Active Vision Strategies in Predation

Abstract

Visual predation requires precise and accurate behaviour, for which many predators have evolved excellent visual skills. However, an animal's visual abilities are greatly affected by how it moves its eyes, known as active vision. Insects have immobile eyes but can direct their gaze by moving their heads and bodies. This thesis examines three predatory insects with different predatory strategies, to understand the extent to which active vision can be used in predation. The first experimental chapter considers the African praying mantid, Sphodromantis lineola. Praying mantids are stationary terrestrial predators, which use their extremely mobile necks to visually track prey until it is within reach. By using statistical models, we identified what factors elicited strikes and, importantly, their success rate. The timing of head movements greatly increased the chances of strike success, with earlier movements increasing the success rate. The second experimental chapter addresses how darting robber flies, Psilonyx annulatus, aerially attack static prey. Prior to attacking, darting robber flies translate their body around a central point, assessing their prey. After assessment, they attack from a position correlated with the target's absolute size, not its angular size. Prey is beyond the robber fly's stereopsis range during the period of assessment. Assessments of differently sized targets have similarities with the behaviour exhibited by jumping insects, which use motion parallax, a form of active vision, to assess jump distance, suggesting darting robber flies also use motion parallax to predate. The final experimental chapter considers killer flies, Coenosia attenuata, which chase moving targets aerially. Killer flies use a combination of gravity and wing acceleration to increase their speed when chasing prey from above. This increased speed restricts the flies' ability to steer. However, killer flies create strong looming stimuli which may trigger their prey to produce evasive manoeuvres, thereby slowing down. Moreover, by travelling faster towards their prey, killer flies may avoid losing track of it, a real danger when chasing moving prey with low- resolution eyes potentially avoided thanks to active vision. By employing active vision, each of the predators considered can achieve impressive performances, despite relying on very different strategies to capture prey. The use of active vision can increase the success of already excellent visual predators and improve the performance of predator with limited vision. However, active vision can also substantially alter predatory behaviour, leading to a trade- off between the advantages in visual perception active vision can bring and the disadvantage in behavioural efficiency of active vision strategies.