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DUNG BEETLES TURN IN WINGS FOR A LONG, DRY WALK
Newcastle-Upon-Tyne, UK
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For most dung beetles, wings are pretty handy when they need to get to a steaming pile of dung in a hurry. But not having wings has allowed one species that specialises in elephant dung to evolve a pretty nifty respiration system that might help it survive in arid environments. Lacking wings, the flightless dung beetle that lives in the Addo Elephant Park has a tightly sealed elytra (wing case) that forms a cavity where the wings would normally be. According to Marcus Byrne and Frances Duncan of the University of Witwatersrand, South Africa, this cavity may help the beetle conserve water (p. 1309).
This isn't a new theory. Beetles breathe by pumping air in and out of their bodies through small valves called spiracles. The problem is that when these valves are open, the beetle can loose valuable water by evaporation. All flightless beetles were thought to tackle this problem by breathing in through the spiracles at the front of their bodies, and out through the ones in the subelytral cavity at the back, lifting the elytra to release the carbon dioxide waste. `This is a bit like breathing into your cupped hands' explains Byrne, because as the cavity becomes humid it helps prevent water diffusing out of the beetle.
But when the team looked to see whether the beetles exhaled through their rear ends `instead of air coming in at the front and out at the back, it came in and out at the front' says Duncan! Maybe the insects didn't use the subelytral cavity during respiration at all. The team decided to see by monitoring the gases inside the subelytral cavity as the beetle breathed.
First they fitted a rubber skirt around the beetle's waist so that they could monitor the gases released from the insect's front and back ends. Then they drilled tiny holes in the beetle's elytral case, to monitor carbon dioxide and oxygen levels inside the cavity, and to see how the spiracles below were involved in breathing.
What they found was a `rather weird respiratory cycle'. The spiracles within the subelytral cavity respired cyclically, keeping oxygen below normal atmospheric levels, but raising the level of trapped CO2 until the beetle pumped the gas out of the elytral cavity and back through its body to be released at the front end. So the subelytral cavity behaves like a CO2 store, but why might this help the beetle?
Byrne and Duncan suggest that by storing CO2 outside its respiratory system, beneath the elytra, the beetle might be able to exhale less often, reducing the time that its spiracles are open to the environment and minimising water loss. And the low concentration of oxygen inside the insect's CO2 tank may also help to draw oxygen in around the edges of the elytra, allowing the insect to hold its breath for longer.
Winged beetles have the advantage over flightless beetles in searching for dung, but the remarkable respiratory system of the flightless dung beetle may give it the upper hand in arid areas. Nevertheless, it's still a long walk to the elephant takeaway when you've got no wings!
References
Byrne, M. J. and Duncan, F. D. (2003). The role
of the subelytral spiracles in respiration in the flightless dung beetle
Circellium bacchus. J. Exp. Biol.
206,1309
-1318.
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