Substrate-induced chromatic variations in three species of leaf and stick insects (Insecta Phasmatodea)

Mimicry, particularly chromatic adaptation, is a defining characteristic of leaf and stick insects, which comprise phasmids (Insecta Phasmatodea). This study investigates substrate-induced color changes in three phasmid species: Extatosoma tiaratum, Hesperophasma sp. “La Ciénaga”, and Phyllium gantungense “Rizal”. For each species, two experimental groups were exposed to a grey-green lichen (Evernia prunastri) as a substrate, while two cages were set up with an artificial pink material. Control groups were reared on elmleaf blackberry (Rubus ulmifolius) alone; all groups were composed by at least 20 individuals. Using digital pictures, the RGB color channels of juveniles and adults were analyzed to quantify color variations. We revealed significant species-specific variations in color development responses related to substrates. In E. tiaratum we did not observe a significant difference between adults reared over different substrates. However, digital image analyses revealed that E. tiaratum exhibited significant chromatic changes during development, especially among 11 juvenile females, suggesting a potential genetic predisposition for substrate-specific color adaptation. In the species P. gantungense, we did not find color changes during the experiment, indicating a lack of phenotypic plasticity in response to substrate variation. Hesperophasma sp. "La Ciénaga" showed minimal color changes; however, untreated juveniles naturally darkened as they matured, a trend that was inhibited in substrate-treated groups, indicating a potential influence of the substrate on natural color development. These findings highlight the species-specific nature of substrate-induced color variation in stick insects. Investigating the genetic and physiological mechanisms underlying these changes is essential for understanding the evolutionary dynamics and ecological adaptations of these insects. Further research in this area could reveal insights into the broader implications of phenotypic plasticity and environmental adaptation in phasmids.