The moisture in dried-out elephant droppings isn't much to live on in a dry environment, so desert-dwelling dung beetles have had to evolve clever ways to conserve water. In a tour-de-force investigation of beetle breathing, scientists have discovered that dung beetles came up with an unexpected strategy: They breathe out of only one body opening while at rest.
Like all insects, dung beetles breathe through a set of tiny holes called spiracles. Scientists believed that the beetles inhale through spiracles on the underside of their midsection. From there, the theory went, air flows into a moist cavity beneath the wings before being exhaled through the abdominal spiracles on the side of their rear segment. That's the way cockroaches and locusts do it, and researchers thought dung beetles did the same thing.
To better understand the respiratory pattern of these desert sanitation engineers, scientists at the University of the Witwatersrand in South Africa devised an elaborate contraption. They wrapped a latex sheet around the bugs to isolate gas exchange for each of the body compartments, and they superglued sampling tubes to measure airflow over individual spiracles. The beskirted beetle looks "ready for takeoff," says entomologist Marcus Byrne.
The experiments revealed that resting beetles breathe almost exclusively through a single spiracle (the right mesothoracic, in case you're wondering), Byrne and co-author Frances Duncan report in the 15 August issue of the Journal of Experimental Biology. With only one hole open, air likely passes through the branching respiratory system, into the cavity beneath the wings, then back through the body and out whence it came. By keeping the other 15 spiracles closed, the beetle may retain more moisture in the air it exhales. But the researchers don't yet know exactly how this strategy saves water.
"This work will become a classic in insect physiology," says zoologist Laura Fielden of Truman State University in Kirksville, Missouri. She says the study shows researchers knew a lot less about beetle breathing than they'd thought, and it should challenge them to do more rigorous studies like this one.