Repugnatorial glands and aposematism in terrestrial isopods (Isopoda: Oniscidea: Armadillidiidae, Porcellionidae)

Abstract Invertebrates exhibit a wide diversity in defensive secretions used to deter predators. Terrestrial isopods possess lobed glands, thought to secrete defensive substances, particularly through uropods or lateral plates. Our study explores the defensive mechanisms of species of Armadillidium Brandt, 1833 with distinctive aposematic colourations, investigating the secretion properties and histological characteristics of their tegumental glands. Histological analysis revealed the presence of lobed glands across all examined species, with notable interspecific variation in abundance and morphology. While all species produced sticky, viscous secretions from uropod glands, only Armadillidium gestroi Tua, 1900 and Armadillidium klugii Brandt, 1833 discharged an unpleasant odour from lateral plate glands when exposed to cold. These findings suggest that aposematic species of Armadillidium may signal their distastefulness honestly through both chemical and visual cues. Cryptic species or those lacking detectable odour, however, may mimic toxic species without producing chemical defences or can produce substances with an unpleasant taste. The diversity of gland morphology and secretion properties appears to be linked to predator avoidance strategies, such as clinging, running, or rolling, across different isopod ecomorphotypes.