A Recoverable Summer-Temperature Signature in Relative Wing Size of the Damselfly Lestes sponsa Out-of-population inverse prediction and spatial-block validation of an environmental morphological signal

Environmental effects on insect wing morphology are well established, and geographic variation in Lestes sponsa wing size and shape has already been described [1-9]. Here we ask a distinct inverse question: can summer temperature be recovered from wing morphology of a population excluded from model fitting? We reanalyzed the open 17-population L. sponsa geometric-morphometric dataset [1] and linked population coordinates to CHELSA 1981-2010 climatologies [22]. The source-defined wing ratio (wing centroid size/body size) was evaluated as a low-dimensional relative-wing-size channel, alongside Procrustes landmark geometry. Leave-one-population-out (LOPO) prediction recovered August temperature from wing ratio in both forewings (r=0.780, MAE=1.61 °C) and hindwings (r=0.774, MAE=1.63 °C). Full 14-landmark geometry generalized poorly, whereas exploratory localization concentrated fitted temperature-associated deformation near LM9; LM9 alone was not predictive. To address spatial dependence [23,24], we applied geographic-block cross-validation. Under the strict five contiguous-latitude-block design, performance declined (forewing r=0.428, hindwing r=0.419), but permutation tests of the full blocked workflow remained better than random (P=0.0125 and 0.0195). We therefore identify a recoverable summer-temperature signature in a sparse, low-dimensional component of damselfly wing morphology dominated by relative wing size. The analysis does not establish that temperature directly causes the observed morphology, nor that the wing is a literal biological thermometer. Its novelty lies in treating wing morphology as an inverse environmental prediction problem and testing transfer to unseen populations. Keywords: Odonata; Lestes sponsa; relative wing size; geometric morphometrics; temperature; inverse prediction; environmental signature; spatial cross-validation; phenotypic plasticity