Caterpillars detect flying wasps by hairs sensitive to airborne vibration

1. Intact caterpillars (Barathra brassicae) react to the airborne vibrations of an approaching wasp (Dolichovespula media) by cessation of movement or by squirming. These reactions are no longer observed after removal of the eight filiform hairs on the caterpillars' thorax. When directly attacked, both intact and operated caterpillars squirm and often regurgitate gut-contents. 2. The selective advantage of detecting a wasp's approach with the vibration sensitive hairs was quantitatively determined. A cage contained a wasp colony and intact or operated caterpillars as the only prey: it was found that intact caterpillars which react to the wasp's approach are attacked about 30% less frequently than operated animals. Of all caterpillars that are attacked, about 20% are killed. Of all caterpillars killed 30% were intact, while 70% lacked their filiform hairs (P < 0.001). 3. Air vibration generated in the near field of a tethered wasp in flight was measured at different distances and directions. The dominant frequency in the spectral energy distribution of flight-generated vibration was about 150 Hz. Maximal medium displacements were below and behind the wasp (Figs. 2 and 3). This vibration amplitude was above threshold for the filiform hairs (at 150 Hz) up to a distance of 70 cm from the wasp. 4. The influence of the caterpillars' response on the hunting strategies of wasps predating or parasitizing on them is discussed.