Stabilization of altitude and speed in tethered flying gypsy moth males: influence of (+) and (‐)‐disparlure

ABSTRACT. Changes in lift and thrust were elicited in tethered male gypsy moths, Lymantria dispar L. (Lepidoptera, Lymantriidae), by visual pattern elements moving radially either towards or from the point directly beneath their body, if the sex‐pheromone, (+)‐disparlure, was present. The sign of these changes was such as to counteract the pattern movements, which were generated by a rotating spiral beneath the moth. By restricting the area of spiral visible to the moth to either transverse or longitudinal sectors, flight altitude was affected by the centrifugal/centripetal movements in the lateral sectors, whereas flight speed was affected by those in the frontal sector. It is deduced that in free flight these compensatory reactions are responsible for the stabilization of flight altitude and speed, respectively. Surprisingly, without pheromone present these responses were usually not detectable: a wide range of flight altitude and speed was then observed. In the presence of (+)‐disparlure, however, these responses were always strongly pronounced, the animal keeping within a narrow range of speed and altitude. These compensatory reactions were blocked by the attraction‐inhibiting (‐)‐disparlure if presented in racemic mixture with the (+) form: the range of speed and altitude shown by the moth was then the same as without any pheromone. Under closed‐loop conditions, the mean flight speed was reduced by the racemic mixture as well as by (+)‐disparlure alone, however.