Collision avoidance behaviour in a pair of flying locusts (Locusta migratoria L.)

Migratory locusts; Locusta migratoria L. flying in a swarm would encounter spatiotemporally complex visual cues such as translating, receding and looming stimuli, produced by self-motion as well as object motion in the environment. A rapidly approaching conspecific or a predator represents a looming object approaching on a collision course and is involved in triggering urgent collision avoidance behaviours. To avoid predators and collision with conspecifics, and to navigate through complex environments, locusts must produce appropriate collision avoidance manoeuvres. Flying locusts have evolved the ability to not only avoid predation but also effectively navigate within the swarm without constantly colliding with one another. Collision avoidance and predator evasion in response to looming stimuli are important in many animals and in locusts, the key elements in the neuronal pathway underlying this behaviour are the lobula giant movement detector (LGMD) and its postsynaptic component, the descending contralateral movement detector (DCMD). Previous studies have suggested that the LGMD/DCMD pathway allows each locust within a dense swarm to remain sensitive to approaches of individual objects including conspecifics and flying predators, approaching frequently from many directions or along the same trajectory and to produce appropriate collision avoidance behaviours. Collision avoidance responses of a rigidly tethered locust presented with a looming object have been studied previously. However, behavioural strategies for collision avoidance within a group of conspecifics are yet unknown. Avoidance behaviour exhibited by a single locust may or may not differ from that of an individual in a group. Further, salient cues produced by objects on a collision course (looming) can be influenced by each animal’s position relative to the object and/or its position within a group. In my first objective of this thesis, I exposed locusts (L. migratoria L.) to a computer generated looming object in the presence of a live and dead conspecific separately. This first experiment was done to determine if collision avoidance behaviour of a locust: Locust 1 (L1) or Locust 2 (L2), is affected by the presence of a conspecific. As my second objective, the responses of a pair of flying locusts placed in differing relative positions in a wind tunnel were studied during presentation of the same looming object. This second experiment was done to determine if collision avoidance behaviour of a locust is affected by the relative position of a conspecific. From the results, I looked at different spatio-temporal characteristics of L1 and L2 collision avoidance behaviour and their dependency on the presence as well as on different relative positions of a conspecific in the vicinity. Results from Experiment 1 showed that the types of collision avoidance responses, some components of six degrees of freedom of L1 and L2 and also the timing of the onset and duration of the initial avoidance response of L2, were affected by the presence of a conspecific. According to Experiment 2, the avoidance responses and three translational degrees of freedom of L1 and L2 were also affected by the relative position of the conspecific and its own position, respectively. Also, I found that the timing of the onset and the duration of the initial avoidance response of L2 were affected by its own position in the wind tunnel. Both locusts’ responses to the looming stimuli were more robust in the presence of a live conspecific and less pronounced in the presence of a dead locust. Thus, results further suggest that locusts use visual cues from the looming objects as well as an immediate conspecific to generate appropriate avoidance responses. Taken together, the results of my study indicate that a locust’s collision avoidance behaviour can be affected by the presence as well as the relative position of a conspecific in the vicinity