Chemical Secretion Use Varies Among Species but Lacks a Straightforward Relationship With Other Defenses in...

Chemical secretion deployment is a widespread defensive strategy in arthropods, typically aimed at surviving encounters with predators. However, the ecological forces shaping variation in its use across species remain unclear. We quantified chemical secretion behavior in 11 sympatric species of Prionostemma within the understudied arachnid order Opiliones. We surveyed species in two distinct forests in Costa Rica and tested their secretion propensity when stimulated, and whether these differences correlate with interspecific variation in morphology, leg-loss levels, or social behavior. We found that species varied in their tendency to release chemical secretions, forming a continuous gradient from taxa that rarely secreted to those that secreted in nearly all encounters. However, this variation was not explained by body size or leg loss. Species with similar morphology or comparable leg loss displayed contrasting patterns of secretion. Among the seven sympatric species with available social data, aggregation patterns showed no straightforward association with secretion propensity: species spanned the full range of secretion behavior, regardless of whether they frequently formed roosting aggregations with conspecifics or heterospecifics or roosted alone. Instead, each species combined secretion propensity, leg loss, body size, and aggregation tendencies in distinct ways, indicating a mosaic of defensive strategies rather than a single axis of trait covariation. Together, these results suggest that chemical defense in this group of tropical arachnids is evolutionarily labile and shaped by species-specific ecological and social histories. This work also provides a foundation for future research on the chemical and behavioral mechanisms underlying chemical defensive diversification in arthropods.