Evidence suggests that modified setae of the crab spiders Stephanopis spp. fasten debris from the background

Crab spiders (Thomisidae) are known by their ability to change their body colouration via change in epithelial pigments. However, the crab spider genus Stephanopis appears to match the colouration of the bark they are sitting on by having debris attached to its dorsal cuticle. The functional morphology, colouration, and evolution of this phenomenon were investigated in Stephanopis cf. scabra and S. cambridgei. Analysis under the microscope revealed that debris originated from the bark they were sitting on. Using scanning electron microscopy, three different types of setae likely related in the retention of debris were found in S. cf. scabra and one in S. cambridgei. These setae are branched and possess barbs, unlike the more filiform setae found in other crab spider species. In addition, the presence of debris improved the brightness background matching of spiders against the bark, but not hue and chroma matching. Ancestral character state reconstruction suggested that presence of debris evolved two to three times within Thomisidae. The evolution of both masking and colour change among crab spiders indicates that they are under a strong selection to avoid detection.