eyes, including eye size and ommatidial density, are therefore closely linked to the ecological roles and behavioural of different taxa. Subtle morphological variations also occur within a single genus. Here, we analysed, for the first time, the morphometric characteristics, focusing particularly on the compound-eye structure of three congeneric ground beetle species, Nebria kratteri, a strictly forest-dwelling species, Nebria germari, a species adapted to cold and humid habitat with a currently limited and fragmented distribution and Nebria diaphana, a ground-dwelling and cold-adapted beetles, to identify morphological differences associated with their ecological habits. We also examined sexual dimorphism by comparing morphological and ocular traits between males and females of these species. Our results revealed clear differences between species in body and eye morphology. N. kratteri had larger body and eye dimensions, while N. germari and N. diaphana had smaller body and eye dimensions, but longer antennae. Significant sexual dimorphism was observed in all species, particularly in N. kratteri, where the females were larger and had more developed eyes. These findings provide new insights into the relationships among body morphology, sensory adaptation, and ecological specialisation in mountain-dwelling carabid beetles, contributing to a better understanding of adaptive differentiation within the genus Nebria.