Abdominal ruddering and the control of flight in the hawkmoth

Hawkmoth flight follows from a complex interplay between sensory input and motor output. In these, and many other flying insects, their wings, abdomen and legs, all play critical roles in regulating the flight trajectory. To explore how each of these contributes to flight control we used a programmable system on a chip (PSOC) that can both record and stimulate motor pathways. In this study, we are particularly interested in the extent to which abdominal ruddering controls flight trajectory. To explore this issue, we displayed vertically moving visual stimuli to tethered moths and recorded the activation pattern of abdominal flexors. We found that up/down abdominal flexion is driven by a pattern of motor output whose frequency is positively correlated with the speed of the vertical stimulus. There is a strong coupling, therefore, between the abdominal rudder motion and the visual input. We further found that we could stimulate such ruddering motions with a pattern of digital 3 volt pulses that draw very little current (< 100 uA). This stimulus, applied to the ventral nerve chord just caudal to the abdominal ganglia controlling flexion, could be programmed to get a variety of abdominal motions. We also show that we can use the PSOC in this manner to alter the forces (and path) of an insect in tethered and nearly free flight.