It all started with a grasshopper in a linear accelerator at the Argonne National Laboratory located in Illinois. The critter was being beamed with synchrotron X-rays – a mighty machine for such a little bug. Why did they do it?
"We were running experiments testing the effect of anesthetics at 2am," study author Jon Harrison, professor of environmental physiology at Arizona State University, told IFLScience. "We overdosed a grasshopper who happened to be head-up, and I saw the air sacs expand in real time in front of my eyes. I thought I was hallucinating. Then we started flipping hoppers over and saw the same thing. It was very cool and exciting."
"We were totally caught off guard," added Jake Socha, a professor at Virginia Tech's College of Engineering. "We didn’t think that an insect’s orientation relative to gravity would make any difference whatsoever."
And yet, it did. Air sacs in the grasshoppers’ heads expanded when upright while air sacs in the abdomen contracted. When head-down, the opposite was true – a peculiar discovery.
"The air sacs in both ends of the animal responded in the same way. Whichever one was on bottom, those sacs would be compressed, and in contrast, those on top were inflated," said Socha.
"This happens quickly, and is more dramatic when the grasshopper is anesthetized, which caused us to realize that they were actively fighting the effects of gravity," added Harrison.
To deepen their understanding of the physiological effects of gravity on invertebrates, the team performed a series of imaging and radio-tracing experiments on both anesthetized and alert grasshoppers. Insects like grasshoppers have open circulatory systems – their blood is confined to vessels for only a portion of its travel through the body. For the rest, it flows into a central cavity and mixes with other fluids, which make contact with the bug’s internal tissues and organs. This fluid is collectively called "hemolymph."
In a surprise discovery, the team found that American grasshoppers (Schistocerca americana) can alter their internal pressure with a flexible valve system. In fact, the insects were able modulate different pressures in different regions of their body. Anesthetized grasshoppers, on the other hand, had no such control.
"We showed multiple responses of insects to body orientation, including changes in heart rate, breathing rate, and functional valving between body segments. One implication is that insects are likely altering all of these whenever they change position, which helps us appreciate their sophistication," said Socha.
The team found that the insect’s heart rate slowed when upside-down after 5 minutes and beat faster when right-side-up, similar to vertebrates. Grasshoppers have a tube-like heart to control blood pressure and the findings suggest they have physiological mechanisms to counteract orientation relative to gravity in order to deal with the change.
"Insects are being used for molecular studies of gravity-sensing mechanisms important for understanding biomedical issues in humans, and now we know that they have quite similar systemic responses to gravity as we do (just like they respond similarly to many drugs)," said Socha.
Gravity is one of the most ubiquitous environmental effects on living organisms, note the researchers in the Proceedings of the National Academy of Sciences, but the specifics of how they perform these feats are yet to be uncovered.
"We think it has something to do with the flexibility of the tissues inside the body: they might be able to create internal compartments by moving tissue around. But we don’t really know – that’s our next thing to figure out," said Socha.
Another question on the table (or accelerator) is if these effects disappear in tiny insects and get more dramatic as insects increase in size?
"For other researchers working on insects, you might have to be careful about what position the insect is in when doing experiments," added Harrison. "We don't know how widespread these newly discovered gravity effects are – we need to test lots of other species."
And sometimes, said Socha, that means using "one of the world’s most high-powered X-ray machines to look inside something as small and humble as an insect."