Altitudinal Variation in Forewing Size, Estimated Epithelial Cell Traits, and Flight-Muscle Sarcomeres in Pteromalidae and Eulophidae from Northwestern China

How insect organs change along environmental gradients is a central ecological question, yet most altitudinal size work stops at the organismal level. We asked how the chalcidoid forewing varies along a 278–4236 m transect in Northwestern China, using 167 field-collected Pteromalidae and Eulophidae after averaging technical replicates to one row per specimen. Three nested levels were scored: organ size (forewing area), estimated epithelial cell size and number (trichome density proxy), and, in two species, flight-muscle sarcomere geometry derived from longitudinal transmission electron microscopy of high- versus low-elevation conspecifics. After accounting for nested genus- and species-level variation, the forewing area increased significantly with the elevation among Eulophidae, whereas the positive trend in Pteromalidae was not significant. The estimated epithelial cell area increased significantly with the elevation in both families, while the estimated cell number showed no independent elevational relationship. High-elevation sarcomeres were wider in both species examined and significantly shorter in Pachyneuron aphidis; Neochrysocharis formosus showed no independent elevational relationship. Thus, epithelial cell enlargement was the most consistent cellular correlate of elevation, whereas organ- and muscle-level responses were more taxon-specific.