Why do horseflies need polarization vision for host detection? Polarization helps tabanid flies to select sunlit dark host animals from the dark patches of the visual environment

Abstract

Horseflies (Tabanidae) are polarotactic, being attracted to linearly polarized light when searching for water or host animals. Although it is well known that horseflies prefer sunlit dark and strongly polarizing hosts, the reason for this preference is unknown. According to our hypothesis, horseflies use their polarization sensitivity to look for targets with higher degrees of polarization in their optical environment, which as a result facilitates detection of sunlit dark host animals. In this work, we tested this hypothesis. Using imaging polarimetry, we measured the reflection–polarization patterns of a dark host model and a living black cow under various illumination conditions and with different vegetation backgrounds. We focused on the intensity and degree of polarization of light originating from dark patches of vegetation and the dark model/cow. We compared the chances of successful host selection based on either intensity or degree of polarization of the target and the combination of these two parameters. We show that the use of polarization information considerably increases the effectiveness of visual detection of dark host animals even in front of sunny–shady–patchy vegetation. Differentiation between a weakly polarizing, shady (dark) vegetation region and a sunlit, highly polarizing dark host animal increases the efficiency of host search by horseflies.

Zebra stripes, tabanid biting flies and the aperture effect

“…On the other hand, stripes might act more centrally at the decision-making level of host-finding behaviour in horseflies. Little is known about the precise cues used by horseflies to visually segment their scene into host versus background and how this may feed into their in-flight decisions (but see [22][23][24]). However, it remains likely that at distances greater than 2 m from a zebra, black and white stripes fall below the resolving power of the tabanid eye (based on an estimated ommatidial acceptance angle (Δρ) of 1°(MJ How 2019, unpublished data) and an average stripe width of 35 mm [25]; figure 1c i ).…”

“…During the final moments of approach (between approximately 0.5–1 m from the horse [ 13 ]), the appearance of the target will change to reveal the black and white striped pelage ( figure 1 c ii). This visual transformation could somehow disrupt the tabanid's decision-making process, perhaps by diverging away from an expected search image or by obscuring the visual coherence of the single target host [ 23 , 26 ].…”

Standardising visual control devices for Tsetse: East and Central African Savannah species Glossina swynnertoni, Glossina morsitans centralis and Glossina pallidipes

“…palpalis gambiensis on the blue portion of targets [ 52 ]. This suggests that other complex fly visual phenomena may be at play [ 59 ] and serves to underline that colour preferences using this sticky method of enumeration should be interpreted with caution [ 42 , 52 ]. Also, as noted by Vale [ 56 ], blue-black targets generally perform better for savannah species, including G .…”

Horsefly reactions to black surfaces: attractiveness to male and female tabanids versus surface tilt angle and temperature

“…This visual attraction is adverse if the lured insects either lay their eggs onto the black surface or cannot escape from the polarised signal and perish due to dehydration. This phenomenon is called polarised light pollution (Horváth et al 2009(Horváth et al , 2014c. Solar panels are usually tilted and oriented toward south (on the northern hemisphere of the Earth) to maximize the energy yield.…”

“…Horizontally polarised light reflected from surfaces on the ground means water for water-seeking male and female tabanids (Horváth et al 2008 ; Kriska et al 2009 ). If the shiny black target lies above the ground and reflects light with high degrees and various directions of linear polarisation, the optical signal means host animal for female tabanids seeking a blood meal (Egri et al 2012a ; Horváth et al 2017 ). Based on these two different kinds of polarotaxis in tabanids, efficient L-shaped polarisation horsefly traps have been developed (Horváth et al 2014b ).…”