Plasticity in the Visual System Is Correlated With a Change in Lifestyle of Solitarious and Gregarious Locusts

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Abstract
We demonstrate pronounced differences in the visual system of a polyphenic locust species that can change reversibly between two forms (phases), which vary in morphology and behavior. At low population densities, individuals of Schistocerca gregaria develop into the solitarious phase, are cryptic, and tend to avoid other locusts. At high densities, individuals develop instead into the swarm-forming gregarious phase. We analyzed in both phases the responses of an identified visual interneuron, the descending contralateral movement detector (DCMD), which responds to approaching objects. We demonstrate that habituation of DCMD is fivefold stronger in solitarious locusts. In both phases, the mean time of peak firing relative to the time to collision nevertheless occurs with a similar characteristic delay after an approaching object reaches a particular angular extent on the retina. Variation in the time of peak firing is greater in solitarious locusts, which have lower firing rates. Threshold angle and delay are therefore conserved despite changes in habituation or behavioral phase state. The different rates of habituation should contribute to different predator escape strategies or flight control for locusts living either in a swarm or as isolated individuals. For example, increased variability in the habituated responses of solitarious locusts should render their escape behaviors less predictable. Relative resistance to habituation in gregarious locusts should permit the continued responsiveness required to avoid colliding with other locusts in a swarm. These results will permit us to analyze neuronal plasticity in a model system with a well-defined and controllable behavioral context.