Locust with a backpack that helped apply mechanical stress to the exoskeleton
Jan-Henning Dirks
When the gravity acting on them is increased, locusts adapt. Locusts placed in a centrifuge to mimic the conditions of hypergravity grew tougher legs than those living normally – but not all of them survived the process.
Many biological materials, such as bone and wood, can adapt and become stronger under physical strain, but it isn’t clear whether animals with shell-like exoskeletons can adapt in the same way as those with internal skeletons. Karen Stamm and Jan-Henning Dirks at the City University of Applied Sciences in Bremen, Germany, studied this by placing locusts inside a specially designed centrifuge to stress-test their exoskeletons using simulated hypergravity.
The locusts were assigned to one of four gravity conditions: 1g – which is typical gravity at sea level and didn’t involve a centrifuge – and 3g, 5g or 8g conditions, all of which did involve centrifuging the insects. After two weeks, the researchers removed the locusts’ hind legs and tested how much force was required to bend them.
Stamm and Dirks also fitted some locusts with weighted backpacks to mimic the 3g, 5g and 8g conditions, but some of these locusts struggled to keep their balance with the added weight, and others found it difficult to move at all, so the researchers focused on the centrifuged locusts instead.
They found that the 3g group had legs nearly 1.7 times as stiff as the 1g group. The locusts in the 5g group had legs about as stiff as the 1g group, unless they were given what the researchers termed a “lunch break” between 12 and 1pm every day – then they had similar properties to the 3g group, aside from a slightly lower survival rate. Most of the 8g locusts died, although a lunch break kept more of them alive.
“My interpretation is that 8g is just too much. These high mechanical forces put the animals under a lot of stress,” says Richard Weinkamer at the Max Planck Institute of Colloids and Interfaces in Germany. “A bit of relaxation and a cosy lunch and the will to live is back – the struggle can go on.”
These results could help answer fundamental questions about how biological materials in general adapt and evolve under stress, as well as helping engineers design materials that can adapt to their conditions, Weinkamer says.
“Insect exoskeletons are in many ways different to bone endoskeletons, so finding this ‘universal ability to adapt’ [is] absolutely fascinating,” says Dirks. “The follow-up questions can most likely keep us busy for many decades.”
In the future, he and his colleagues intend to test whether the same effects are seen in different body parts of insects and in different species, as well as trying to understand the mechanisms behind these changes.
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