Olfactory performance explains duality of antennal architectural designs in Lepidoptera

Male attraction by females through sex pheromones is widespread among Lepidoptera, and antennae are key olfactory organs during male orientation. Broadly speaking, two designs of antennae coexist in Lepidoptera: complex (pectinate) or stick-like (filiform) ones. Pectinate antennae have attracted attention because of their multiscale geometry, assumed to outperform filiform. Yet, the filiform design is by far more common. We compare the olfactory performance of the two designs using modelling, particle image velocimetry on three-dimensional-printed scaled-up models and computational simulations. In terms of absolute odour capture, pectinate antennae perform better at nearly all flying speeds. However, when considering drag, filiform designs are more energy efficient than pectinate ones at low-flight speeds, while the reverse holds at high speeds. This is owing to the differential scaling of drag and molecule capture with flight speed. According to our results, small and slow moths would bear filiform antennae whereas big and fast moths would have pectinate ones, which is the general trend observed in nature. We discuss exceptions to this general pattern and how species could evolve from one design to the other by investigating the influence of the antennal structural elements.