Nocturnal insects use optic flow for flight control

Abstract

To avoid collisions when navigating through cluttered environments, flying insects must control their flight so that their sensory systems have time to detect obstacles and avoid them. To do this, day-active insects rely primarily on the pattern of apparent motion generated on the retina during flight (optic flow). However, many flying insects are active at night, when obtaining reliable visual information for flight control presents much more of a challenge. To assess whether nocturnal flying insects also rely on optic flow cues to control flight in dim light, we recorded flights of the nocturnal neotropical sweat bee, Megalopta genalis, flying along an experimental tunnel when: (i) the visual texture on each wall generated strong horizontal (front-to-back) optic flow cues, (ii) the texture on only one wall generated these cues, and (iii) horizontal optic flow cues were removed from both walls. We find that Megalopta increase their groundspeed when horizontal motion cues in the tunnel are reduced (conditions (ii) and (iii)). However, differences in the amount of horizontal optic flow on each wall of the tunnel (condition (ii)) do not affect the centred position of the bee within the flight tunnel. To better understand the behavioural response of Megalopta, we repeated the experiments on day-active bumble-bees (Bombus terrestris). Overall, our findings demonstrate that despite the limitations imposed by dim light, Megalopta-like their day-active relatives-rely heavily on vision to control flight, but that they use visual cues in a different manner from diurnal insects.

Figure 1.

The effect of changes in horizontal optic flow cues in the tunnel on the groundspeed of M. genalis (black boxes) and B. terrestris (grey boxes). Box limits represent the 25th and 75th percentiles of the data, dotted lines indicate the median, whiskers extend to the rest of the data, crosses indicate outliers. Both species increase their groundspeed when horizontal motion cues are minimized, but unlike Megalopta, bumble-bees do not fly faster when horizontal motion cues are removed from one wall. Significance codes: **p < 0.01; ***p < 0.001; n.s., not significant.

Figure 2.

The effect of changes in horizontal optic flow cues on centring in (a) M. genalis and in (b) B. terrestris. Thick black lines indicate the tunnel walls; light grey lines indicate the midline of the tunnel and the pattern (check or stripe) indicates the position of the patterns. Other details as in figure 1. In the check/stripe condition, bumble-bees fly closer to the stripe pattern; we see no such effect in Megalopta.