When it comes to flying, bigger isn't always better, especially in the insect world. In fact, mango stem borers (Batocera rufomaculata) that eat poorly in their youth experience stunted growth but also fly better than their bigger, well-fed peers. The reasons these larger beetles struggle to stay in the air are unknown, but it could lie in the beetles’ respiratory systems. Insects breathe through a network of tracheae – air-filled tubes that pipe oxygen directly to their cells. Bigger beetles need longer tubes, meaning that oxygen molecules take more time to reach the cells. So, Tomer Urca and Gal Ribak of Tel Aviv University, Israel, teamed up with Eran Gefen of the University of Haifa, Israel, to figure out whether larger mango stem borers might struggle to deliver oxygen to their cells in a timely manner. Because flying takes a lot of energy and requires a lot of oxygen, the larger beetles would then have a harder time supplying their muscles with enough oxygen to sustain flight.
Urca and colleagues first looked for differences in the size and number of airways in individual mango stem borers. The scientists weighed deceased beetles of various sizes, filled their airways with water and then weighed them again. The change in mass allowed the researchers to determine the total volume of each beetle's airways. It turned out that airways took up more space in the bodies of larger mango stem borers. The scientists wondered whether the extra airways were enough to help the bigger beetles move oxygen through their airways as quickly as their smaller peers. To check, they placed living beetles in an air-filled chamber and measured how much carbon dioxide each insect breathed out. Then, the researchers reduced the amount of oxygen entering the chamber until they found the lowest oxygen levels the beetles could handle and still breathe normally. That concentration indicates how quickly an insect can move oxygen through its body. It turned out that the size of the mango stem borer had no effect on the amount of oxygen it needed to sustain itself, suggesting that extra airways pay off by helping large mango stem borers deliver oxygen to their cells on time.
While the scientists carried out that experiment, they also noted each beetle's sex, and decided to check for differences between male and female beetles. They noticed that female beetles could breathe normally at lower oxygen levels than the males. Because adult female mango stem borers often fly with eggs in their abdomens – the large, rear sections of their bodies – the team figured that females might need to move more oxygen through their airways while they carry eggs. At first the scientists thought that eggs might compress the airways in the females’ abdomen, making it more difficult for them to breathe. But when they measured the volume of the female's abdomens, the researchers realized that the insects’ abdomens expanded to allow room for both the eggs and the airways. The researchers decided that the female beetles must be compensating for the added weight of the eggs instead.
To fly, heavier beetles need to generate more power and move oxygen to their flight muscles more quickly. For both sexes, the flight muscles reside in the beetle's less expandable thorax – the middle section between the head and abdomen – so the scientists decided to measure the beetles’ thoraxes as well. Larger beetles had relatively spacious thoraxes, but their extra airways took up more of the available space. The scientists think that larger beetles must have smaller flight muscles than expected for their size to make room for all the tracheae they need. Breathing room is necessary, but it comes at a cost.
