One obvious pattern in animal biology is that insects and other arthropods are generally smaller than vertebrates. Why is this? More than 300 million years ago, in the late Paleozoic, giant insects up to 10-fold larger than those alive today roamed the earth. Recent findings from geologists have shown that the atmosphere had substantially more oxygen than today (31% vs. 21% today). These findings suggest that insects are small today (and were giant in the Paleozoic) because of the way they delivery oxygen to their tissues. This project tests this hypothesis by using fruit chafer beetles, the largest beetles living in the world today. Physiological and imaging experiments will be conducted to determine whether larger insects invest a greater fraction of their body as respiratory structure, and experience associated negative consequences (or trade-offs) such as reduced proportional sizes of the brain or digestive tract. A newly developed neural stimulating system will be used to assess respiratory function during the high demands of flight. 3D x-ray tomography and confocal microscopy will be used to measure the volume and structure of the respiratory system, brain, digestive tract, and muscles. Using such technologies, this project will test whether larger beetles must reduce the size and function of essential organs (such as the brain) in order to deliver oxygen to their tissues ? the limited by oxygen delivery hypothesis. Thus, this project has the potential to explain one of the most broad-scale patterns in animal biology. Understanding such physical limits on animal design is critical for development of new technologies for engineering micro-injection systems that utilize bio-inspired fluidic systems, and for understanding the potential effects of atmospheric changes on animals. This research project will also include educational and training components, including development of a web-based K-12 teaching resource, and research training for graduate, undergraduate, and high school students.