Hoverflies use a time-compensated sun compass to orientate during autumn migration

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.

Keywords: flight simulator; hoverfly; insect migration; navigation; orientation; time-compensated sun compass.

Figure 1.

Hoverfly flight simulator. (a) Active migrants were collected from the Puerto de Bujaruelo (black arrow), a 2273 m pass on the French–Spanish border in the Pyrenees. (b) Flight simulator schematics and visual field presented to the hoverfly (light grey: full visual angle 116°). (c) A tethered Scaeva pyrastri. (d) A still taken from a flight simulator video and representative data extraction steps: after loading the stack of photos corresponding to an experiment, contrast is increased for all photos by decreasing maximum levels until background detail disappears; (i) the hoverfly body is outlined on one frame by manually drawing an ellipse over the abdomen and a circle over the thorax, then unifying the shapes; (ii) the resultant angle between thorax and abdomen is checked by eye; (iii) this outline creates a kernel which is rotated for each image to find the angle of best fit. (e) Circular histogram displaying the data from a 5 min experiment of a single hoverfly. Each blue dot represents three recorded angles, the red arrow indicates overall mean direction while the length of the arrow indicates r, the measure of flight directedness, ranging from 0 (random distribution) to a maximum of 1 at the edge of the circle. (f) Virtual flight path for the hoverfly observed in (e). Axes show distance travelled in kilometres under a constant 5 ms⁻¹ flight speed. Flight speed is estimated from mark-recapture experiments on migratory hoverflies carried out by Aubert et al. between the Col de Bretolet on the Switzerland–France border and the Col de la Golèse in France [21]. Hoverflies frequently completed the 3 km distance in 10–15 min indicating a minimum flight speed of 3–5 ms⁻¹. (Online version in colour.)

Figure 2.

Hoverfly orientation in a flight simulator under various conditions and diurnal activity patterns. (a) Hoverfly orientation in a flight simulator under clear-sky conditions. Blue lines indicate the vectors of individual hoverflies with colour indicating the species and length corresponding to the r value (r = 1 at the outer radius). Red arrows indicate the group weighted mean direction with length depicting R* relative to 2.5 at the outer radius. Dashed circles represent the significance intervals of 95% (inner) and 99% (outer). Red bars outside of the circle indicate 95% weighted confidence intervals. (b) Diurnal activity pattern of hoverfly migrants as a rate of southbound movement over the mountain pass and mean temperature. (c,d) the results of (a) split by time of day: (c) hoverflies flown before 13.53 (first 15) and (d) hoverflies flown after 13.57 (last 15). (e) Under restricted views conditions with the sun obscured but light intensity, chromatic and polarization patterns visible. (f) Hoverflies that had undergone a 6 h advanced clock shift. Numerical results are presented in table 1. (Online version in colour.)