How wings first appeared and then were lost in insects is not well understood. The proposed research will study transitions in insect wing structure within a functional context. The focus of the research is stick insects, which exhibit tremendous diversity of wing morphology and flight capacity. Initial studies have found flight performance to be correlated with relative wing size. This study samples wing morphology and flight biomechanics of stick insects from the field, and uses an integrative approach to compare wing morphology and flight biomechanics among different species and to analyze the history of these traits in an integrative, multi-dimensional manner. Anticipated results include demonstrations of how stepwise transitions of wing morphology correlate with changes in the flight performance, and the role of intermediates between flying and non-flying forms. Understanding of the transition between flying and flightless forms will be relevant to other fields, for example, to the design of micro air vehicles. In schools and museums, scientific results from this study can be used to help people understand the process of how insects gained or lost wings. Furthermore, this study enhances scientific communication between the U.S. and Malaysia, and continues to provide research opportunities to undergraduate students in both countries.

Project Report

The evolutionary origins of insect flight are unclear. In particular, we lack knowledge about the process of transition between wingless and winged insects. In order to solve this problem, one may address the utility of intermediate-sized wings, and then prove that such an intermediate state links the evolutionary transition between winged and wingless forms. Different stick insects are ideal for this study because a great variation of wing size is preserved in this group. With this award, we focused on the mechanics of flight using different sized wings, and used the findings to understand stick insect flight along ecological and evolutionary dimensions. First, we found the mechanics and utility of flight change with relative wing size (i.e. the proportion of wing size to body size). We used high-speed filming to study detailed movement of wings and other body parts during flight. By comparing the flapping movement of different sized wings, we found that the the wing stroke differs among species of different sized wings. For example, relatively longer wings tend to move with a greater degree of freedom, but the wingbeat frequency is determined by relative wing size and various other factors, such as body size. Also, the means by which other body parts, e.g., the abdomen and legs, coordinate with wing stroke movements in flight differs among species of different sized wings. By showing great differences in basic kinematics of different sized wings, these results indicate that the fundamental mode of wing movement can be influenced by relative wing size, and further suggests a complex interaction of morphological and mechanical features may underpin insect flight origins. Only species with larger wings are capable of ascending in flight, and as shorter wings have reduced capacity to support the insects’ body weight, such taxa descend during flight. This behavior is similar to gliding with rudimentary wing flapping, and may be particularly useful in canopy environments as characterized the initial origins of insect flight. We also tested whether the aforementioned pattern was affected by the evolutionary history of sampled lineages, and found it to be consistently independent of any relatedness among species. For a phasmid species living in tropical Southeast Asia, we also found the phenotypic transition from fully-developed to miniaturized wings is correlated with increasing altitude, and that flight biomechanics also changed with relative wing size in a pattern similar to that described above. Overall, we demonstrated significant changes in flight performance and aerodynamic function associated with different wing sizes, and examined this pattern of variation taking both evolutionary history and ecological factors into account. Our results identified key variables important to the evolutionary transition between winged and wingless morphologies in stick insects, and more generally suggested that such changes can be relevant in arboreal enviroments characteristic of initial flight evolution.

National Science Foundation (NSF)
Division of Environmental Biology (DEB)
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Elizabeth Friar
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University of California Berkeley
United States
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