UC Berkeley

Many unresolved questions in animal flight evolution relate to the transition between flightless and volant forms. Functional analysis of transitional modes using anatomical intermediates may help to assess the biomechanical underpinnings to such transitional processes. The group of stick insects exhibits tremendous diversity in wing sizes, which is potentially correlated with selection gradient for wing size. This dissertation work uses stick insects as a model system to address the ecological context of controlled aerial behaviors and the evolutionary consequences of flight loss.

Chapter 1 explores the behavioral context of controlled aerial descent in the newly hatched larvae of an Australian stick insect Extatosoma tiaratum, which exhibit ephemeral ant-mimicry and hyperactive dispersal activities. Through lab experiments, we documented the ontogenetic variation in various taxic behaviors and voluntary drop as an escaping response. Chapter 2 explores the visual ecology of directed aerial descent in larval E. tiaratum. We investigated how visual contrasts are used as locomotor references during directed aerial descent. Our results suggest the use of vertically constant contrast edges is a major component of directional targeting. The utility of contrast as visual cues was further shown to be dependent on both the heterogeneity of the visual environment and the quality of perceived signals. Chapter 3 addressed the biomechanics of aerial righting behaviors in larval E. tiaratum. Through high--speed filming and three--dimensional motion reconstruction, the results showed highly controlled leg movements are involved in righting maneuvers. Through posture control, the insects achieve effective righting rotation. More subtle leg movements were also shown in stroke--like patterns, which are instantaneously correlated with whole--insect rotation. This study provided useful information for understanding the ecology and evolution of controlled aerial behaviors in invertebrates. Chapter 4 addressed the functional consequences of progressive wing size reduction along an altitudinal gradient in three populations of a stick insect Asceles tanarata native to Malay Peninsula. We investigated how wing and body morphology change along the altitudinal gradient, and further studied the biomechanics of flight in each flight morph. The results indicate the reduction of fight apparatus leads to changes in wing design and wing kinematics. Reduced flapping fight performances resemble parachuting and gliding. Due to both altered kinematics and flight trajectories different from conventionally recognized flapping flight, the aerodynamics of wing flapping is characterized by large advance ratio and reduced half--stroke asymmetries. Although the reduction in flight performance is closely correlated with the reduction of wing size, wing aerodynamics shows more complex pattern along the performance gradient.