Wetenschap - 19 oktober 2017

Mosquitoes escape our tactile senses

tekst:
Roelof Kleis

Mosquitoes that have sucked up blood take off in such a way that we do not notice. They have developed a special method for this.

© Florian Muijres, montage: James Gathanay

The method was discovered by a team of researchers led by Wageningen zoologist Florian Muijres. The result is described in the latest issue of the Journal of Experimental Biology. Muijres has been researching the flight behaviour of insects such as fruit flies for quite some time. He uses high-speed cameras that register their movements at thousands of frames per second.

Heavy
This time, his cameras were pointed at mosquitoes. After taking a blood meal, they carry a load roughly equal to their body weight, doubling its total. One would expect that they need to push off in order to take off. Our skin is very sensitive to touch, sensitive enough to notice a fruit fly pushing off. So how is it possible that we don’t feel a mosquito leave with our blood?

anopheles-gambiae-mosquito-malaria-vector-parasite.jpg

To find out, Muijres and his colleagues recorded the flights of 63 different mosquitoes in high detail. The critters were filmed using three perpendicularly set up cameras at 13,500 frames per second. Half of the insects were allowed to suck up blood before take-off. The amount of on-board blood was determined for each mosquito through measuring and weighing.

Unusual
Muijres used the images to investigate the flight of the mosquitoes. The result is remarkable, he explains. ‘Most flying animals use their legs to push off and start flying once they are airborne. Mosquitoes have an unusual approach and do it the other way around: they start flying before pushing off.’ Complex calculations have proven that the mosquito gets almost two-thirds of its take-off from its wing force.

The push-off force is thus spread over a larger area and a longer time.
Florian Muijres

During the take-off, the mosquito uses its long legs in such a way that the push-off is even less tangible. Muijres: ‘The mosquitoes lay their legs flat like a leaf and slowly stretch them. The push-off force is thus spread over a larger area and a longer time.’ Compared to fruit flies, which Muijres has also extensively studied, the mosquitoes barely use their legs during take-off. The mosquito uses a mere 13% of the maximum push-off force of the fruit fly.

Amplitude
Muijres’ images also show how the mosquito gets its heavy blood cargo in the air after filling up. It actually uses a fairly easy trick: ‘When the mosquito has to carry blood, it enlarges its wing beat. Under normal circumstances, it moves its wings 20 degrees forward and 20 degrees backward. With a blood cargo, this amplitude is doubled. It really is a simple adjustment.’

This peculiar push-off method allows the mosquito to get away unnoticed. The push-off force remains below the tactile trigger limit of mammals – including that of humans. According to Muijres, this knowledge will not immediately lead to applications, but it is certainly useful. ‘Look at it like an arms race between mosquitoes and us humans. The mosquitoes want blood, and we want to fight them. Over the ages, the mosquitoes have developed this strategy to escape. Knowledge on how they do this could help us settle the arms race to our advantage.’


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