Blood-red colour as a prey choice cue for mosquito specialist predators

Specialist predators are innately and distinctively proficient at targeting specific prey types. This is enabled by behavioural, perceptual and cognitive mechanisms that can only be understood using carefully designed experiments. Evarcha culicivora is an East African jumping spider that feeds on vertebrate blood acquired indirectly by actively targeting blood-carrying female mosquitoes as preferred prey. Here we asked whether these spiders use the colour red to identify this prey. In Objective 1, we used spectrophotometry to document the changing redness of mosquitoes in the first 17 h following blood meals. In Objective 2, we used vision-based choice tests to document how E. culicivora's preference for blood-carrying mosquitoes changes over time (using an interval comparable to that in Objective 1). We found the strongest preference for blood-carrying mosquitoes during the first 6 h after those mosquitoes had fed on blood and that this time interval corresponded to when mosquitoes exhibit the reddest coloration. Based on this, we then took mosquitoes that had never fed on blood and manipulated their colour using a blend of red food dye to make them appear blood-fed (i.e. by matching their spectral properties to mosquitoes that had fed on blood within the previous 6 h). We also created grey-dyed mosquitoes that matched the spectral properties of those that had never fed on blood. In Objective 3, there was no evidence of E. culicivora visually discriminating between our red-dyed mosquitoes and blood-carrying mosquitoes; this indicates that, to E. culicivora, our red-dyed mosquitoes appropriately resembled blood-carrying mosquitoes. Finally, in Objective 4, we show that E. culicivora consistently preferred red-dyed mosquitoes over grey-dyed mosquitoes, supporting the hypothesis that E. culicivora uses some aspect of red coloration to identify their preferred prey. We discuss how these findings relate to cognitive processes, colour-based communication and sensory exploitation