Abstract
The sun is the most reliable celestial cue for orientation available to daytime migrants. It is widely assumed that diurnal migratory insects use a ‘time-compensated sun compass’ to adjust for the changing position of the sun throughout the day, as demonstrated in some butterfly species. The mechanisms used by other groups of diurnal insect migrants remain to be elucidated. Migratory species of hoverflies (Diptera: Syrphidae) are one of the most abundant and beneficial groups of diurnal migrants, providing multiple ecosystem services and undergoing directed seasonal movements throughout much of the temperate zone. To identify the hoverfly navigational strategy, a flight simulator was used to measure orientation responses of the hoverflies Scaeva pyrastri and Scaeva selenitica to celestial cues during their autumn migration. Hoverflies oriented southwards when they could see the sun and shifted this orientation westward following a 6 h advance of their circadian clocks. Our results demonstrate the use of a time-compensated sun compass as the primary navigational mechanism, consistent with field observations that hoverfly migration occurs predominately under clear and sunny conditions.
Highlights
Migratory insects dominate aerial bioflows in terms of diversity, abundance and biomass [1]
Hoverflies flown in the sun compass experiment, with the sun visible, landscape cues obscured and other celestial cues attenuated by photographic diffuser, headed almost due south (Moore’s modified Rayleigh (MMR) test: θ = 188.2°, n = 30, R* = 1.294, p < 0.01; figure 2a)
Our methodology differed from butterfly experiments in the use of a magnetic tethering system, attenuation of celestial cues with a diffuser layer and a lack of laminar airflow that may have each contributed to poorer orientation responses
Summary
Migratory insects dominate aerial bioflows in terms of diversity, abundance and biomass [1]. Together with stable isotope analysis, suggest that individuals leaving northern Europe may travel thousands of kilometres to potential overwintering sites around the Mediterranean basin and North Africa [30,31] This appears to be achieved by a combination of high-altitude wind-assisted flight and, when facing headwinds, low-level flight within the flight boundary layer (FBL) [32,33]. In addition to estimates of flight speed, radar studies have revealed sophisticated strategies used by migratory hoverflies, including the ability to select favourable winds, to partially compensate for wind drift and to orientate towards seasonally preferred directions, during autumn migration [35,36]. We predict that hoverflies would continue to orientate southwards in a seasonally favourable direction when given a view of the sky including the sun but excluding geographical features, and that when subjected to the clock-shift procedure, hoverflies would predictably shift their orientation in accordance with the solar azimuth at their perceived time
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