Sex differences in olfaction-mediated visual acuity in blowflies and its consequences for gender-specific trapping

The behaviour of 650 female Calliphora vicina Robineau-Desvoidy (Diptera: Calliphoridae) was examined in a wind tunnel using odour, in combination with six artificial visual stimuli, ranging from a simple black square to a three-dimensional model of a dead mouse. The carcasses of laboratory mice were used to provide a natural odour and visual source, and a blend consisting of dimethyl trisulphide, mercaptoethanol, and o-cresol was used to provide a synthetic lure. Significant differences were found in attraction to these odour sources: 90% of the flies oriented upwind to the natural source and 62% to the synthetic lure. No significant differences were found in upwind orientation towards different visual stimuli, but flies showed significantly more landings if the visual cues provided a vertical contrast against the background. A horizontal contrast gave no difference in landing rate compared to treatments without visual cues. In a field study, the blowfly genera Pollenia, Calliphora, and Lucilia were caught. The overall blowfly catch was significantly higher when an odour lure was present (Pollenia: 3x, Calliphora: 15x, Lucilia: > 79x). A significant three-way interaction between visual cue, genus, and gender was found. The saprophagous Lucilia and Calliphora showed a gender-specific response to visual stimuli, whereas the parasitic Pollenia did not. A 2:1 female:male sex ratio was found for Calliphora spp. and a 12:1 sex ratio for Lucilia spp. The data suggest that landing responses of male and female saprophagous blowflies, and consequently trap catches, result from olfaction, but also from gender-specific visual responses when under the influence of odour.