The motor apparatus of head movements in the Oleander hawkmoth (Daphnis nerii, Lepidoptera)

Head movements of insects play a vital role in diverse locomotory behaviors including flying and walking. Because insect eyes move minimally within their sockets, their head movements are essential to reduce visual blur and maintain a stable gaze. As in most vertebrates, gaze stabilization behavior in insects requires integration of both visual and mechanosensory feedback by the neck motor neurons. Whereas visual feedback is derived from the optic flow over the retina of their compound eyes, mechanosensory feedback is derived from their organs of balance, similar to the vestibular system in vertebrates. In Diptera, vestibular feedback is derived from the halteres - modified hindwings that evolved into mechanosensory organs - and is integrated with visual feedback to actuate compensatory head movements. However, non-Dipteran insects including Lepidoptera lack halteres. In these insects, vestibular feedback is obtained from the antennal Johnstons organs but it is not well-understood how it integrates with visual feedback during head movements. Indeed, although head movements are well-studied in flies, the underlying motor apparatus in non-Dipteran taxa has received relatively less attention. As a first step towards understanding compensatory head movements in the Oleander hawkmoth Daphnis nerii, we image the anatomy and architecture of their neck joint sclerites and muscles in using X-ray microtomography, and the associated motor neurons using fluorescent dye fills and confocal microscopy. Based on these morphological data, we propose testable hypotheses about the putative function of specific neck muscles during head movements, which can shed light on their role in neck movements and gaze stabilization.