Geographic phenotype variation in neotropical South American grasshoppers (Acrididae: Orthoptera): patterns along environmental gradients.

The understanding of phenotypic variability across a geographic space constitutes a central issue in present evolutionary biology. As a rule of thumb, body size and trait variation mirror the adaptation to the environment within a species range as a result of adaptation to local environments, or to neutral drift, or both. This study examines phenotypic variability, static allometry, and sexual size dimorphism (SSD) along environmental gradients in 3 South American grasshoppers with distinct ecological requirements: the terrestrial Dichroplus elongatus (Giglio-Tos) (winged) and Dichroplus vittatus (Bruner) (wing dimorphic), and the semiaquatic Cornops aquaticum (Bruner). Dichroplus elongatus follows Bergmann's rule and a converse Allenian pattern, likely driven by developmental rates and biomechanical compensation in cooler environments. In D. vittatus, body size and scaled appendages (following Allen's rule) are strongly influenced by precipitation and thermal range, showing sex-specific responses where females prioritize fecundity. Cornops aquaticum, which displays a converse Bergmann pattern, does not show any evidence for Allen's rule or its converse, suggesting semiaquatic habitat shifts toward hydrodynamic constraints. SSD was female-biased in all species. However, static allometric patterns critically moulded the phenotype, adapting proportional investment in locomotor versus reproductive structures in a habitat-specific manner. Thoracic dimensions in D. vittatus followed the converse of Rensch's rule due to high female sensitivity to environmental conditions, whereas the more stable semiaquatic habitat of C. aquaticum reduced sex-specific climatic responses. These results underscore that orthopteran phenotype evolution is a complex interaction between habitat use (terrestrial vs semi-aquatic), life history strategies, dispersal modes, and sex-specific selective pressures.