See the light: Electrophysiological characterization of the Bolwig organ's light response of Calliphora vicina 3rd instar larvae

The anatomy and development of the larval cyclorraphous Diptera visual system is well established. It consists of the internal Bolwig organ (BO), and the associated nerve connecting it to the brain. The BO contributes to various larval behaviors but was never electrophysiologically characterized. We recorded extracellulary from the Bolwig nerve of 3rd instar Calliphora vicina larvae to quantify the sensory response caused by BO stimulation with light stimuli of different wavelengths, intensities and directions. Consistent with previous behavioral experiments we found the BO most sensitive to white and green, followed by blue, yellow, violet and red light. The BO showed a phasic–tonic response curve. Increasing light intensity produced a sigmoid response curve with an approximate threshold of 0.0105 nW/cm2 and a dynamic range from 0.105 nW/cm2 to 52.5 nW/cm2. No differences exist between feeding and wandering larvae which display opposed phototaxis. This excludes reduced BO sensitivity from causing the switch in behavior. Correlating to the morphology of the BO frontal light evoked the maximal reaction, while lateral light reduced the neural response asymmetrically: Light applied ipsilaterally to the recorded BO always produced a stronger response than when applied from the contralateral side. This implies that phototacic behavior is based on a tropotactic mechanism.

Research highlights▶ The larval Calliphora visual system is feasible for electrophysiology. ▶ Spectral sensitivity is reflected in phasic–tonic response curves. ▶ The sensitivity of the visual system is identical in feeding and wandering larvae. ▶ The system is direction sensitive.