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.

Migratory hoverflies orientate north during spring migration

“…Flight simulator experiments in autumn migrant hoverflies show that they fail to orientate south when the sun is obscured indicating that other available cues are not utilised, at least in this experimental set up (11). Surprisingly, here we find that spring hoverflies also orientate to the north when the sun is obscured, leaving a question as to the cue being used for orientation under these conditions.…”

“…Insect movements spread vital ecological roles over large areas including pollination, pest control, decomposition, and nutrient transfer (1,(4)(5)(6). Various approaches have been employed to investigate their orientation during migration including vertical looking radars to monitor insects at high altitudes, flight simulator experiments to assess orientation in a controlled environment and vanishing bearings to assess butterfly orientation within a natural setting (1,(7)(8)(9)(10)(11)(12). Radar studies in migratory hoverflies have demonstrated seasonally favourable directions of movement, south in autumn and north in spring (7).…”

Genome‐wide transcriptomic changes reveal the genetic pathways involved in insect migration

“…Another trait seen in migrants is their ability to orientate in seasonally beneficial directions and migratory hoverflies have recently been shown to use an innate time‐compensated sun compass and to partially compensate for wind drift (Gao et al, 2020; Massy et al, 2021). Surprisingly, despite these differences between migratory and summer hoverflies, investigations into migrant morphology using wing morphometrics have so far revealed little change to non‐migratory stages, leaving the full extent of the underlying differences between forms an open question (Raymond et al, 2014).…”

“…Flying within the FBL lessens the impact of headwinds with migration often observed along coasts or through mountain passes where topology serves to concentrate migrant number, for example in the mountain pass of Bujaruelo in the Pyrenees (Figure 1b-d). This feature of hoverfly migration allows the collection and study of individuals actively expressing the migratory syndrome (Massy et al, 2021).…”

Migratory hoverflies orientate north during spring migration

“…Insect movements spread vital ecological roles over large areas, including pollination, pest control, decomposition and nutrient transfer [1,[4][5][6]. Various approaches have been employed to investigate their orientation during migration, including vertical-looking radars to monitor insects at high altitudes, flight simulator experiments to assess orientation in a controlled environment and vanishing bearings to assess butterfly orientation within a natural setting [1,[7][8][9][10][11][12].…”