Study shows how insects react to darkness while flying

Fruit flies and mosquitoes maintain controlled flight after a sudden drop in light. Honeybees, under similar conditions, interrupt their flight and collide in about 100 milliseconds. The result comes from a study with Drosophila melanogaster, Culex pipiens and Apis mellifera.

The research evaluated flight in continuous low light and after sudden darkening mid-flight. Flies and mosquitoes performed a defined maneuver. After a visual-motor delay of approximately 45 milliseconds, they increased their wingbeat frequency, pitch angle, reduced speed, and changed flight direction. The sequence occurred within a 250-millisecond observation window.

The bees exhibited the opposite behavior. In the sudden darkening trial, researchers recorded collisions in all 217 events evaluated. The change in trajectory occurred, on average, 98 ± 39 milliseconds after the loss of light. The individuals tilted their bodies downwards and struck the bottom or wall of the container.

Sensory factors

The study links this difference to two sensory factors. Flies and mosquitoes have greater sensitivity to light. Furthermore, dipterans possess halteres, gyroscopic organs used to measure body posture without relying solely on vision. Bees do not have these structures.

In tests under continuous darkness, flies and mosquitoes were also able to take off and fly. The frequency of takeoff decreased in relation to light. Flies took off 15 times less. Mosquitoes took off six times less. Even so, the trajectories indicated stable flight up to the filmed volume, located 10 cm from the walls of the chamber.

The response to the sudden loss of light involved rapid kinematic adjustments. Flies and mosquitoes raised their bodies by about 10 degrees and reduced their forward horizontal speed. In flies, 74% of events reached a complete stop in this velocity component, with a mean braking time of 85 ± 14 milliseconds. In mosquitoes, 49% of events reached this stop, with a mean time of 96 ± 35 milliseconds.

Vertical dynamics showed a different pattern. Mosquitoes stopped vertically in 70% of the events. Flies did so in 56% of the events. In both cases, the average vertical stop time was close to 70 milliseconds.

Partial drop in brightness

The researchers simulated a partial drop in brightness. In this case, the light decreased tenfold, from 34 to 3,4 lux. The scenario aimed to mimic a transition from a lit area to a shaded area. Flies and mosquitoes showed a response similar to that observed in complete darkness, especially at the beginning of the maneuver.

The partial response maintained the sequence of increased wing frequency, pitch elevation, braking, and change of direction. However, the final result of the orientation differed. In complete darkness, the insects finished the maneuver with an almost random direction. In partial darkness, there was a preference for the initial flight direction.

In flies, leg extension was a significant part of the response. In 74 out of 76 responses to step-darkness, the insects extended their legs. The average time was 52 ± 15 milliseconds. The study suggests a relationship with increased moment of inertia and aerodynamic drag, as well as a possible pre-landing or pre-impact function. Mosquitoes kept their legs extended under all conditions.

Removing awns, structures present on the antennae, did not prevent flight in the dark. In flies, ablation reduced response rates to darkness. In mosquitoes, there was no significant change in the evaluated criteria. The result indicates a possible difference in the role of antennae in flight control mechanisms between the two species.

More information at doi.org/10.1242/jeb.251675