Scientists have discovered that three species of dung beetle walk with a "galloping" gait not seen in any other insect.
The vast majority of insect species walk with what is known as an alternating tripod gait - steadily moving forward three legs at a time.
It is not clear why these flightless beetles have shifted so radically from the usual way of walking.
The researchers wrote: "Like a bounding hare, the beetles propel their body forward by synchronously stepping with both middle legs and then both front legs."
They beetles appeared simply to "drag" their back legs, they added.
"We noticed that [one species we were looking at] was kind of bobbing along in a peculiar manner," he told BBC News.
"At first we thought that maybe there was something wrong with it - that maybe it had some damage to its hind legs.
"And then we noticed that all of the beetles of that species move like that most of the time."
The researchers used high-speed camera to film the beetles' motion in detail.
They then carried out tests in the lab, tracing the exact movement of the insects' legs as they walked across sandpaper.
If the group of species the team was studying, known as Pachysoma, they found three species to have this same unusual gait.
'A real puzzle' Dr Bill Sellers, who studies biomechanics at the University of Manchester, described the findings as "very peculiar".
He explained that almost all of about one million insect species that have been described to science walk with the classic tripod gait.
"They form a triangle with their legs," he explained. "So they have the left-front, right-middle and left-back legs on down at once, and then the opposite ones."
"It's a very easy gait to control, and we know it's quite efficient."
"So it's difficult to see why these beetles do what they do - it's a real puzzle."
Dr Smolka and his colleagues carried out miniature time trials in their lab to see if this beetle was able to move any faster than beetles that walked normally.
"But it turned out that the normally walking insects were up to 50% faster," he said.
"What we're going to look at in the future is if it has something to do with their navigation."
Their latest studies have suggested that the galloping beetles are able to stabilise their vision better when they are on slippery surfaces.
"So it could be that if you're moving really fast [in a tripod gait], your head just moves from side to side so much that vision could be blurred," Dr Smolka explained.
"Whereas in the galloping beetles, vision is always stabilised towards the front.
The vast majority of insect species walk with what is known as an alternating tripod gait - steadily moving forward three legs at a time.
It is not clear why these flightless beetles have shifted so radically from the usual way of walking.
The researchers wrote: "Like a bounding hare, the beetles propel their body forward by synchronously stepping with both middle legs and then both front legs."
They beetles appeared simply to "drag" their back legs, they added.
- The fact that dung beetles feed on faeces makes them useful recyclers. There are three main types of dung beetles: rollers, tunnellers and dwellers. The dwellers actually live in the dung.
- The three species featured in this study are dung ball rollers. The three species with the galloping gait are Pachysoma endroedyi, Pachysoma hippocrates and Pachysoma glentoni.
Dr Jochen Smolka, from the
University of Lund in Sweden, who carried out the research in South
Africa, was studying wingless desert dung beetles in order to find out
how they navigated to their burrows.
"At first we thought that maybe there was something wrong with it - that maybe it had some damage to its hind legs.
"And then we noticed that all of the beetles of that species move like that most of the time."
The researchers used high-speed camera to film the beetles' motion in detail.
They then carried out tests in the lab, tracing the exact movement of the insects' legs as they walked across sandpaper.
If the group of species the team was studying, known as Pachysoma, they found three species to have this same unusual gait.
'A real puzzle' Dr Bill Sellers, who studies biomechanics at the University of Manchester, described the findings as "very peculiar".
He explained that almost all of about one million insect species that have been described to science walk with the classic tripod gait.
"They form a triangle with their legs," he explained. "So they have the left-front, right-middle and left-back legs on down at once, and then the opposite ones."
"It's a very easy gait to control, and we know it's quite efficient."
"So it's difficult to see why these beetles do what they do - it's a real puzzle."
Dr Smolka and his colleagues carried out miniature time trials in their lab to see if this beetle was able to move any faster than beetles that walked normally.
"But it turned out that the normally walking insects were up to 50% faster," he said.
"What we're going to look at in the future is if it has something to do with their navigation."
Their latest studies have suggested that the galloping beetles are able to stabilise their vision better when they are on slippery surfaces.
"So it could be that if you're moving really fast [in a tripod gait], your head just moves from side to side so much that vision could be blurred," Dr Smolka explained.
"Whereas in the galloping beetles, vision is always stabilised towards the front.
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