Horsehair worm infection reshapes host trait architecture via allometric and sex-specific allocation shifts

1. Parasites can profoundly modify host phenotype, yet most studies emphasize behavioral manipulation rather than changes in morphology. From a functional perspective, organismal performance depends on proportional investment among traits, suggesting that parasitism may alter trait architecture through changes in allometry rather than simply reducing body size. 2. Here we test whether infection by Chordodes horsehair worms (Nematomorpha) reorganizes host morphology in two mantis species. Using multivariate analyses (PERMANOVA) and allometric models (ANCOVA), we quantified how infection affects scaling relationships, sexual dimorphism and multivariate trait structure occupancy. 3. Infection altered proportional changes among traits rather than causing uniform shrinkage. In the Chinese mantis Tenodera sinensis, thoracic and wing traits showed reduced scaling and sexual dimorphism contracted, whereas in the giant Asian mantis Hierodula patellifera, appendage traits were primarily affected, and dimorphism was amplified or reversed. Thus, parasite effects were sex-specific and species-dependent. 4. These patterns closely parallel parasite-induced feminization and trait reallocation documented in rhizocephalan–crab systems, suggesting a general mechanism whereby parasites redirect developmental investment. 5. Our results demonstrate that parasites reorganize functional morphology through allometric reallocation, thereby modifying performance-related traits and potentially altering evolutionary trajectories. Integrating parasitism into trait-based ecology is therefore essential for understanding how organismal form, function and diversification are linked.