CUTICULAR FLUORESCENCE IN EXTANT AND EXTINCT CHELICERATES

The cuticle of scorpions (Chelicerata: Arachnida) fluoresces under ultraviolet light due to the presence of the hyaline layer in the exocuticle. The adaptive significance of this phenomenon has been debated; however, no attempt has been made to document the occurrence of fluorescence among other chelicerates. A systematic study of modern chelicerates revealed fluorescence to be ubiquitous within the unsclerotized integument across all groups, while horseshoe crabs (Xiphosura) and some harvestmen (Opiliones: Gonyleptidae) also exhibit cuticular fluorescence. A hyaline cuticular layer is confirmed in xiphosurids and gonyleptids through SEM and histological slides and shown to be absent in the cuticles of groups that do not fluoresce. Fossil material does not fluoresce, however the cuticular structure is shown to have been altered during taphonomy. The occurrence of fluorescence in xiphosurids indicates that scorpions inherited this trait and that it may have no direct adaptive significance. These results support hypotheses that scorpions are basal arachnids. We conclude that cuticle fluorescence is due to the structure of the hyaline layer and that it is a plesiomorphic rather than derived trait among arachnids. This has implications for the phylogeny of chelicerates and the distribution of cuticular fluorescence among fossil taxa.