Insects can be pesky creatures, with flies and mosquitoes able to skillfully avoid attempts at swatting them away. Their ability to fly — and manoeuvre dexterously — is made possible by some remarkable adaptations they possess, including a clutch and even a gearbox, according to research from a team of Indian scientists.
Insects are the most successful organisms to inhabit the earth, notes Sanjay P. Sane of the National Centre for Biological Sciences (NCBS) in Bengaluru. They were the first living things that evolved the ability to fly. Mastery of the air, along with miniaturisation of their bodies, gave them incredible access to ecological niches that other animals could not reach.
But such miniaturisation also made flight more challenging. The laws of aerodynamics demand that the smaller an animal becomes, the faster it must flap its wings in order to fly, remarked Dr. Sane.
Houseflies and fruit flies flap their wings at between 200 to 300 times a second, with each wing stroke being carried out in about four to five milliseconds. A blink of an eye, on the other hand, takes about 150 milliseconds. “We are extremely slow compared to insects,” he said.
To remain airborne, flies must synchronise the movement of their wings as well as those of a pair of sensory organs (one on each side) known as halteres. The halteres, which are essential in giving insects a sense of their orientation as they fly, move in the opposite direction as the wings.
In a paper published this week in the Proceedings of the National Academy of Sciences (PNAS), Dr Sane, along two students in his lab, examined how flies coordinate the movement of their wings and halteres at such high rates of flapping. Nerve cells would be far too slow for this purpose.
When the wing of a dead fly was manually manipulated, the wing on the other side also moved as did both halteres, showing that mechanical linkages were involved. Using the soldier fly, Hermetia illucens , Tanvi Deora, a graduate student and the paper’s first author, was able to painstakingly figure out what those connections were.
A structure in the insect’s thorax, the scutellum, linked its wings. The wing and haltere on each side were connected by another structure, the subepimeral ridge.
However, a fly is also able to move just one wing. During courtship, for instance, a male fruit fly vibrates a wing to create a distinctive sound for attracting females.
It turned out that the insects utilised a ‘clutch’ mechanism to engage and disengage a wing from the thoracic coupling used to coordinate wing and haltere movement. This clutch was controlled by nerve cells, said Dr. Sane.
In another remarkable similarity with automobile transmission systems, flies also had the insect equivalent of a gearbox at the base of each wing. An imaging system set up by Amit Kumar Singh, the paper’s second author and now a graduate student in Australia, showed the gearbox in action.
This gearbox has four modes, with one mode used during the insect’s resting state and the other three during flight. The flight modes set how far up and down the wings could move, thus varying the force they created.
The ability to change modes in mid-flight appeared to be under the insect’s neuronal control, according to Dr. Sane. A fly may be setting different modes on its two wings so that it could turn and manoeuvre rapidly, he added. Bees and beetles too use similar mechanisms for flight, said Ms. Deora.
