Developing photoreceptor-based models of visual attraction in riverine tsetse, for use in the engineering of more-attractive polyester fabrics for control devices

Riverine tsetse transmit the parasites that cause the most prevalent form of human African trypanosomiasis, Gambian HAT. In response to the imperative for cheap and efficient tsetse control, insecticide-treated ‘tiny targets’ have been developed through refinement of tsetse attractants based on blue fabric panels. However, modern blue polyesters used for this purpose attract many less tsetse than traditional phthalogen blue cottons. Therefore, colour engineering polyesters for improved attractiveness has great potential for tiny target development. Because flies have markedly different photoreceptor spectral sensitivities from humans, and the responses of these photoreceptors provide the inputs to their visually guided behaviours, it is essential that polyester colour engineering be guided by fly photoreceptor-based explanations of tsetse attraction. To this end, tsetse attraction to differently coloured fabrics was recently modelled using the calculated excitations elicited in a generic set of fly photoreceptors as predictors. However, electrophysiological data from tsetse indicate the potential for modified spectral sensitivities versus the generic pattern, and processing of fly photoreceptor responses within segregated achromatic and chromatic channels has long been hypothesised. Thus, I constructed photoreceptor-based models explaining the attraction of G. f. fuscipes to differently coloured tiny targets recorded in a previously published investigation, under differing assumptions about tsetse spectral sensitivities and organisation of visual processing. Models separating photoreceptor responses into achromatic and chromatic channels explained attraction better than earlier models combining weighted photoreceptor responses in a single mechanism, regardless of the spectral sensitivities assumed. However, common principles for fabric colour engineering were evident across the complete set of models examined, and were consistent with earlier work. Tools for the calculation of fly photoreceptor excitations are available with this paper, and the ways in which these and photoreceptor-based models of attraction can provide colorimetric values for the engineering of more-attractively coloured polyester fabrics are discussed.Author summary: Tsetse flies transmit the parasites that cause sleeping sickness. Tsetse control can contribute to disease control thanks to cheap and efficient ‘tiny targets’ that attract tsetse using a panel of blue fabric, a highly attractive colour for the flies. However, the modern blue polyesters employed are only about half as attractive as traditional phthalogen blue cottons. It will be possible to engineer more-attractive polyesters using techniques based on those already employed for fabric colour matching to the human eye. However, because fly photoreceptors differ to those of humans, these methods must be modified to evaluate colour from the fly’s eye view. This paper continues recent work attempting to explain tsetse attraction to differently coloured fabrics using the calculated responses of fly photoreceptors to those fabrics. In particular, this paper investigates several different assumptions about the sensitivities of tsetse photoreceptors and the ways in which their responses are processed. Regardless of these assumptions, common principles for the engineering of attractive fabrics were determined. The tools provided with this paper, along with fabric engineering methods already in use, will permit the engineering of more-attractively coloured polyesters for the increased efficiency of tsetse and sleeping sickness control.