Tracking energy expenditure in insect flight: from the contractile proteins to the animal's wake

Insects are amongst the most diverse, successful and economically important orders on earth and flight is key to their success. Flight is one of the most energetically expensive modes of locomotion and there are few aspects of an insect's ecology, behaviour and physiology that are not affected by its energetic demands. During all modes of locomotion, muscles convert chemical energy (ultimately derived from food) into mechanical work that is ultimately transferred to the environment to produce movement. Ideally, to achieve a full understanding of the system, we need to be able to trace the transfer of energy between all levels of organisation from the contractile proteins to the momentum transferred to the animal's wake and relate this to the animal's locomotor performance, morphology and ecology. This has not yet been achieved for any mode of locomotion. However, by combining research expertise in muscle physiology and locomotor energetics at Leeds and fluid dynamics at Oxford it is achievable in insect flight. The overall aim of this proposed research is to use an integrative, multidisciplinary approach to determine, in insect flight, the transfer of energy from biochemical potential energy, through the muscles, to the surrounding air. This will be achieved by tracking the transduction of energy by quantifying the following. First, we will determine the whole organism metabolic rate by measuring the rates of oxygen consumption and carbon dioxide production during tethered flight in a wind tunnel. Second, we will measure the muscle's metabolic rate by measuring the total enthalpy during contraction - this is the sum of the mechanical work generated by the flight muscles and the heat that is liberated due to the inefficiencies of the contraction. The mechanical work generated by the muscles will be determined by simulating the muscle length change and activity pattern during flight. At the same time, we will use a thermopile to measure the heat liberated both during and after the contraction and determine the efficiency of the crossbridges, the efficiency with which the mitochondria re-synthesise ATP by oxidative phosphorylation and the inefficiencies arising due to the costs of muscle activation. Finally we will determine the efficiency of the wings in transferring the work generated by the flight muscles into useful energy in the air. This will be done using a technique called Particle Image Velocimetry (PIV) that allows the velocities of air flowing around the wings and in the wake to be quantified. By selecting insects with either synchronous or asynchronous flight muscles, closely related species with different ecologies, unrelated species demonstrating convergent ecological and morphological evolution and geometrically similar species across a range of body sizes, we will identify the main cause or causes of differences in locomotor efficiency across a range of sizes, guilds and taxonomic groups. We will be able to explain differences in overall efficiency of locomotion in terms of the underlying processes: the efficiency of the crossbridges, the efficiency of the mitochondria in re-synthesising ATP, the aerodynamic efficiency of the wings and differences in the ability to store energy in muscle elasticity. Together, our results will provide an unprecedented understanding of energy expenditure in this diverse and ecologically important group.

The factors that determine the overall efficiency with which chemical energy is converted into mechanical work that is ultimately transferred to the environment to produce movement, have not yet been quantified for any mode of locomotion. We plan to use an integrative approach to track the transfer of energy from the contractile proteins to the surrounding air and to quantify energy losses at each stage of this process. Specifically, we will quantify: (1) the efficiency with which high-energy phosphates are generated by oxidative phosphorylation in the mitochondria; (2) the efficiency with which the contractile proteins generate mechanical work by ATP hydrolysis in the cross-bridge cycle and (3) the efficiency with which mechanical work from the flight muscles is transferred into useful aerodynamic work. We will test the hypothesis that differences in muscle operating frequency, type of flight muscle, elastic energy storage in the muscles, and wing morphology will affect the efficiency of steps within the energy transduction chain and hence will affect the overall efficiency of locomotion. By selecting insects with either synchronous or asynchronous flight muscles, closely related species with different ecological niches, unrelated species demonstrating convergent evolution and geometrically similar species across a range of body sizes, we will identify the main cause or causes of differences in locomotor efficiency across a range of sizes, guilds and taxonomic groups. Together, our results will provide an unprecedented understanding of energy expenditure in this diverse and ecologically important group.

Obtaining an integrative understanding of locomotor energetics and its interaction with the environment is of broad scientific relevance and will have impact on the aeronautic industry, the general public and on the researchers employed on the grant, in addition to the benefits to the academic community (see Academic Beneficiaries) APPLIED LINKS WITH THE POTENTIAL TO IMPROVE HEALTH, DEVELOP THE 3Rs, BENEFIT INDUSTRY AND CONTRIBUTE TO CONSERVATION The knowledge gained in this project will help in the development and refinement of computational models of muscle contraction. Our work is focused on healthy muscle tissue, but understanding how normal tissue works is central to developing an understanding of malfunctions that occur during ageing and disease. The fundamental contractile mechanics of insect flight muscle, especially its response to stretch, bears remarkably similarity to the contraction of cardiac muscle. Our findings will be of relevance to biomedical researchers seeking to understand the operation and energetics of cardiac muscle contraction. Developing accurate computational models of muscle contraction will allow some animal experiments to be replaced and in other cases reduced as model simulations may allow research efforts involving animal research to be better designed. Our data linking wing morphology and aerodynamic performance of flapping wings will be important to engineers developing autonomous micro air vehicles (MAVs) for exploration, surveillance and rescue work in situations where manned flights could be unsafe or expensive. NATO Research and Technology Organization engineers are adopting a bio-inspired approach to the design of MAVs. Knowledge of how the combination of wing shape and motion relate to flight performance and efficiency will guide design optimization. The UK has been at the forefront of advances in our understanding insect flight aerodynamics since the pioneering work of Weis-Fogh and Ellington (Cambridge) and more recent work by the Oxford Animal Flight Group. Our research will help to maintain and promote the UK as a leader in insect flight research and make the UK an attractive prospect for MAV development funding. There has recently been a notable increase of interest in the changes in the distribution of organisms in response to climate change and the use of insects as indicators of biodiversity. Some insects have considerably modified their ranges while others have not. Flight energetics is an important factor that could influence the dispersal of adult insects but has not been considered to date. An understanding how flight energetics differ between different species could become a powerful tool alongside ecological and developmental factors in explaining current changes in distribution and predicting which species are likely to be adversely or favourably affected by further changes in climate and the implications that this might have for conservation and the spread of insect-borne diseases. IMPACT ON THE GENERAL PUBLIC Animal locomotion is a topic that consistently arouses public interest. We are committed to using our research to inspire young audiences to take an interest in science. Our work will have a positive impact by informing the general public about technological advances in science and the applications of biological research. We will engage with the public through open lectures, school visits and a special exhibition at Leeds City Museum. OTHER SPECIFIC IMPACTS Specific beneficiaries include the two PDRAs who will develop their scientific careers with BBSRC funding. They will be involved in a research project that crosses discipline boundaries in biology and engineering. They will benefit from working closely with laboratories in two different leading institutions (as verified by the 2008 RAE). The research will also impact on the training of undergraduates who will benefit from carrying out final year resresearch projects within our laboratories.