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doi:10.1038/nindia.2015.7 Published online 20 January 2015

Mechanical toolbox aids snappy flight of flies

Vijay Shankar Balakrishnan

A scanning electron microscopy image of the fly wing hinge showing the levers.
© Sane, S. P. et al.




A mechanical toolbox in the thorax of flies helps them move their wings in perfect synchrony, according to a new study1. Researchers at the National Centre for Biological Sciences, Bangalore, say the study throws light on an important aspect of evolution and biomechanics – how wing movements of flies are so fast yet precise.

The aerodynamic success of tiny flies – taking off, flapping wings 100 times a second, making twists and turns, or hovering in mid-air – has been of intrigue to researchers. “Their coordination seems much too perfect for a system that relies on the nervous system, on time scales of a few milliseconds,” says Sanjay Sane, the lead author of the paper.

Flies overcome this problem with the help of their thorax – an inverted-pear-shaped part between their head and abdomen. The thorax has a pair of specialised gyroscopes on either side, plus a clutch and a gearbox, just like in a car.

Gyroscopes are top-like spinning tools used in aircraft to look for orientations. In flies, similar club-shaped outgrowths called halteres on the hind of the thorax, do this job. Within the hind (or scutellum), the base of the halteres neighbour with the base of the wings. In fact, the halteres are evolutionarily modified hind wings that vibrate and help mediate balance during flight.

Sane’s group observed the wing and haltere movements of the flies – both in free flight and when they were tethered to a pedestal. “We began to closely observe haltere movements using high-speed videography in soldier flies,” Sane says.

Researchers (clockwise from top) Sanjay Sane, Amit Singh & Tanvi Deora




The halteres not only coordinate with themselves but also with the wings to execute the precise flight mechanics. Using surgical procedures and scanning electron microscopy, Sane’s group found a thick strip of 'cuticle' that links the wings, and also the halteres to the wings. This toolbox controls the coordination in flies independent of their nervous system, they found. The linkage makes the halteres move forward when the wings flap backward, like in a breast-stroke swim or, according to Sane, “like water-treading”.

The group also found that a different system controls the wing-haltere movement. Just around the base of the wings and halteres on either side, there are three movable levers – radial stop, pleural wing process and PteraleC. They act next to an immovable part called sub-epimeral ridge that sits under the base of the wing. The ridge connects the base of the wings to the base of the halteres, and the other movable parts act as gears, together as a clutch-and-gearbox set up. The action of this set up facilitates the coordinated swift in the flight. And this action is paired in time, just like in a car. However, unlike in a car, the components are kept on opposite sides.

One limitation of the study is that the methods employed don't explain higher-level control of the gearbox and clutch mechanisms, according to Ty Hedrick, who studies controlled movement of animals at the University of North Carolina, Chapel Hill, NC, USA. Hedrick says, however, that the study is interesting because Sane’s group has used “old" tools to reach broad conclusions. It could inspire engineers to make fast and precise movements in miniature machines.

Sane says the team would now try to understand how and how much this system varies between insect species.


References

1. Deora, T. et al. Biomechanical basis of wing and haltere coordination in flies. P. Natl. Acad. Sci. USA (2015) doi: 10.1073/pnas.1412279112