Nocturnal adaptations of moths

Moths make up the third most diverse insect group in New Zealand, and their day/night habits are also diverse. While most moths are nocturnal (active at night), others are diurnal (active during the day) or crepuscular (active at twilight). Each of these lifestyles has its own advantages. Being active at night means less competition for food and being less likely to be detected and eaten, whereas daylight means there is more light to see and more energy from the Sun. Crepuscular animals can get the best of both worlds.

Nocturnal navigation

Nocturnal moths first evolved long before humans invented bright lights. They learned to navigate by using distant celestial objects such as the Moon and stars. Moths navigate by positioning themselves and flying on a fixed angle relative to these celestial light sources. If the position of the Moon or stars is not obvious, moths instead use geomagnetic signals – the Earth’s magnetic field.

Why are nocturnal moths attracted to the light?

It is not yet fully understood why moths appear attracted to bright lights, but there are several hypotheses. It could be that certain lights (such as candles) emit specific wavelengths that attract moths – perhaps like a female’s pheromones, which attract male moths. Another possibility is that artificial ultraviolet light is associated with a food source. Many night-blooming flowers reflect ultraviolet light. However, this isn’t necessarily how moths seek out these flowers. These flowers release high levels of carbon dioxide (CO₂) at night. Moths can detect these CO₂ emissions and use them to find the flowers.

The leading idea is that moths use starlight to accurately fly long distances. By maintaining a constant angle towards an object like the Moon or the stars, moths can fly in a straight line. Because the Moon or stars are so far away, the change in the light’s angle is negligible, but when a moth flies toward a close-up artificial light, the angle changes dramatically. Thinking it is a star or the Moon, the moth will keep changing its flight angle to keep going in a straight line (keeping a constant frame of reference). The changes mean the moth continually turns towards the light, causing it to spiral and circle around the light source instead. (Heath moth traps take advantage of this downward spiralling behaviour.)