Gravity is one of the most universal environmental effects on living systems. It tethers us to the earth and creates our tides.
For humans, physiological reactions to gravity occur daily. Just imagine you’re flipped upside down and standing on your head. After a few seconds, you’d feel pressure in your head due to an increased blood flow.
Now we know that gravity also affects the smaller inhabitants of our planet.
A new study published in the journal Proceedings of the National Academies of Sciences by Jake Socha from Virginia Tech, US, shows that insects experience similar physiological effects of gravity.
“This project started by seeing some weird things in X-ray images and asking questions,” says Socha. “No one expected that a small insect would have any type of response due to their gravitational orientation.”
This is because insects have an open circulatory system, which means that their blood isn’t contained in arteries and veins and that organs float in one central cavity. It was believed that this body structure existed without the same strong effects of gravity on closed cardiovascular systems seen in larger vertebrates.
Socha and his team studied the effect of gravity on Schistocerca americana, commonly known as the American grasshopper, and discovered an active response called functional compartmentalisation.
Analysis of X-rays revealed that air sacs located in the grasshoppers’ heads had greatly expanded when the insects were head-up (upright) while air sacs in the abdomen were smaller.
When the animals were head-down, the opposite occurred: the air sacs in the head decreased in size while those in the thorax greatly expanded.
Socha compared this effect to diving into a deep swimming pool. As a person dives lower down into the water, there is more pressure. This same concept applies to the grasshopper’s body: the part that’s lower, or beneath, the rest of the body has higher blood pressure, thus the air sacs are compressed.
However, when the insect is awake, the response is different. The air sacs change less in response to orientation.
“Our findings suggest that animals had control of the inside of their bodies,” says Socha.
“Earlier this year, we published a paper with a similar finding. We analysed beetles and found they had active body responses to compensate for forces on their bodies. So, we were interested in the other physiological responses of other animals.”
Classical understanding of open circulatory systems is that blood flows freely within the body, like liquid in a bottle, and that pressures inside the body would all be similar. Socha’s team discovered that the grasshoppers could alter internal body pressures with a flexible valving system.
“This was remarkable,” he says. “We had been seeing odd occurrences in X-rays, so we had ideas that something was going on.
“Finding this gave us the evidence to conclude that grasshoppers do have a mechanism to counteract gravity, which is counterintuitive to most scientists.”
The researchers also found that grasshoppers’ heart rates change with orientation just as humans’ do. An example is that dizzy feeling from standing up too quickly because gravity impedes blood flow to the brain. Your heart pumps harder to overcome this gravity effect.
“We have multiple indicators pointing to the grasshoppers responding to its body orientation,” says Socha.
“They respond physiologically to its orientation relative to gravity and have mechanisms inside its body to be able to deal with it. Grasshoppers are able to change their heart rate, respiratory rate, and functionally compartmentalise their bodies to control pressure.”
Ian Connellan is editor-in-chief of the Royal Institution of Australia.
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