If you ever lit a candle in the wilderness during nighttime, you would likely find moths (and other nocturnal insects) irresistibly drawn to the candle flame. This phenomenon is so common that there’s a phrase for it, “like a moth to a flame.” For centuries, scientists wondered what attracts insects to flames [1]. Some believe insects mistake flame light for an indicator of gaps between leaves, or a celestial compass like the moon. Others propose that the heat of the light draws them in, or that the intense light blinds them, causing erratic flight patterns. To answer this age-old question, researchers at Imperial College London recorded and analyzed high-resolution flight trajectories of insects under artificial lights. Surprisingly, their findings point to a new explanation: the dorsal light response (DLR) [1–3], a highly conserved behavior that keeps the dorsal side, or the back, of insects facing the brightest visual region. Flying insects need to stay upright to maintain correct flight orientation. Throughout the long history of insect flight evolution, the sky has consistently been the brightest region in the visual field, making it a reliable cue for determining which way is up. Thus, by leveraging DLR, flying insects can adjust their flight orientation with reference to the sky. This capability is especially critical for small insects flying with a relatively low inertia because the gravitational force they experience can often be masked by other forces exerted by wind or turbulence, so unlike humans and plants, gravity cannot be used as a reliable cue of direction. Let’s take a closer look at how the researchers arrived at this unexpected conclusion of DLR. The first clues for DLR emerged from initial field experiments in Costa Rica, where the team used infrared high-speed cameras (Footnote 1–2) to record insect flight under various lighting configurations. Upon analyzing the recordings, researchers observed three unusual behaviors that are common in conditions with light but not in complete darkness (Figure 1). When the insects are at the same horizontal level as the light source, they orbit around it. If the insects are above “Like a Moth to a Flame” 「飛蛾 A New Explanation of Figure 1 Insect flight trajectories demonstrating the three unusual behaviors observed during field experiments: orbiting (left), inverting (middle), and stalling (right). 圖一 研究人員在野外實驗觀察到的三種不尋常昆蟲飛行軌跡：繞圈（左）、上下顛倒（中）和失速（右）。
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