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1 Biology, University of Newcastle, Newcastle upon Tyne, United Kingdom
2 School of Biological Sciences, University of Nebraska at Lincoln, Lincoln, Nebraska, United States
3 Biology, Newcastle University, Newcastle upon Tyne, United Kingdom
* To whom correspondence should be addressed. E-mail: claire.rind{at}ncl.ac.uk.
Locusts have two large collision-detecting neurons, the descending contralateral movement detectors (DCMDs) that signal object approach and trigger evasive glides during flight. We wanted to investigate if vision for action, when the locust is in an aroused state rather than a passive viewer, significantly alters visual processing in this collision detecting pathway. To do this we used two different approaches to determine how the arousal state of a locust affects the prolonged periods of high frequency spikes typical of the DCMD response to approaching objects that trigger evasive glides. First, we manipulated arousal state in the locust by applying a brief mechanical stimulation to the hind leg; this type of change of state occurs when gregarious locusts accumulate in high density swarms. Second, we examined DCMD responses during flight, because flight produces a heightened physiological state of arousal in locusts. When arousal was induced by either method we found that the DCMD response recovered from a previously habituated state, that it followed object motion throughout approach, and most importantly that it was significantly more likely to generate the maintained spike frequencies capable of evoking gliding dives even with extremely short intervals (1.8 s) between approaches. In tethered flying locusts 41% responded with a glide to images of approaching objects, separated by 1.8s. When we injected the neuronal octopamine receptor antagonist, epinastine, into the haemolymph responsiveness declined to 12% suggesting that octopamine plays a significant role in maintaining responsiveness of the DCMD and the locust to visual stimuli during flight.
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