Polarization properties in the scorpion's exoskeleton

Scorpions are arthropods with a broad geographic distribution. They live in environments ranging from deserts to tropical rainforests. Several ecomorphotypes have evolved in scorpions, including species that live under stones or in leaf litter, burrow in hard soil, and move through sandy habitats. Besides the obvious external morphology, their exoskeletons could potentially reflect ecological adaptations through specialization to substrates and niches. While the polarimetric properties of other arthropods, such as beetles, have been studied to understand biological traits, there are no reports on scorpions. To address this lack of information, we examined the polarimetric characteristics of the exoskeleton in four scorpion species using the Mueller polarimetry approach. Polarimetric readings were obtained first in reflected scattering geometry with incident polarized light at 635 nm; the studies were extended to the analysis of fluorescence emission near 500 nm produced by 405 nm UV-polarized light excitation. Although polarization effects in scorpions are not evident to the naked eye, Mueller matrix analysis and derived polarimetric metrics indicate that the exoskeleton of scorpions behaves primarily as a partially depolarizing surface with interspecific differences. Darker species with weaker fluorescence showed stronger depolarization, whereas lighter species with higher fluorescence exhibited lower depolarization. Notably, D. withei, the darkest species examined, displayed the strongest depolarization. This species exhibits a pelophilous ecomorphotype, characterized by low superficial activity and a predominantly burrowing lifestyle. Overall, this study applied the Mueller matrix method to characterize the polarization properties of scorpion exoskeletons and demonstrated differentiated interspecific polarization properties. Whether these depolarization features have an adaptive biological role, however, remains an open question that deserves further study.