Theoretical Morphospace Analysis of Neuropteran Wings Reveals Little Evidence of Optimization for Flight Performance

Abstract The wing is the key evolutionary innovation of pterygote insects and wing morphology is commonly envisaged as finely attuned to functional performance. Here, we use a theoretical morphospace approach to analyse the evolution of disparity and functional optimality in neuropteran wings, thus, investigating how wings are adapted for flight and how varied factors constrain wing shape. Analysing 738 wings from the Permian to the present neuropterans, we construct a theoretical morphospace with 1092 hypothetical shapes. These theoretical wings were subjected to functional analysis, creating performance landscapes for aerodynamic traits. We used a Pareto ranking scheme to create an optimality landscape for the trade-off between functional traits, which identifies wings with a convex leading edge as optimal. After projecting neuropteran wings onto this optimal landscape, we find that neuropterans do not occupy this optimal region. Aerodynamic constraint is evident solely in increasing aspect ratio, increasing the lift-to-drag ratio and therefore flight efficiency. Phylomorphospaces reveal a large overlap between different neuropteran clades, reflecting convergence on these same functionally sub-optimal wing shapes. Forewings and hindwings are subject to different evolutionary constraints with forewings more aerodynamically related and hindwings more phylogenetically related. The theoretical morphology approach facilitates morphological analyses without prior assumptions of adaptive optimality.