The tarantula Grammostola anthracina uses type IV urticating setae in passive defence: evidence from its presence on the ootheca

Abstract New World tarantulas present a unique and conspicuous defensive mechanism: the release of urticating setae. The morphological differentiation of the types of setae suggests their distinct use, with two defensive mechanisms proposed: active defence against potential predators and passive defence against other arthropods, through the incorporation of setae into the moulting mat and/or ootheca. Tarantulas from Uruguay present three types of urticating setae with different morphologies (types I, III, and IV). It has been proposed that type I is used in passive defence and that type III in active defence; however, the use of type IV remains enigmatic. This study aims to elucidate the use of the type IV urticating setae. For this, we analysed oothecae of Grammostola anthracina, a species that presents type III and IV setae, quantifying them, and comparing the proportions between each type. Differences were found in the number of setae incorporated into the ootheca: type IV urticating setae are present in a higher proportion compared to type III, which suggests their use mainly in passive defence. We also tested the effects of type IV urticating setae incorporated on oothecae on phorid larvae and ants.