People have long wondered why flying insects can be seen spiraling around light sources at night. Among other suggestions was that the critters were used to navigating by the Moon, and got confused, or that they were seeking heat.

An ingenious new study shows that the navigation idea is not completely wrong, but the insects instead use sky glow, even at night, as a major clue for how to orient themselves: by keeping their dorsal (back) to a point or diffuse light source, for millions of years, then they would keep their legs pointed down and they would fly the way they want.

However, these researchers found that if they placed a light bulb in roughly the center of an otherwise darkened, enclosed space inside a tent with flying insects, then most (but not all) species of nocturnal insects flying above or to the side of the light tended to orient their bodies so that their dorsal side was towards the light— so that they were flying sideways or upside down! Thus disoriented, they would flitter around, confused as to which way was up.

This also explains why it is so easy to catch nocturnal flying insects by shining a bright light onto a sheet or blanket laid on the ground: convinced by hundreds of millions of years that “light = up”, a large fraction of the critters fly **upside down** towards the lighted surface and careen onto it, out of control.

Caution: This study has not yet been replicated or peer-reviewed, but if it holds up, then it unveils a very simple and inexpensive fix for both ever-worsening light pollution and the collapse of our global insect populations: simply put shielding around ALL exterior light fixtures at night, so that NO light is emitted either upwards or sideways. (This is known as a Full cut-off (FCO) lighting.)

Larger animals like birds, reptiles, and mammals can simply use gravity to tell them which way is up. Insects, by contrast, are apparently so small that the air itself acts like a viscous medium, and tends to overpower the cues from gravity, much like scuba divers can get confused as to which way is up — unless they follow cues like air bubbles and where the diffused light from the surface comes from.

“The largest flying insects, such as dragonflies and butterflies, can leverage passive stability to help stay upright ^{30, 31}. However, the small size of most insects means they travel with a lower ratio of inertial to viscous forces (Reynolds number) compared with larger fliers³². Consequently, smaller insects, such as flies, cannot glide or use passive stability, yet must still rapidly correct for undesired rotations³³. Multiple visual and mechanosensory mechanisms contribute to the measurement and correction of undesired rotations, but most measure rotational rate rather than absolute attitude ^{26, 28, 32, 34}. In environments without artificial light, the brightest portion of the visual environment offers a reliable cue to an insect’s current attitude.”

“Inversion of the insect’s attitude (either through roll or pitch) occurred when the insect flew directly over a light source (Fig. 1 c & Supp. Video 3), resulting in a steep dive to the ground. Once below the light, insects frequently righted themselves, only to climb above the light and invert once more. During these flights, the insects consistently directed their dorsal axis towards the light source, even if this prevented sustained flight and led to a crash.”

The researchers report that certain types of insects did **not** appear to get confused by lights at night: Oleander Hawkmoths (Daphnis nerii) and fruit flies (drosophila).

Here is the link to the preprint:

https://www.biorxiv.org/content/10.1101/2023.04.11.536486v1.full