Evolutionary constraints shape the diversity of microinsects' wing morphology.

Miniaturization profoundly alters animal morphology, particularly locomotory structures like insect wings. Larger insects possess membranous wings optimized for flight dominated by inertial forces, while microinsects have highly derived bristled wings with a reduced membrane, adapted to viscous interactions. Distantly related microinsects share striking similarities in some aspects of wing architecture, such as elongated bristles or narrowed wing blades, while features such as venation or proportion of bristled wing area vary widely. The basis of these differences remains unknown. Although insect wing morphology is largely shaped by evolutionary history, the role of evolutionary constraints in macro-to-micro wing transition has not been examined. For the first time, we combined morphological analyses with evolutionary modelling to explore how selection for wing optimization during miniaturization is constrained by evolutionary inertia in key wing features. Analysing 39 bark beetle species, ranging greatly in size, we found that some modifications, like bristle elongation or wing narrowing, exhibit very low evolutionary constraints, enabling rapid adaptation to miniaturization. In contrast, traits like venation development or bristled area proportion were highly constrained, requiring longer evolutionary timescales to adapt. Our findings provide novel insights into the origins of wing-architecture diversity in microinsects, emphasizing the role of evolutionary constraints in modulating the transition from macro- to micro wings.