Evolution in 3D

Abstract

This thesis explores the mechanisms underlying motion vision in the praying mantis (Sphodromantis lineola) and how this visual predator perceives camouflaged prey. By recording the mantis optomotor response to wide-field motion I was able to define the mantis Dmax, the point where a pattern is displaced by such a distance that coherent motion is no longer perceived. This allowed me to investigate the spatial characteristics of the insect wide field motion processing pathway. The insect Dmax was found to be very similar to that observed in humans which suggests similar underlying motion processing mechanisms; whereby low spatial frequency local motion is being pooled over a larger visual area compared to higher spatial frequency motion. By recording the mantis tracking response to computer generated targets, I was able to investigate whether there are any benefits of background matching when prey are moving and whether pattern influences the predatory response of the mantis towards prey. I found that only prey with large pattern elements benefit from background matching during movement; and above all prey which remain un-patterned but match the mean luminance of the background receive the greatest survival advantage. Additionally, I examined the effects of background motion on the tracking response of the mantis towards moving prey. By using a computer generated target as prey, I investigated the benefits associated with matching background motion as a protective strategy to reduce the risk of detection by predators. I found the mantis was able to successfully track a moving target in the presence of background My results suggests that although there are no overall benefits for prey to match background motion, it is costly to move out of phase with the background motion. Finally, I examined the contrast sensitivity of the mantis wide-field and small target motion detection pathways. Using the mantis tracking response to small targets and the optomotor response to wide-field motion; I measured the distinct temporal and spatial signatures of each pathway. I found the mantis wide-field and small target movement detecting pathways are each tuned to a different set of spatial and temporal frequencies. The wide-field motion detecting pathway has a high sensitivity to a broad range of spatio-temporal frequencies making it sensitive to a broad range of velocities; whereas the small-target motion-detection pathway has a high sensitivity to a narrow set of spatio-temporal combinations with optimal sensitivity to targets with a low spatial frequencymotion.