Download Full Image
The answer, supported by findings released in the journal Proceedings of the National Academy of Sciences (PNAS) by scientists from Midwestern University, Glendale, Ariz., ASU and the Argonne National Laboratory, Argonne, Ill., lies in how insects breathe – and, more specifically, in how limited their leg space is.
Unlike vertebrates, whose well-developed circulatory system connects in series to a respiratory system, insects depend on series of fine, passive tubes, called trachea, to deliver oxygen to their cells. Tracheae extend from a series of holes called spiracles, located on the surface of an insect’s abdomen. Oxygen diffuses from the spiracles’ openings, along the length of the trachea, directly to the active tissues. The system clearly works, as insects are one of the most diverse and populous organisms on the planet.
So why the scale-down in size since the Paleozoic?
Lead author Alexander Kaiser, who teamed up with fellow researchers Michael Quinlan of Midwestern University, and C. Jaco Klok and Jon Harrison at ASU, hypothesized that how big an insect could grow pivots around their tracheal system.
The researchers examined the respiratory systems of four separate species of the darkling beetle (a group whose larvae include mealworms), whose sizes ranged from an eighth of an inch to 1.25 inches in length. Working in tandem with John Socha and Wah-Keat Lee of the Argonne National Laboratory, the group imaged the bugs’ trachea with a phase-contrast synchrotron X-ray phase-contrast imaging system.
“This type of synchotron facility was developed for ‘big physics questions’ but is now being increasingly used for biological questions,” says Harrison, a professor in ASU’s School of Life Sciences. “For me, it was absolutely incredible to be able to observe the tracheae and air sacs inside living insects, in exquisite detail, contracting and pulsing with life.”
What the authors found is that modern bugs can become “too big for their britches.” Larger beetles “devote a greater fraction of their body volume to their gas exchange structures” – 4.8 percent versus the 0.5 percent found in smaller beetles – and that the trend toward more trachea in larger insects is greatest in their legs, reaching up to 18 percent in the largest species studied.
This pattern of increased investment in the tracheal system in larger insects was completely unexpected, Harrison says.
“In vertebrates, regardless of body size, lung and heart masses are a constant fraction of body mass, while many components of the respiratory system, for example the capillaries, are reduced in larger animals,” he says.
So how can overall body size pivot on a proverbial hollow leg?
The largest insect known to exist today is a mere 6 inches in length: the South American Titanic longhorn beetle, Titanus giganteus. To grow larger would mean more trachea, based on the research of Kaiser and his colleagues. And, while an exoskeleton eliminates the need for bones and yields more interior leg room to work with, you can only stuff so many hollow tubes into an organism’s extremities before the tubes start to crowd out the muscles that need them.
In other words, it’s hard to run from a predator if there’s nothing but air in your legs.
Moreover, because of their jointed skeletons, insects have narrowed points of connection between their bodies and their extremities, so there was nowhere else to go but down. This narrow portal at the joint, the number and width of the trachea needed to supply larger leg muscles, plus the lower oxygen levels, means that modern insects lack the leg room to grow larger.
So how could a rise in atmospheric oxygen allow insects to be bigger?
Previous work at Duke, Stanford and in Harrison’s lab in the College of Liberal Arts and Sciences at ASU has shown that insects produce smaller tracheae when reared in higher oxygen levels. Thus, Harrison says, higher atmospheric oxygen levels could allow insects to grow larger before filling up their legs with tracheae.
“A next important step will be to see whether this trend really occurs in the Earth’s largest insects, and in the modern species descended from the Paleozoic giants,” Harrison says. “One thing comparative biology has taught us is that evolutionary innovation tends to allow life to overcome physical limitations.”